In the last decade, Hybrid Photon Counting (HPC) X-ray detectors [1] like the PILATUS have transformed synchrotron research. They provide noise-free detection and enable new data acquisition modes. The most current HPC detector family EIGER2 enables even more ambitious X-ray science. These detectors combine all advantages of previous generations while offering new acquisition features and improved performance: maximum count rates of 10⁷ photons/sec per pixel, small pixels of 75 µm × 75 µm, two energy-discriminating thresholds, and frame rates up to 2 kHz with zero dead time (<100 ns) between exposures.

EIGER2 detectors were designed and optimized for the demands of synchrotron applications, and they are available for the laboratory as well. Equipped with CdTe sensors they provide high quantum efficiency at energies up to 100 keV, making them ideal for hard x-ray diffraction applications. Two energy thresholds allow for reduction of high-energy background such as from cosmic radiation, higher harmonics, or unwanted sample fluorescence. These benefits advance established X-ray diffraction methods in general like crystallography including powder diffraction as well as scattering techniques. Fast and gated measurements become possible and empower new fields of research, by enabling e.g., time-resolved or pump-probe techniques such as in laser-heating or fast compression and decompression experiments [2].

We will demonstrate the advantages of the HPC CdTe technology for high-pressure X-ray research. We show results from characterization and application measurements carried out in the laboratory and at synchrotron beamlines (ESRF, DLS, BSRF, APS) using loan detectors and the recently installed EIGER2 CdTe systems.

[1] Förster, A. et al. (2019), Philos Trans R Soc Math Phys Eng Sci, 377, 20180241

[2] Shen, G et al. (2017), Rep. Prog. Phys. 80, 016101

Insufficient coverage of reciprocal space may impede space group determination [1, 2], render revealing a crystal structure impossible [3] and conceal or wrongly reveal fine details such as disorder or unusual charge density distribution. [4] Standardised quality checks demand the diffraction pattern to be complete up to a certain resolution, usually 0.6Å^-1, for a good reason. [5]

While in majority of X-ray diffraction experiments modern area detectors and multi-axis goniometers allow to quickly scan the reciprocal space, the signal will not be observed if the beam does not have access to sample altogether. [6] This can be caused by a presence of Diamond Anvil Cell (DAC) or any other device which absorbs the beam around the sample. Deficiencies of such kind can be usually fixed by the means of symmetry. [7] The problem stands firm in case of compounds growing in low-symmetry crystal systems. A necessity to collect at least half or quarter of the whole pattern discourages investigators from conducting diffraction experiments, as suggested by the Cambridge Structural Database (CSD) [8] statistics.

Here we would like to present a comprehensive set of statistics describing a completeness of data in high-pressure experiments. Presented values have been calculated using a series of numerical simulations performed in a custom software. An influence of internal symmetry, crystal orientation and diamond anvil cell geometry on a final data completeness is meticulously analysed. Examples of experimental strategies leading to e.g. complete dataset for monoclinic sample and incomplete dataset for cubic sample are presented. Experimental strategies aiming to increase obtained completeness in various conditions are suggested. While similar estimations have been already suggested, [9] to the best of out knowledge our work is the first comprehensive study of this kind.

[1] Arnold, H., Aroyo, M. I., Bertaut, E. F., Billiet, Y., Buerger, M. J., Burzlaff, H., Donnay, J. D. H., Fischer, W., Fokkema, D. S. et al. (2005). International Tables for Crystallography, Volume A, 5th Edition (reprint), Space-group symmetry, edited by T. Hahn. pp 44–54. Springer.

[2] Sheldrick, G. M. (2015) Acta Cryst A 71, 3.

[3] Yogavel, M., Gill, J., Mishra, P. C. & Sharma, A. (2007) Acta Cryst D 63, 931.

[4] Takata, M. & Sakata, M. (1996) Acta Cryst A 52, 287.

[5] Spek, A. L. (2020) Acta Crystallogr. Sect. E 76, 1.

[6] Merrill, L. & Bassett, W. A. (1974) Review of Scientific Instruments 45, 290.

