In the Cu-Sb-O ternary system, CuSb₂O₆ is the most intensively studied compound, owing to its unusual structural and magnetic behaviour. Jahn-Teller distortions from the Cu²⁺ cause an axial elongation of the Cu-O octahedra to give rise to a monoclinic structure (s.g. P2₁/n)[1,2]. At high temperatures, this material undergoes a second order phase transition to the tetragonal phase (s.g. P4₂/mnm), isostructural to room temperature structures of CoSb₂O6 and NiSb₂O₆[3]. This modification may only be possible through an intermediate orthorhombic modification in Pnmm as defined through systematic symmetry reduction [4]. Through the doping of CuSb₂O₆ with Co and Ni, this structural transition can be investigated.

Neutron, lab X-ray and synchrotron single crystal and powder diffraction have been used to study phase transitions in both solid state solutions. In the Cu_1-xCo_xSb₂O₆ system, it was found that two phases exist between compositions x = 0.2 and 0.5, with a Cu-rich monoclinic phase and a Co-rich tetragonal phase⁴. By contrast, the Cu_1-xNi_xSb₂O₆ system exhibits a single-phase region from x = 0.4, where only the tetragonal phase remains. A phase transition can be observed in the solid solution where the monoclinic phase becomes tetragonal at high temperature. The orthorhombic intermediate can only be observed through Synchrotron powder diffraction.

X-ray absorption spectroscopy indicates that there has been partial reduction of Cu²⁺ to Cu¹⁺ in the higher doping concentrations of Cu_1-xNi_xSb₂O₆ with neutron diffraction on these materials confirming a net oxygen deficiency in the materials. Compounds with similar structures have also been investigated, including NiSb_2-xSn_xO₆ and ZnSb_2-xSn_xO₆, which also show a net oxygen deficiency in the structure. At higher temperatures, these materials also indicate a mixed occupation of Ni and Sb on the 2a and 4f sites, that suggest the material is undergoing a high temperature phase transition to the rutile phase.

Over the past 25 years, the field of negative thermal expansion (NTE) materials has grown from a scientific curiosity observed in a small number of oxide families to a vibrant field encompassing numerous different compositions, structures and mechanisms. Successful prediction and synthesis of new compositions that may show NTE by substituting atoms in known structure types has significantly expanded the number of materials that display this property. However, control of expansion and phase transition behavior as a function of temperature and pressure remains a challenge in many families of NTE materials.

This talk will focus on materials in the scandium tungstate (A₂M₃O₁₂) family, in which the M-site generally contains Mo or W, while the A-site can be substituted by trivalent cations ranging in size from Al³⁺ to the smaller lanthanides. Compositions in which the A and/or M-site are substituted by aliovalent cations have also been reported, which may adopt cation ordered structures. In this family, NTE is observed in an orthorhombic structure, but many compounds show a reversible phase transition to a structurally related denser monoclinic polymorph with positive expansion upon cooling or when pressure is applied. We recently found that strategic choice of A-site cations can be used to suppress undesired phase transitions to lower temperatures and higher pressures.

The CeT₂X₂ (T: transition element, X: p-element) intermetallics are intensively studied for their magnetic properties and exotic ground states. Determination of crystal structure is usually the basic characterisation for further research, but this is not the case in CePt₂Al₂ and selected homolog compounds, which are structurally unstable at low temperatures. This structural instability influences then the magnetic and electronic properties and plays the key role.

CePt₂Al₂ is a new member of this family, therefore we focused on structural properties in broad temperature interval 3 – 500 K. At room temperature the single-crystal X-ray diffraction study shows that the crystal structure is orthorhombic and modulated (Cmme(a00)000, with q⃗= (0.481, 0, 0)). This is uncommon in this family of compounds, which are usually tetragonal at room temperature. The high temperature X-ray powder diffraction was used for structure determination above room temperature and reveals structural transition to a tetragonal structure, which could be presumably described by CaBe₂Ge₂ structural type. This transition exhibits 50 K hysteresis and creates a domain structure in the sample. During the transition both tetragonal and orthorhombic phases coexist and their ratio is dependent on cooling rate.

The structural phase transition study is complemented by measurement of physical properties such as a specific heat, magnetization, and transport measurements in the temperature range between 0.5 and 300 K. Specific heat and magnetic susceptibility show an antiferromagnetic order below 2 K. On the basis of electrical resistivity and other bulk measurements, CePt₂Al₂ can be considered as a Kondo lattice material, for which the reduction of free magnetic Ce³⁺ moment is typical. The presence of a modulated crystal structure opens the possibility of a charge density wave state in CePt₂Al₂ as observed in (Re)Pt₂Si₂.

In cuprate materials, copper-oxide based perovskites, high-temperature superconductivity microscopically intertwines [1] with the pseudogap phase [2,3], charge-density-wave (CDW) order [4-5], as well as electronic nematic phases [6]. The mechanisms underlying the emergence of superconductivity, the nature of the pseudogap phase and the symmetry properties of the density-wave states remain to be clarified.

In the La-Sr-Cu-O system the ubiquitous presence of twin domains [7] prevents to unambiguously establish the true nature of CDW order. Under these conditions, in fact, the diffraction signatures of a uniaxial stripe CDW order [4] are indistinguishable from those of biaxial structures in which the charge density is simultaneously modulated along two perpendicular directions.

Here we report on our high-energy (100 keV) X-ray diffraction experiments carried at the P21.1 beamline at PETRA III (DESY) showing that applying uniaxial pressure to La_1.88Sr_0.12CuO₄ (LSCO) it is possible to resolve the domain degeneracy and thereby uncover the underlying charge stripe structure. We find that the resulting charge stripe ordering vector is perpendicular to the uniaxial stress direction. We discuss the average symmetry transition from the high temperature tetragonal (HTT) to the low-temperature orthorhombic (LTO) showing signatures of a possible breaking of the lattice centering and its link to the symmetry of the CDW order. Using a first-of-its kind dataset of CDW peaks, collected with a 2D single-photon counting detector, we attempt to resolve the underlying structure modulation in terms of in-plane and out-of-plane ionic displacements and discuss our finding within the bounding limits imposed by the evidence of an extended stacking-type disorder along the c-axis.

[1] Fradkin E., Kivelson S. A., Tranquada J. M., (2015) Rev. Mod. Phys. 87, 457–482.[2] Norman M. R., Pines D., Kallin C., (2005) Advances in Physics 54, 715–733.

[3] Daou R., Chang J., LeBoeuf D., Cyr-Choinière O., Laliberté F., Doiron-Leyraud N., Ramshaw B. J., Liang R., Bonn D. A., Hardy W. N., Taillefer L., (2010) Nature 463, 519–522.

[4] Tranquada J. M., Sternlieb B. J., Axe J. D., Nakamura Y., Uchida S., (1995), Nature 375, 561–563.[5] Wu T., Mayaffre H., Krämer S., Horvatic M., Berthier C., Hardy W. N., Liang R., Bonn D. A., Julien M., (2011), Nature 477, 19.

[6] Murayama H., Sato Y., Kurihara R., Kasahara S., Mizukami Y., Kasahara Y., Uchiyama H., Yamamoto A., Moon E. G., Cai J., Freyermuth J., Greven M., Shibauchi T., Matsuda Y., (2019) Nature Communications 10, 3282.

[7] Braden M., Heger G., Schweiss P., Fisk Z., Gamayunov K., Tanaka I., Kojima H., (1992) Physica C 191, 455.

The charge-density wave (CDW) is a modulation of the density of conduction electrons in metals, which is coupled to displacements of the atoms according to a wave with the same wave length (same wave vector) as the modulation of the conduction band [1]. The classical CDW is the stable state of quasi-1-dimensional (1D) metallic crystals at low temperatures. The latter possess crystal structures with obvious 1D features, like chains of atoms supporting 1D electron bands, and these materials display a highly anisotropic electrical resistance, with the low resistance in the direction of the 1D atomic chains. The mechanism is Fermi-surface nesting (FSN), where a single wave vector q connects to each other different parts of the Fermi surface. A modulation of the atomic positions according to a wave with this wave vector q leads to a lowering of the electronic energy. The accompanying elastic strain then leads to an optimal magnitude of the atomic displacements, the latter which can be measured by x-ray diffraction (XRD).

More recently, CDWs have been found in metals whose crystal structures and physical properties lack obvious 1D features [2, 3]. Mechanisms have been put forward, that provide alternative explanations for the formation of CDWs. In particular this includes q-dependent electron-phonon coupling (EPC). The EPC peaks at a particular wave vector q. A static modulation with this wave vector then leads to a lowering of the energy of the system, which is not purely electronic, but should be attributed to the EPC terms.

Here, we present comprehensive studies towards CDWs in the materials La₃Co₄Sn₁₃, CuV₂S₄ and Er₂Ir₃Si₅ with strong electron correlations [4–6]. Temperature-dependent transport and thermodynamic properties are correlated with temperature dependent diffraction studies. First-order and second-order CDW phase transitions are identified. Crystal structures of the CDW phases are determined, while considering changes of the symmetry along with the development of twinned crystals at the phase transitions. Crystal structures are successfully used to identify the atomic mechanisms of CDW formation and to explain peculiar electronic and magnetic properties. La₃Co₄Sn₁₃: Pm-3n (No. 223) to I2₁3 (No. 199; 8-fold supercell); CuV₂S₄: Fd-3m (No. 227) to Imm2(s 0 0) (No. 44.1.12.4) and Er₂Ir₃Si₅: Ibam (No. 72) to I-1(s₁ s₂ s₃) (No. 2.1.1.1).

[1] Monceau, P. (2012). Electronic crystals: an experimental overview. Adv. Phys. 61, 325-581. Doi: 10.1080/00018732.2012.719674

[2] Chen, C.-W., Choe, J. & Morosan, E. (2016). Charge density waves in strongly correlated electron systems. Rep. Prog. Phys. 79, 084505.

[3] Ramakrishnan, S. & van Smaalen, S. (2017). Unusual ground states in R₅T₄X₁₀ (R = rare earth; T = Rh, Ir; and X = Si, Ge, Sn): a review. Rep. Prog. Phys. 80, 116501. Doi: 10.1088/1361-6633/aa7d5f.

[4] Ramakrishnan, S., Schönleber, A., Hübschle, C. B., Eisele, C., Schaller, A. M., Rekis, T., Bui, Ng. Hai An, Feulner, F., van Smaalen, S. Bag, B., Ramakrishnan, S., Tolkiehn, M. & Paulmann, C. (2019). Charge density wave and lock-in transitions of CuV₂S₄. Phys. Rev. B 99, 195140. Doi: 10.1103/PhysRevB.99.195140

[5] Welsch, J., Ramakrishnan, S., Eisele, C., van Well, N., Schönleber, A., van Smaalen, S., Matteppanavar, S., Thamizhavel, A., Tolkiehn, M., Paulmann, C. & Ramakrishnan, S. (2019). Second order structural and CDW transitions in single crystals of La₃Co₄Sn₁₃. Phys. Rev. Mater. 3, 125003. Doi: 10.1103/PhysRevMaterials.3.125003

[6] Ramakrishnan, S., Schönleber, A., Rekis, T., van Well, N., Noohinejad, L., van Smaalen, S., Tolkiehn, M., Paulmann, C., Bag, B., Thamizhavel, A., Pal, D. & Ramakrishnan, S. (2020). Unusual charge density wave transition and absence of magnetic ordering in Er₂Ir₃Si₅. Phys. Rev. B 101, 060101(R). Doi: 10.1103/PhysRevB.101.060101

Lead titanate (PbTiO₃) is a prototype ferroelectric material. At high temperatures, it presents the ideal centrosymmetric cubic perovskite structure with Pm-3m symmetry. Cooling down to 770K, a first-order phase transition from the paraelectric phase to the non-centrosymmetric tetragonal P4mm is observed[1]. Its high para-ferroelectric phase transition temperature would be promising for its technological application. However, the high anisotropy (6% c/a at room temperature) combined with its positive thermal expansion in cooling, makes it impossible to produce this ceramic in the form of bulk. Doping the Pb or Ti site of the perovskite are options to decrease the anisotropy only enough to makes possible bulk production[2]. In this work, we study the effect of isovalent substitution of Pb⁺² by Ca⁺² in paraelectric to ferroelectric phase transition by structural, dielectric and ferroelectric properties.

The bulk ceramics with stoichiometry Pb_0.6Ca_0.4TiO₃ were synthesized by solid-state reaction, uniaxially and hydrostatically pressured and sintered by the conventional method. The electric permittivity as a function of frequency and temperature was carried out in an impedance analyzer (Agilent - 4294A) from 20K up to 600K with the sample in Linkan furnace and APD cryostat. The ferroelectric polarization versus electric field loops were carried out using a Sawyer-Tower circuit and APD cryostat. To perform the Synchrotron X-ray diffraction (SXRD) patterns collection a piece of the bulk was crushed into thin powder and annealed at 600K for 5 hours to remove the residual strain. The patterns were collected at XPD and XRD1 beamlines at the Brazilian Synchrotron Laboratory. The high-temperature piece of experiment was performed in reflection geometry using Arara’s furnace, while the low-temperature piece of experiment was performed in transmission mode using Oxford Cryoject.

The structural analyses were performed by Rietveld Refinement method using GSASII and the model P4mm from the pristine PbTiO₃ compound. Heating up, the linear expansion of the a lattice parameter is observed while c contracts until 360K, then an abrupt change happens and c presents a linear expansion. This structural anomaly temperature is relatively close to the paraelectric-ferroelectric phase transition is observed at ~390K by ferroelectric and dielectric properties [3]. However, differently to the pristine PbTiO₃ compound, this phase transition happens from a tetragonal non-centrosymmetric to another tetragonal but centrosymmetric (therefore paraelectric) symmetry – which can be the same phase of CaTiO₃-high temperature (I4/mcm)[4].