[7] Binns, J., Kamenev, K. V., McIntyre, G. J., Moggach, S. A. & Parsons, S. (2016) IUCrJ 3, 168.

[8] Bruno, I. J., Cole, J. C., Edgington, P. R., Kessler, M., Macrae, C. F., McCabe, P., Pearson, J. & Taylor, R. (2002) Acta Cryst B 58, 389.

[9] Casati, N., Genoni, A., Meyer, B., Krawczuk, A. & Macchi, P. (2017) Acta Cryst B 73, 584.

Pressure dependence of crystal and molecular structure and NLO response of L-Arg homologue salts

Rejnhardt P. ¹, Zaręba J.K. ², Katrusiak A. ³, Daszkiewicz M. ^1
1Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Division of Structure Research, Wrocław, Poland
²Wrocław University of Science and Technology, Faculty of Chemistry, Wrocław, Poland
³Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, Poznań, 61-614, Poland

Second harmonic generation (SHG) in organic crystals is a subject of extensive investigation for years. The absence of an inversion centre in such crystalline materials is mandatory in order to observe second-order non-linear response, although many other features play an important role and must be taken into account for synthesis of non-linear materials. For example, an occurrence of delocalized π electrons and intramolecular donor-acceptor charge transfer between two molecular subparts (functional groups) is necessary and it leads to large hyper-polarizability β. So, this kind of materials are good candidates for second harmonic generation studies.

In light of the above, the burning question is – in which direction one should look for materials with great SHG response? In the author’s concept, conformation of molecules in crystals is a key factor to obtain high SHG signal. Generally, the most important parts of molecule in the SHG context are donor – acceptor groups. With this reason, L-arginine analogues were chosen in these studies. They have shorter carbon chain in comparison to the L-arginine. We observed that relative distance between functional groups have a key role for enhancement of the SHG signal. What is more, inorganic anions in salts have a great influence on conformation of carboxyl and guanidinium groups. Some of them make single hydrogen bond with cation, what leads to appear more degrees of freedom for donor-acceptor groups. It is possible, that more degrees of freedom for aforementioned groups causes more suitable alignment of intramolecular charge transfer vectors in the crystal, which gives higher non-linear response.

The goal of the study is to verify experimentally, how the high pressure can modulate SHG response in one phase to avoid SHG changes connected to phase transitions and juxtapose them to the results for L-arginine homologues. So, correlation of molecular conformation and crystal structure with SHG is shown. This knowledge give us opportunity to describe theoretically which conformation can give the highest SHG response for particular materials.

We present crystal and molecular structures of 7 new salts (2Cl^–, 2Br^–, 2I^–, 2NO₃^–, Cl^–, I^–, 4NO₃^–) of (S)-2-amino-3-guanidinopropanoic acid, which is an analogue of L-arginine. Crystal structures were determined by X-ray diffraction at room and low-temperature conditions (100 K). Since earlier studies have shown that external pressure can tune SHG signal, diamond anvil cell was used to investigate the pressure dependence of SHG response.

The result of SHG measurements revealed that monochloride salt of (S)-2-amino-3-guanidinopropanoic acid has better optical non-linear properties than L-arginine chloride, 3·I_KDPvs. 0.3·I_KDP. This fact can be associated with shorter carbon chain (S)-2-amino-3-guanidinopropanoic acid and thus closer intramolecular distance between p-electron rich carboxyl and guanidinium groups than in L-arginine. What is more, the SHG response is more than 2 times better in 2.8 GPa pressure than in standard pressure.

Ice VII is one of the crystalline ices that stably exist above 2 GPa at room temperature. Oxygens form a bcc-type lattice and each oxygen is bound to neighbouring oxygens via hydrogen bonds. Hydrogens are disordered among the four sites on the oxygen-centred tetrahedra with equivalent probability resulting in their occupancy of 0.5 shown in Fig. 1. This simple cubic structure model is widely adopted but the true structure of ice VII is yet to be known. Strictly speaking, the oxygen sublattice is not bcc, and two models with oxygen displacements along <100> [1] and along <111> [2] are postulated. We reinvestigated the site disorder of oxygens (and hydrogens) in ice VII by neutron diffraction using modern high-pressure apparatuses.