Moreover, uniaxial anisotropy was observed in the ferroelectric phase, in the direction (001). This anisotropy became isometric at the paraelectric phase.

To conclude, from 40% of Ca-doping in PbTiO₃ ceramic, we characterized a tetragonal ferroelectric to a tetragonal paraelectric phase transition in which the crystallite anisotropy (or strain) was an important feature to characterize the polar transition. This was also the first report of tetragonal-paraelectric phase in Ca-modifield PbTiO₃ ceramics.

[1] B. Jaffe, W.R. Cook, H. Jaffe, Nom-metallic Solids: Piezoelectric Ceramics, 1971.

[2] J. Zhao, et al., A combinatory ferroelectric compound bridging simple ABO3 and A-site-ordered quadruple perovskite, Nat. Commun. 12 (2021) 1–9.

[3] F. Regina Estrada, M. Henrique Lente, D. Garcia, The normal to diffuse phase transition crossover from thermal expansion analysis in calcium modified lead titanate, Ferroelectrics. 534 (2018) 50–55.

[4] M. Yashima, R. Ali, Structural phase transition and octahedral tilting in the calcium titanate, Solid State Ionics. 180 (2009) 120–126.

Valence tautomers are characterized by different distributions of electron density, where metal-to-ligand electron transfer accomplishes interconversion between tautomers [1]. These compounds are unique model systems that can help to study electron transfer mechanisms and find applications as sensors and displays or storage and fast optical switching devices. Wherein, the valence tautomeric interconversion can be thermally, magnetically or radiatively driven. Among transition metal complexes cobalt complexes with redox-active ligands have been shown to undergo a valence tautomeric interconversion between high-spin and low-spin forms [2, 3].

This study is devoted to optical and x-ray structure characterization of novel (N-cyclohexyl-2-iminopyridine)(3,6-di-tert-butyl-o-benzosemiquinonato)(3,6-di-tert-butyl-catecholato) (Co2) cobalt complex. We have monitored the transition induced both by temperature and laser stimuli. Complexes were dissolved in toluene. X-ray pump-probe study was performed at the Super-XAS beamline of the Swiss Light Source, Villigen, PSI. Green nanosecond laser with 532 nm wavelength was operated at 150 kHz repetition mode. For each energy point we accumulated an X-ray fluorescence signal for different delays after laser excitation pulse with 20 ns time resolution. Then principal component analysis was applied for the whole data set.

Figure 1. (a) Co K-edge XANES measured for Co2 sample at 213K and 293K in a solution, (b) transient difference signal after laser excitation for Co2 and (c) temperature dependence of UV-Vis spectrum for the Co2 sample in toluene.

Fig. 1 shows XANES and optical results for one of the studied cobalt complexes. According to XANES, change in the oxidation and spin state of cobalt can be observed when temperature decreases below 240 K (Fig.1(a)). Time-resolved transient difference shown in figure 1b can be compared to the static difference obtained at low and high temperatures (Fig.1b). Kinetics of the transient signal decay for 213 K can be approximated by a monotonic exponential decay with characteristic time 490 ns. Low-temperature UV-Vis spectra show intensity decreasing of broad band at 800 nm and increase of bands at 600 nm and 395 nm (Fig.1(c)). The band at 700 – 850 nm shows the transition in the high spin Co^II tautomer and has likely a metal-to-ligand charge transfer nature. The peak at ~600 nm characterise the low spin Co^III tautomer and caused by the ligand-to-metal charge transfer. The appearance of isosbestic points during cooling is strong evidence that only two different species are present in solution [2]. Obtained results confirm the presence of a valence tautomeric transition in the cobalt complex under study. Our measurements indicate that where Co^II is transformed into Co^III under temperature decrease while reverse transition can be induced both under the influence of temperatures and laser radiation.

[1] Pierpont, C. G. (2001). Coord. Chem. Rev. 216, 99.

[2] Adams D.M., Hendrickson D.N. (1996). J. Am. Chem. Soc. 118, 11515.

[3] Ash R., Zhang K., Vura-Weis J. (2019) J. Chem. Phys. 151, 104201.

Rare-earth iron borate RFe₃(BO₃)₄ crystals are studied worldwide lately owing to their perspective magnetoelectric and multiferroic properties [1]. A major part of these single crystals was grown by flux method using Bi₂Mo₃O₁₂ as a solvent [2, 3]. In this work temperature-dependent structural behavior of RFe₃(BO₃)₄ (R = Ho, Y, Sm, Nd) single crystals were studied by X-ray structure analysis. The chemical composition was verified by X-ray energy-dispersive elemental analysis. Additional structure information was obtained by Mössbauer spectroscopy on ⁵⁷Fe nuclei.

Bi atoms entered the composition of all the crystals during the growth process and the final compositions of single crystals studied are Ho_0.96Bi_0.04Fe₃(BO₃)₄, Y_0.95Bi_0.05Fe₃(BO₃)₄, Sm_0.93Bi_0.07Fe₃(BO₃)₄, and Nd_0.91Bi_0.09Fe₃(BO₃)₄.

Unit cell parameters for R = Ho, Y, Nd were measured over 30–500 K. Parameters a,b of the crystals with R = Ho, Y are descending with temperature lowering, whereas a,b parameters of Nd-crystal do not change strongly. A sharp jump of a,b for R = Ho and Y was registered demonstrating presence of structural phase transition. At the same time, c (T) dependence has the similar character for all three crystals (R = Ho, Y, Nd) – c parameter decreases with lowering temperature to 80–100 K and then grows smoothly down to 30 K.

Structure of Ho_0.96Bi_0.04Fe₃(BO₃)₄, Y_0.95Bi_0.05Fe₃(BO₃)₄, Sm_0.93Bi_0.07Fe₃(BO₃)₄, and Nd_0.91Bi_0.09Fe₃(BO₃)₄ was determined at several temperatures in 90–500 K temperature range to study temperature-dependent structure peculiarities, in particular, changes during the structural phase transition for R = Ho, Y. The temperature of the phase transition T_str= 365 К for R = Ho and T_str= 370 К for R = Y was stated on the basis of systematic absences analysis and temperature dependence of a,b parameters. Inclusion of Bi atoms with a larger ionic radius leads to T_str lowering in comparison with powder samples without Bi [4]. The structure of the compounds with R = Ho, Y was refined in sp. gr. R32 above T_str and in sp. gr. P3₁21 below it. Structure of crystals with R = Sm, Nd belongs to sp. gr. R32 at all temperatures studied. There is a slight steady decrease of specific distanced in (R,Bi)O₆ trigonal prisms, FeO₆ octahedra, BO₃ triangles, and Fe–Fe helicoidal chains with temperature lowering in sp. gr. R32. When going to lower-symmetry sp. gr. P3₁21 (for R = Ho, Y) and with further temperature decreasing non-uniform changes in the bond lengths are observed. Equivalent atomic displacement parameters U_eq decrease with temperature lowering. However, U_eq of oxygen atoms O1 and O2 as well as ones of boron atoms B2 and B3 (sp. gr. P3₁21 labels) are highly sensitive to a structural phase transition, demonstrating fluctuations around T_str.

Debye (T_D) and Einstein (T_E) characteristic temperatures for cations in the crystals with R = Ho, Y, Sm, Nd were calculated. Both T_D and T_E values are close for the same type of cations. T_D and T_E for R and Fe atoms in sp. gr. R32 are close to the corresponding values in sp. gr. P3₁21, and there is a significant change in T_D, T_E values for B atoms after a phase transition.

Gamma-resonance measurements on ⁵⁷Fe nuclei showed that the hyperfine parameters of the Mössbauer spectra correspond to Fe³⁺ ions in an octahedral oxygen environment. Quadrupole splitting Δ temperature dependence demonstrates complex behaviour and is in good agreement with X-ray diffraction results.

[1] Kadomtseva, A.M., Popov, Yu F., Vorob'ev, G.P. et. al. (2010) Low Temp. Phys. 36. 511.

[2] Bezmaternykh, L. N., Kharlamova, S. A. & Temerov, V. L. (2004) Crystallogr. Rep. 49 (5). 855.

[3] Gudim, I.A., Eremin, E.V., Temerov, V.L. (2010) J. Cryst. Growth. 312. 2427.

[4] Hinatsu, Y., Doi, Y., Ito, K., Wakeshima, M. & Alemi, A. (2003) J. Solid State Chem. 172, 438.

While polynomial coefficients cannot explain the physical parameters associated, the Debye-Einstein-Anharmonicity (DEA) model [1, 2] adequately describes the temperature-dependent vibrational energy in the Grüneisen first-order approximation for lattice thermal expansion of a crystalline solid. In the DEA model, the Grüneisen parameter accounts for the isothermal and the anharmonicity parameter for the isochoric anharmonicity. Beside such advantages in DEA that concomitantly holds both quasi-harmonic and low-perturbed anharmonic [3] terms, this model is limited to explain metric thermal expansions close to phase transitions. For instance, framework material |Na₈I₂|[AlSiO₄]₆ sodalite [4] shows Landau-type tri-critical phase transition at 1080(6) K driven by tilt mechanism. The DEA model strikingly departs from the evolution of lattice parameters from 820(10) K up to the T_c. The kentrolite-type Pb₂In₂Si₂O₉ exhibits a second order phase transition at 778(5) K due to group-subgroup driven coordinate changes; again, the thermal expansion between 580(10) K and T_c cannot be modeled using DEA. Starting from the Landau theory for phase transitions [5-7], we propose a model that considers additional energy contributions integrated into the DEA, leading to metric parameter calculations close to phase transition. Adding the gliding (G) function to the temperature-dependent changes of internal energy essentially extends the general description as DEA+G. Thus, for the temperature-dependent metric parameter (M_i(T)) the Grüneisen first-order approximation can be expressed as:

?? (?) = ?0,? + ?????(?) + ??? (?) (1)

with M_0,i as the zero-point metric parameter of the i^th phase and be the contributed by DEA energy. is expressed as:

??? (?) = ?_? ?? = ?_? ?_? 3? ?_? ? ?^−?(??−?)? (2)

where N is the number of atoms, κ_G (1/K_o) the compressibility, a_G the isochoric anharmonicity parameter, k_B the Boltzmann constant, T_C the phase-transition temperature, λ the purview parameter, and n the transition exponent. While κ_G could be calculated from the bulk moduli associated with the Debye- and Einstein-thermoelastic terms, the only variable parameter is the isochoric anharmonicity in the range of a_G ~ 10^-5 K^-1 [3]. The function keeps the properties that describe the abrupt change of the metric parameters of a given material close to phase transition. The new approach (DEA+G) shows excellent fits for thermal expansion of both |Na₈I₂|[AlSiO₄]₆ and Pb₂In₂Si₂O₉ (Figure 1), and the model can be further justified using more relevant cases.

Chirality is a property that real images are non-superimposable on their mirror images. The importance of chirality has commonly been known through drug incidents of thalidomide all over the world ₁ ^1, ₂ ². Chirality exists not only molecules, crystals, membranes and other objects in nature. Crystal chirality is derived from not only molecular chirality but also helical arrangement of molecules in crystals. In the latter case, even if achiral molecules are put in a right-handed or a left-handed helical arrangement in crystals, the crystals occur chirality. It has already been known that the same amount of left-handed and right-handed crystals are obtained when chiral crystals composed of achiral molecules are grown ₃³. Among crystals composed of achiral molecules, about 8% of them are chiral crystals, so it is very important to grow chiral crystals that have particular chirality. However, it is extremely difficult to grow only right-handed or left-handed crystals from achiral molecules. In this study, we succeeded in growing right-handed or left-handed crystals from achiral molecules.

We have focused on Triglycine sulfate (TGS) crystals composed of glycine and sulfuric acid (Figure 1(a)). We found that TGS with particular chirality has grown by doping with L-, or D-alanine (Figure 1(b)). L-alanine-doped TGS (LATGS) crystals showed left-handedness, while D-alanine-doped TGS (DATGS) crystals showed right-handedness (Figure 2). This is an extremely interesting phenomenon. We discuss that this phenomenon is derived from polarity because TGS is ferroelectricity. The relationship between chirality and polarity helps the elucidation of the explicit mechanism of preferred chirality of TGS crystal by alanine.

HP research on Si started more than 50 years ago and since then several allotropes, displaying a wide variety of physical properties, have been reported. The narrow-bandgap semiconductorSi-III with BC8 structure (originally believed to be semimetal) can be obtained from the high-pressure tetragonal metallic phase, Si-II, formed during compression of common silicon according to Si-I→Si-II. Such a transformation during decompression can be either direct, Si-II→Si-III, or with an intermediate step Si-II→Si-XII→SiIII. Our in situ studies of pure Si in oxygen-free environment indicated that in the absence of pressure medium, Si-I remains metastable at least up to ~14 GPa, while the pressure medium allows reducing the onset pressure of transformation to ~10 GPa. Upon heating Si-III at ambient pressure a hexagonal structure, named Si-IV, was observed. This allotrope was believed to be a structural analogue of the hexagonal diamond found in meteorites (called also lonsdaleite) with the 2H polytypestructure. Calculations have predicted several hexagonal polytypes of Si and of other Group-IV elements to be metastable, such as 2H (AB), 4H (ABCB) and 6H (ABCACB). Exhaustive structural analysis, combining fine-powder X-ray and electron diffraction, afforded resolution of the crystal structure. We demonstrate that hexagonal Si obtained by high-pressure synthesis correspond to Si-4H polytype (ABCB stacking), in contrast with Si-2H (AB stacking) proposed previously. The sequence of transformations Si-III→Si-IV(4H)→Si-IV(6H) has been observed in situ by powder X-ray diffraction. This result agrees with prior calculations that predicted a higher stability of the 4H form over 2H form. Further physical characterization, combining experimental data and ab-initio calculations, have shown a good agreement with the established structure. Strong photoluminescence emission was observed in the visible region, for which we foresee optimistic perspectives for the use of this material in Si-based photovoltaics. The study of silicon allotropic transformation in Na-Si and K-Si systems at high pressure led to new open-framework allotrope of Si, Si24 with zeolite structure and promising optoelectronic properties.