Single-crystal and powder neutron diffraction patterns were collected at the D9 at the ILL in France and at the BL11 (PLANET) at the MLF J-PARC in Japan, respectively. Both measurements were conducted at approximately 298 K and 2 GPa. The single-crystalline specimen were directly crystallised from an alcohol-water mixture (D₂O:MeOD:EtOD = 5:4:1 in vol. ratio) in a newly-developed diamond anvil cell [3]. Powder specimen were prepared from pure D₂O in situ using the MITO system [4]. Fine powder crystals were obtained through solid-solid phase transitions (ice I_h+III→II→VI→VIII→VII).

Single crystals of ice VII were obtained by cyclic heating and cooling at a pressure above 2 GPa. The collected diffraction patterns were analysed by the maximum entropy method. The obtained scattering length density map exhibited anisotropic distribution from the average site. A derived pair-distribution function resembles that calculated from the average structure model in the long-r region while it does not match in the short-r region. This inconsistency is considered to be caused by the correlation between local structures.

[1] Kuhs, W. F., Finney, J. L., Vettier, C. & Bliss, D. V. (1984). J. Chem. Phys. 81, 3612–3623.

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[3] Yamashita, K., Komatsu, K., Klotz, S., Fernández-Díaz, M. T., Fabelo, O., Irifune, T., Sugiyama, K., Kawamata, T. & Kagi, H. (2020). High Press. Res. 40, 88–95.

[4] Komatsu, K., Moriyama, M., Koizumi, T., Nakayama, K., Kagi, H., Abe, J. & Harjo, S. (2013). High Press. Res. 33, 208–213.

Improvement to macroscopic features such as better conductivity, weather resistance, and improved capacity and speeds for digital storage is in high demand for the ever-growing electronic-device market. A key point in this development is to relate these features with the atomic and electronic structure of the material in interest, regardless of knowledge area (e.g. chemistry, physics, engineering). With this in mind, we intend to develop an infrastructure that supports both control and analysis of such structural properties based on uniaxial pressure and advanced synchrotron-based techniques. For this, a compact cell [1] allows a consistent uniaxial strain to sub-millimeter-sized samples over a temperature range between 0.3 and 325 K. Its clever design guarantees easy access to the sample, which also allows electrical contacts into it and provides a great solid angle for XRD experiments. In addition to it, the cell also supports magnetic fields up to 30 T. On the other hand, a code based on the xrayutilities [2] python package allows an efficient description of the material in interest via its composition, crystalline structure and, most essentially in our case, its elastic properties. Therefore, it plays an essential role in tracking or predicting the sample's structural changes depending on its orientation and the uniaxial strain direction, as is shown in Fig. 1, where a uniaxial stress sweep was applied to sapphire (Al₂O₃) to obtain its lattice parameters at each state.

Here, we exhibit the status of this uniaxial strain infrastructure present at the EMA beamline of Sirius [3], which can be used with other extreme thermodynamic conditions such as high magnetic field (up to 11T) and low temperatures (down to 0.5K). Particularly for XRD experiments, the uniaxial strain cell conveniently sits on top of a 6-circle (4S+2D) diffractometer (Fig. 2) that supports conventional single-crystal experiments and high-resolution Reciprocal Space Mapping (RSM), which are great tools to probe strain and crystal symmetry breaking. This approach will open a plethora of opportunities for fine-tuning of properties of advanced materials, such as the Heusler material Mn₃Ge, also shown here. Given its various crystalline structures with different magnetic ordering (cubic → ferrimagnetic, tetragonal → ferromagnetic with momenta along the c axis, hexagonal → non-collinear antiferromagnetic with Anomalous Hall Effect), it is a particularly interesting material, and it being theoretically possible to change between each structure using pressure in specific directions [4] offer a great potential of exploration.