Protic Ionic Liquids (PILs) are a class of tailorable solvents made up of fused salts with melting points below 100 °C, which are formed through a Brønsted acid-base reaction involving proton exchange[1]. These solvents have applications as lubricants, electrolytes, and many other uses[2]. Although they are quite similar to molten salts, their crystal structures have not been explored in-depth, with only ethylammonium nitrate (EAN) having a reported crystal structure[3, 4].
Ten alkylammonium-based protic ionic liquids at both neat (<1 wt% water) and 90 mol% PIL, 10 mol% water concentrations were selected. Diffraction patterns were collected at the Australian Synchrotron ANSTO while attempting to crystallise the samples by cooling to 120 K. Five samples crystallised (3 neat, 2 dilute), where the temperature of the system was then increased at a rate of 6 K/min to room temperature. From these patterns we have identified a number of crystal phases, identifying their stability ranges and lattice constant variation from 120 K to room temperature.

[1] Hallett, J.P. and Welton, T. (2011). Chemical Reviews. 111, 3508–3576.
[2] Greaves, T.L. and Drummond, C.J. (2008). Chemical Reviews. 108, 206–237.
[3] Abe, H. (2020). Journal of Molecular Liquids. 6.
[4] Henderson, W.A., et al. (2012). Physical Chemistry Chemical Physics. 14, 16041.

Hydrated sulfates are likely to be the dominant water reservoir in the equatorial region of Mars [1] forming massive, stratified deposits [2]. Hence their detailed mineralogical characterisation is critical in order to understand the aqueous history as well as the present-day equatorial water cycle of our neighbouring planet. Due to significant spectral similarities between sulfates of different chemical composition and degrees of hydration, attempts to constrain the exact nature of the polyhydrated sulfates from remote sensing data has proven to be challenging [3]. In-situ analysis by means of Raman spectroscopy and X-ray diffraction in Rover missions, however, appears to be a promising approach to provide insight on the mineralogy of these deposits. In order to facilitate such a phase analysis, there is a major interest in the thermal expansion and vibrational characteristics, and structural stability of candidate minerals at temperatures relevant to the martian surface.

To this end, we carried out a systematic study on various magnesium sulfate hydrates and related compounds and, notably for the first time, managed to synthesise cranswickite-type MgSO₄·4D₂O in the laboratory. Subsequently, we determined its thermal expansion at temperatures ranging from 340 to 20 K by means of time-of-flight neutron powder diffraction at the HRPD instrument (ISIS facility, UK) and from 300 to 80 K using high-resolution synchrotron X-ray powder diffraction at the I11 beamline (Diamond Light Source, UK).

The cranswickite samples studied revealed negative linear thermal expansion // c of approximately the same magnitude as the the positive linear thermal expansion // a, resulting in a net-zero thermal expansion in the ac plane over the entire temperature range under investigation. At 340 K cranswickite (space group C2/c) underwent a polymorphic phase transition to starkeyite (space group P2₁/n). The phase transition proceeded sluggishly and took several hours to complete. Most importantly, the cranswickite-to-starkeyite transition is accompanied by a volume reduction (ΔV ≈ -4.5 %), thus contradicting the general expectation that a less dense polymorph is formed at higher temperatures. To the best of our knowledge such interesting behaviour has so far just been observed for ThSiO₄ and isotypic PaSiO₄, albeitat substantially higher temperatures of approximately 1520 K [4].

[1] Feldman, W., Mellon, M., Maurice, S., Prettyman, T., Carey, B., Vaniman, D., Bish, D., Claire, F., Chipera, S., Kargel, J., Elphic, R., Funsten, H. (2004). Geophys. Res. Lett. 31, 10.1029/2004GL020181.

[2] Roach, L., Mustard, J., Swayze, G., Milliken, R., Bishop, J., Murchie, S., Lichtenberg, K. (2010). Icarus 206, 253-268. 10.1016/j.icarus.2009.09.003.

[3] Bishop, J. L., et al. (2009). J. Geophys. Res. 114, E00D09, 10.1029/2009JE003352.

[4] Keller, C., (1963). Nukleonik 5, 41-48

Spin crossover occurs in octahedral coordination compounds of the 3d⁴-3d⁷ electronic configuration of metal ions. The most spectacular changes are observed in Fe(II) complexes, where the HS→LS (HS – high spin, LS – low spin) transition is associated with shortening of Fe-N distance at about 0.2 Å. Although an ability to change of spin state is an intrinsic feature of the metal ion, the spin crossover properties of bulky, crystalline samples depend on the crystal structure of the coordination compound. Thus, different compositions of first coordination spheres of metal ions or presence in the crystal lattice crystallography unique molecules can result in the complex course of γ_HS(T) dependence (γ_HS(T) – the molecular ratio of molecules in HS form). Most often, a two-step spin crossover can be observed in such a situation. Our studies on iron(II) coordination polymers based on 1,4-di(1,2,3-triazol-1-yl)butane (bbtr) and its derivatives revealed a variety of spin crossover behaviours. [Fe(bbtr)₃](ClO₄)₂ exhibits abrupt spin crossover accompanied by hysteresis loop (T_1/2^¯ = 112 K, T_1/2^­ = 141 K)[1]. Importantly spin crossover in this complex is accompanied by structural phase transition P-3→P-1 depending on the shift of neighbouring polymeric layers. The structural phase transition has not been found in the tetrafluoroborate analogue, and the complex [Fe(bbtr)₃](BF₄)₂ remains in the HS form in the range 10-300 K[2]. The importance of structural changes on spin crossover properties showed our further studies using bbtr derivatives. An application of 1,4-di(5-ethyl-1,2,3-triazol-1-yl)butane (ebbtr) leads to the formation of two-dimensional coordination polymers exhibiting unique spin crossover: “double”[3] and “normal and reverse”[4] transitions. The occurrence of uncommon spin transitions in these complexes is associated with significant structural changes. An application of regioisomeric ligand bbtre leads to forming a three-dimensional coordination network in which the multi-way spin crossover is strongly related to conformational changes of the bridging ligands[5].

Studies of bbtr-based coordination polymers revealed the importance of counterion. Therefore, we have expanded our studies on the application of triflate derivatives. Synthesis performed between Fe(CF₃SO₃)₂·6H₂O and bbtr leads to forming a two-dimensional coordination polymer. The complex crystallizes in R-3 space group. The characteristic feature is the ordering of the half of bbtr bridging molecules and the presence of two crystallographically unique Fe(II) ions. Spin crossover is gradual and complete. Careful analysis of change of Fe-N distances revealed interesting phenomena. Namely, despite one-step spin crossover, both crystallographically unique Fe(II) ions change the spin state in different temperature ranges. Moreover, we have established the occurrence of very slow structural phase transition R-3→ P6₃. This structural transformation is associated with the vanishing of ligand disorder. Details concerning crystal structures of complexes before and after R-3→ P6₃ transformations on the poster.

[1] Bronisz, R. (2005). Inorg. Chem. 44, 4463.

[2] Kusz, J., Bronisz, R., Zubko, M. & Bednarek, G. (2011). Chem. Eur. J. 17, 6807.

[3] Weselski, M., Książek, M., Rokosz, D., Dreczko, A., Kusz, J. & Bronisz, R. (2018). Chem. Commun. 54, 3895.

[4] Weselski, M., Książek, M., Mess, P., Dreczko, A., Kusz, J. & Bronisz, R. (2019). Chem. Commun. 55, 7033.

[5] Książek, M., Weselski, M., Dreczko, A., Maliuzhenko, V., Kaźmierczak, M., Tołoczko, A., Kusz, J. & Bronisz, R. (2020). Dalton Trans. 49, 9811.

[Zn(ebtz)₃](BF₄)₂ (1,2-di(tetrazol-2-yl)ethane) was a first example of coordination polymer based of 2-substituted tetrazole as donor group [1]. Expanding studies on Fe(II) complexes showed that species of [Fe(tetrazol-2-yl)₆]-type core exhibit thermally induced spin crossover (SCO) [2]. Further researches revealed an ability of 2-substituted tetrazole to the formation of coordination compounds in which metal ion (Cu(II), Fe(II)) is surrounded by four tetrazole rings and two alcohol or nitrile molecules. The complexes of the type [Fe(tetrazol-2-yl)₄(RCN)₂] also exhibit SCO, which can be additionally affected by conformational changes of axially coordinated nitrile molecules [3]. It was established that the presence of a wide hysteresis loop in [Fe(ebtz)₂(C₂H₅CN)₂](ClO₄)₂ is related to the reorientation of coordinated propionitrile molecules coupled with significant changes of separation between supramolecular layers [4]. In order to explain the role of coordinated nitrile molecules on spin crossover properties, we have carried out detailed studies depending on systematic exchange of the ones in series of [Fe(ebtz)₂(RCN)₂](BF4₄)₂ [5]. We have focused on uncommon, very slow spin crossover observed in propionitrile derivatives. Measurements of the temperature dependence of magnetic susceptibility revealed thermal quenching of HS form after rapid cooling of the sample at 10 K. Measurements carried out at very slow scan rates showed an occurrence of hysteretic spin crossover (T_1/2^↓= 78 K, T_1/2^↑­=123 K). It allowed to perform isothermal (80 K) time-resolved single-crystal X-ray diffraction studies for the HS®LS transition. Initially, it occurs very slow shrinkage of polymeric chains associated with reduced cell volume at 77% (concerning the difference between cell volumes V_HS - V_LS) and only 16% of iron(II) ions adopt LS form. Then there starts fast and abrupt spin crossover associated with a significant increase of the distance between supramolecular layers, which occurs along the direction of the Fe–nitrile bonds. In this stage, there is a reorientation of propionitrile molecules connected with an increase of Fe-N-C(nitrile) angle from 143.6 to 161.6°. LIESST and r-LIESST studies performed at 14 K on single crystals confirmed that the contribution of switched Fe(II) ions strongly corresponds to the orientation of the nitrile molecule. These studies showed that stabilization of the spin form, produced by light irradiation, is dependent on the lattice-based effects. This property was utilized to manipulate spin crossover parameters by partial exchange of propionitrile by butyronitrile molecules. These studies showed that an increase in a fraction of butyronitrile molecules involves an increase of Fe-N-C(nitrile) angle resulting in a shift of SCO temperatures to higher values and in reduction of the width of the hysteresis loop.

[1] Bronisz, R. (2002). Inorg. Chim. Acta 340, 215.

[2] Bronisz, R. (2007). Inorg. Chem. 46, 6733.

[3] Książek, M., Kusz, J., Białońska, A., Bronisz, R. & Weselski, M. (2015). Dalton Trans. 44, 18563.

[4] Białońska, A. & Bronisz, R. (2012). Inorg. Chem. 51, 12630.

[5] Książek, M., Weselski, M., Każmierczak, M., Tołoczko, A., Siczek, M., Durlak, P., Wolny, J.A., Schünemann, V., Kusz, J. & Bronisz, R. (2020). Chem. Eur. J. 26, 14419.

The Li-ion battery technology completely revolutionized the portable electronic market and today it has become almost impossible to imagine a world without laptops, cell phones, etc.[1] This imposes a great challenge for the Li-ion battery industry, as demands for storing renewable energy and self-sufficiency in private homes are becoming more attractive.[2, 3] This will inevitably put pressure on the demand for both Li and Co, which are very limited resources.2 Despite elimination of toxic transition metals, like Co, has become a general aim for researchers and industry the Li extraction problem is yet to be solved.[4, 5] Here, Na-ion batteries are a great alternative to Li-ion batteries. Two types of materials are especially interesting, namely the O3 and P2 material, first discovered by Delmas and co-workers.[6, 7] Here O3-type has the highest capacity, as this material is synthesized with a higher Na content.[8]

From previous studies, the O3-type material is known to go through several phase transitions going from rhombohedral to monoclinic symmetry. In the beginning of 1980 Na intercalation was established for several O3-materials herein O3-NaCrO₂. The O3-NaCrO₂ is relatively, as great cycling stability and thermal stability has been shown, though upon complete charge, this material becomes disordered, and reversibility is lost.[9, 10] The material has been proposed to form Cr⁶⁺, via a disproportionation from the formation of Cr⁴⁺, during charge which migrates into the interslab forming a tetrahedral environment with oxygen. At end of charge, Cr⁴⁺ is reformed via a comproportionation which is suggested to arrange in a rock-salt structure.[11]

In this work, we set out to follow the structural behavior during charge and discharge in NaCrO₂. We have via operando PXRD confirmed that the material undergoes 3 phase transitions during charge, before the disorder is introduced in the material. Furthermore, we aim to trace the formation of tetrahedral Cr⁶⁺ with both ex situ and operando pair distribution function analysis (PDF) as this is directly linked to Cr-migration This is believed to be the source to the disordering of the material and the misfunctioning as positive electrode material in Na-ion batteries.

References

1. T. Nagaura and K. Tazawa, Lithium Ion Rechargeable Battery, Progress in Batteries Solar Cellsl, 1990.

2. C. Vaalma, D. Buchholz, M. Weil and S. Passerini, Nature Reviews Materials, 2018, 3, 18013.

3. T. M. Gür, Energy & Environmental Science, 2018, 11, 2696-2767.

4. Z.-Y. Li, J. Zhang, R. Gao, H. Zhang, L. Zheng, Z. Hu and X. Liu, Journal of Physical Chemistry C, 2016, 120, 9007-9016.

5. N. Zhang, N. Zaker, H. Li, A. Liu, J. Inglis, L. Jing, J. Li, Y. Li, G. A. Botton and J. R. Dahn, Chemistry of Materials, 2019, 31, 10150-10160.

6. C. Delmas, C. Fouassier and P. Hagenmuller, Physica B & C, 1980, 99, 81-85.

7. C. Delmas, J. J. Braconnier, C. Fouassier and P. Hagenmuller, Solid State Ionics, 1981, 3-4, 165-169.

8. P. F. Wang, Y. You, Y. X. Yin and Y. G. Guo, Advanced Energy Materials, 2018, 8, 1701912.

9. Y. N. Zhou, J. J. Ding, K. W. Nam, X. Q. Yu, S. M. Bak, E. Y. Hu, J. Liu, J. M. Bai, H. Li, Z. W. Fu and X. Q. Yang, Journal of Materials Chemistry A, 2013, 1, 11130-11134.

10. C. Y. Chen, K. Matsumoto, T. Nohira, R. Hagiwara, A. Fukunaga, S. Sakai, K. Nitta and S. Inazawa, Journal of Power Sources, 2013, 237, 52-57.

11. S.-H. Bo, X. Li, A. J. Toumar, G. Ceder and B. C. A. Lawrence Berkeley National Lab, Chemistry of Materials, 2016, 28, 1419-1429.

Traditionally, electrode materials for intercalation-type rechargeable batteries have been crystalline. For crystalline materials, the material structure on the atomic length scale may be probed through traditional diffraction experiments and structure-property relations may be revealed through operando experiments, where the battery is charged and discharged, while irradiated with a proper probe, e.g. intense high-energy synchrotron x-rays. However, when electrode materials disorder, either during battery operation or are disordered ab origine, the limited range of structural coherence diminishes the amount of structural information extractable through traditional diffraction experiments. To probe the material structure on a more local length scale, total scattering combined with pair distribution function (PDF) analysis may be applied to elucidate the atomic structure on a very local level, even under dynamic conditions through operando experiments. [1-2]

This poster presents selected examples on both irreversible and reversible order-disorder phase transitions, for TiO₂-rutile and Na_xFe_1.13(PO₄)(OH)_0.39(H₂O)_0.6, respectively. The phase transitions are induced electrochemically by ion-intercalation and ion-deintercalation during battery operation. Also, an example on an ab origine nanocrystalline material, TiO₂-bronze, is presented. Also for this nanocrystalline material, traditional diffraction methods fall short, when it comes to characterization of the material structure at the atomic length scale. In all three cases presented here, x-ray total scattering and PDF analysis provide unique structural information on the atomic length scale for otherwise crystallographically challenged materials.

Figure 1. Left: Operando X-ray diffraction of Na_xFe_1.13(PO₄)(OH)_0.39(H₂O)_0.6. In the upper part, an overview plot of the scattering data is shown, where the scattering angle in degrees and the time in hours are displayed on the axes. In the lower part, the electrochemical data is shown as the voltage as a function of state of charge (overall Na content). The time and state of charge axes are congruent. Bragg intensity fades during deep discharge but is recovered upon charge. Middle: Ex-situ x-ray diffraction data for the pristine (start), Na-rich (end of 1^st discharge), and Na-poor (end of 1^st charge) Na_xFe_1.13(PO₄)(OH)_0.39(H₂O)_0.6 materials. For the Na-rich material, Bragg intensities fade, and broadening is observed to a rather large extent. Right: Fitted reduced atomic pair distribution functions for the pristine (start), Na-rich (end of 1^st discharge), and Na-poor (end of 1^st charge) Na_xFe_1.13(PO₄)(OH)_0.39(H₂O)_0.6 materials. The experimental PDF is shown as blue circles, the calculated PDF as a red curve, and difference curve of the two is shown as a green curve. [3]

[1] Christensen C. K., Sørensen D. R., Hvam J. & Ravnsbæk D. B. (2019). Nanoscale. 31, 512-520.

[2] Christensen C. K., Mamakhel M. A. Balakrishna A. R., Iversen B. B., Chiang Y.-M. & Ravnsbæk D. B. (2019). Nanoscale, 11, 12347-12357.

[3] Henriksen C., Karlsen M. A., Jakobsen C. L. & Ravnsbæk D. B. (2020). Nanoscale. 12, 12824-12830.

Keywords: Batteries; Operando; Pair Distribution Function Analysis; Order-Disorder Phase Transitions

We acknowledge the Carlsberg Foundation, The Independent Research Fund Denmark, and the instrument center DanScatt for funding.

Methylammonium Tin Halide Perovskites CH3NH3SnX3 (MASnX3, X: halide) are candidates of lead-free light- absorbing materials for photovoltaic devices. They undergo successive phase transitions caused by tilting or distorting of the SnX6 octahedra and orientational ordering of the organic MA cation. The structures of low temperature phases are still controversial due to the difficulty of accurate determination for the orientation of organic cation. In this study, neutron diffraction measurements were performed on MASnX3 with X=I, Br to determine the structures at low temperature phases and elucidate the ordering mechanism of the organic cation through the phase transitions.

Time-of-flight neutron diffraction data were collected using single crystal diffractometer SENJU (BL18) and Super High Resolution Powder Diffractometer SuperHRPD (BL08) installed at Materials and Life Science Experimental Facility, J-PARC. Diffraction patterns observed at five phases of MASnBr3 are distinctly different from each other, showing that the crystal structure changes successively through the four phase transitions. The result requires reconsideration of the structures below 213K in which no perceptible structural changes have been observed between β - γ phases nor δ – ε phases. For MASnI3, three phases with different structures were recognized as previously reported. Structure of the lowest temperature g phase remains uncertain, but a drastic change of the diffraction pattern between β and γ phases indicates that the structural symmetry is reduced considerably from tetragonal to triclinic system. Single crystal structural analysis at the cubic a phase reveals that the center of mass of the MA molecule locates off-center of the cubic unit cell, and nuclear density of the molecule synthesized by the maximum-entropy method shows anisotropic distribution along cubic axis. These tendencies appear more remarkable in MASnBr3, indicating stronger effect of organic-inorganic interaction in the X=Br crystal.

A new cobalt phosphite templated by diprotonated ethylendiamine molecule (C₂N₂H₁₀)[Co(H₂O)₆](HPO₃)₂ has been prepared via slow evaporation method. The hybrid material crystallizes in the orthorhombic system, space group Pbca, with the cell parameters: a=11.1518(9), b=9.8014(8), c=13.3782(8) Å, V=1462.28(19) Å3, and Z=4. The compound exhibits a bidimensional crystal structure formed by an anionic layer with the formula [Co(H₂O)₆(HPO₃)₂]^2- along the a-axis. The ethylenediammonium cations are located within the anionic cavities, through establishing hydrogen bonds network. The layers are made upon isolated Co(H₂O)₆ octahedra and (HPO₃)^2- tetrahedral phosphite anions, which interact through hydrogen bonds. The Infrared spectroscopy presents the characteristic bands of the hydrogenophosphite anion, ethylenediammonium cation, and water molecule. Thermogravimetric analysis (TGA) and catalytic efficiency data for the hybrid compound are investigated and it was found to be very efficient as a catalyst.

Diffraction techniques has been employed to study the molecular reorganisation of three oligonuclear spin crossover (SCO) complexes of the form [Fe(II)_XL^R₄]BF₃·MeCN (X = 2,3,4; L =R-3,5-bis{6-(2,2’-bipyridyl)}pyrazole; R= H, CH3), with a grid-like arrangement [1-2]. A multi-temperature crystallographic investigation exhibits the gradual phase transition in all compounds and a cross-talk between strongly linked metal centres. A systematic comparison between metallogrids suggests that the intramolecular cooperativity results from the complex interplay between grid flexibility and nuclearity. The latter has also a key role in molecular geometry of the ephemeral species formed after light irradiation with a femto-second laser pulse, as observed by time-resolved crystallography. The metallogrid with two non-linked metal centres shows a single molecular rearrangement during the first nanosecond after excitation, while the tetranuclear grid has multiple structural arrangements during the same span of time. More in general, this work exhibits the structural modifications accompanying the thermal and ultra-fast photo-induced spin transition of three metallogrids complexes.

Beta titanium alloys possess several attractive properties for use in load-bearing biomedical implants of large body joints, in particular the high strength combined with low Young’s modulus, biocompatibility and corrosion resistance. Recently developed alloy Ti-35Nb-6Ta-7Zr-0.7O (wt.%) with high content of strengthening oxygen exhibits the high strength of 1000 MPa and the Young’s modulus of 80 GPa. By reducing the content of beta stabilizing elements (Nb, Ta), the high strength of 1000 MPa is preserved due to oxygen strengthening and the Young’s modulus is reduced, reaching value of approx. 60 GPa in Ti-29Nb-7Zr-0.7O (wt.%). The pure beta phase after solution treatment is retained even at these low Nb/Ta concentrations. On the other hand, the phase transformations during heating differ significantly. The ongoing phase transformations in this alloy were investigated by in-situ dilatometry and electrical resistance measurements as well as by ex-situ methods after linear and isothermal heating. The ex-situ methods include: scanning electron microscopy, microhardness measurements and x-ray diffraction. It was found, that the beta phase stability is reduced significantly by reducing the content of Nb/Ta.

The main findings of this complex experimental investigation of several Ti-Nb-(Ta)-Zr-0.7 wt%O may be summarized as follows:

- The yield strength exceeding 1000 MPa and the ductility of approx. 20% was achieved in these alloys.

- The decrease of the content of Nb and Ta resulted in the decrease of the Young’s modulus from 80 GPa at Ti-35Nb-6Ta-7Zr-0.7O to 60 GPa at Ti-29Nb-7Zr-0.7O alloy.

- In-situ dilatometry and electrical resistance measurements suggest ω phase formation in less stable alloys.

- The lower phase stability leads to more homogeneous alpha precipitation at higher temperatures (700°C).

- At lower temperatures (400°C), no phase transformation occurs in highly stabilized alloys while in the least stable alloy, very fine α lamellae and relatively large ω particles (tens of nm) are formed. This is also accompanied by a significant microhardness increase.

- The Ti-29Nb-7Zr-0.7O alloy seems to be a suitable material for orthopaedics and implantation surgery.

The recycling of critical elements has crucial importance to maintain sustainable use of raw materials. Phosphorus(P) is a sought-after limited natural resource due to its wide use in modern agriculture mainly as P-fertilizers. But it causes major problems for the environment such as eutrophication of ecosystems. In the future it could be depleted due to the high demand and declining natural phosphorite ore deposits. Therefore, the phosphorus recovery from mine and agricultural waste waters will be an important factor in preservation of the global consumption. The precipitation of M-struvite (NH₄MPO₄·6H₂O, M²⁺= Mg²⁺, Ni²⁺, Co²⁺) from waste waters is a promising P-recovery route. Besides avoidance of eutrophication due to extraction of excess phosphates and the restoration of the phosphorus resources the recovered M-struvites may be potentially be up-cycled for industrial applications e.g. Co and Ni-phosphate show excellent electrochemical properties for batteries or supercapacitors.

The precipitation process of M-struvites is strongly dependent on the degree of supersaturation, pH and on the exchange ions M²⁺.The influence of these precipitation parameters on the crystal morphology and size of transition metal struvite has been investigated only to a limited extent. An optimization of the reaction conditions could lead to more efficient M-struvite precipitation and significantly improved P-recovery method.

We reveal the effect of different reaction conditions on the crystal shape and crystallite size of M-struvites (NH₄MPO₄∙6H₂O, M = Mg²⁺, Ni²⁺, Co²⁺). Furthermore, we characterize the coordination environment of the crystalline end products and their related phases [Co-dittmarite (COD) NH₄CoPO₄∙H₂O and Co(II)phosphate octahydrate (CPO) Co₃(PO₄)₂∙8H₂O]. Due to the presence of various amorphous phases pH is changing significantly in the different systems. Mg- and Ni-struvite are stable in multiple concentrations of the educts and metal/phosphorus (M/P) ratios in contrast to Co-struvite which forms below M/P ratios of 0.4. A high M/P ratio with high concentrations of the educts decrease the crystallite size and idiomorphism of the crystals while low M/P ratios with low concentrations of the educts increase the crystallite size and the euhedral formation of the crystal planes. In the (Ni, Co)-solid solutions Ni and Co are homogenously distributed in the crystals with similar Ni# as in the aqueous solutions indicating no elemental fractionation in crystallization. Ni and Co-struvite exhibit a more centrosymmetric coordination environment compared to their related phases of COD and CPO determined by EXAFS. The CoO₆ octahedron expands slightly the ideal size of the struvite structure and decomposes to Co-dittmarite. From TEM analysis and pH measurements it is suggested that the crystallization of Ni- and Co-struvite follows a non-classical crystallization theory which consists of multiple nanophases, crystalline or amorphous, on the way to the final crystalline product.

Metastable β titanium alloys are widely used as construction materials in automotive and aerospace industry. These applications demand materials with superior properties, such as high specific strength, good ductility and excellent fatigue and corrosion resistance. The improvement of strength can be achieved through ageing treatment which results in the formation of small precipitates of thermodynamically stable α phase in the metastable β matrix. When the α precipitates are very fine and homogeneously distributed, the strength of the alloy increases without a significant deterioration of the ductility. This fine microstructure can be achieved by employing such heat treatment in which the α phase precipitation is preceded by ω phase formation. ω phase is a metastable phase occurring as nanometric-sized, homogeneously dispersed particles. ω-assisted α phase nucleation results in very fine α phase microstructure.
In this study, very small precipitates of the α phase were studied by scanning electron microscopy (SEM). This observation allowed us to investigate theorientation and spatial relationship between Alpha phase lamellae and Beta phase mattrix. The SEM study was supplemented by the measurement of small-angle x-ray scattering (SAXS) which provides precise information on crystallographic orientation between the β and α lattices and on the shape and size of α precipitates. In order to assess the relationship of microstructure and mechanical properties of the material, Vickers microhardness was measured.