The possibility to induce transformations in solid materials by grinding is known since ancient times [1]. While in those times only simple tools like mortar and pestle were available, laboratories nowadays use automatized milling instruments. Well-known examples for these tools are ball mills, for which shaker and planetary ball mills are the most widely used types [1, 2]. Since 2013, in situ X-ray powder diffraction (in situ XRPD) is applied to study processes taking place in shaker mills [3]. By using high-energy synchrotron radiation, it is possible to monitor transitions of crystalline compounds and the appearance of intermediate crystalline phases during grinding in real-time [3, 4]. Special adaptations of the grinding setup are necessary to fulfill the criteria for the successful performance of such in situ studies [3, 4]. Two aspects are especially important. On the one hand, the material composition and the wall thicknesses of the applied vessels determine the remaining intensity of the diffracted X-rays [5]. On the other hand, the used mill must provide a free pathway for the X-rays, which is not the case for conventional shaker mills [4]. The established way to ensure the fulfillment of both conditions is the use of vessels made from polymethyl methacrylate (PMMA) together with a modified shaker mill [4, 5]. This combination allowed for the successful in situ XRPD monitoring of the syntheses of soft materials by ball milling, like metal organic frameworks or organic co-crystals [3-5]. Recently, the utilization of an alternative vessel design was published for the successful in situ XRPD study of the mechanochemical synthesis of zinc sulfide from its elements [6]. In this case, the vessel was made of a material mix of stainless steel and PMMA. Despite these successful applications, the in situ monitoring of hard materials, which have a high demand towards the mechanical properties of the grinding tools, remain especially challenging. In this work, we present the first in situ XRPD data of the mechanochemically induced transformation of boehmite (γ-AlOOH) to corundum (α-Al₂O₃). So far, the transformation could only be shown by ex situ XRD data [7]. As one of the hardest materials, corundum is especially suited to explore the limits of a grinding system. We will discuss the specific demands, which arise for in situ XRPD of hard materials during ball milling and their technical solutions.

[1] Balaz, P., Achimovicova, M., Balaz, M., Billik, P., Cherkezova-Zheleva, Z., Criado, J. M., Delogu, F., Dutkova, E., Gaffet, E., Gotor, F. J., Kumar, R., Mitov, I., Rojac, T., Senna, M., Streletskii, A. & Wieczorek-Ciurowa, K. (2013). Chem. Soc. Rev. 42, 7571–7637.

[2] Friscic, T., Mottillo, C. & Titi, H. M. (2020). Angew. Chem., Int. Edit. 59, 1018-1029.

[3] Friscic, T., Halasz, I., Beldon, P. J., Belenguer, A. M., Adams, F., Kimber, S. A. J., Honkimaki, V. & Dinnebier, R. E. (2013). Nat. Chem., 5, 66-73.

[4] Halasz, I., Kimber, S. A. J., Beldon, P. J., Belenguer, A. M., Adams, F., Honkimaki, V., Nightingale, R. C., Dinnebier, R. E. & Friscic, T. (2013). Nat. Protoc., 8, 1718-1729.

[5] Halasz, I., Friscic, T., Kimber, S. A. J., Uzarevic, K., Puskaric, A., Mottillo, C., Julien, P., Strukil, V., Honkimaki, V. & Dinnebier, R. E. (2014). Faraday Discuss., 170, 203-221.

[6] Petersen, H., Reichle, S., Leiting, S., Losch, P., Kersten, W., Rathmann, T., Tseng, J., Etter, M., Schmidt, W. & Weidenthaler, C. (2021). Chem. Eur. J. 10.1002/chem.202101260

[7] Amrute, A. P., Lodziana, Z., Schreyer, H., Weidenthaler, C. & Schüth, F. (2019). Science, 366, 485-489.

Pressure has been used as an effective tool to tune the structure and property of materials. The near room temperature superconductive was achieved recently at extremely high pressure with great help from the crystal structure prediction via first principles calculation. But the superconducting mechanism in disordered systems has been less in focus. Superconductivity and Anderson localization represent two extreme behaviors of electrons in condensed matter system. Surprisingly, these two competitive behaviors can occur in the same quantum system, e.g., amorphous superconductor. Although disorder-driven quantum phase transition has attracted much attention, the structure origins remain unclear. Here, by applying high pressure to amorphous Sb₂Se₃, we discover an unambiguous correlation between superconductivity and density up to 65 GPa. Superconductivity first emerges in high density amorphous (HDA) phase above 23 GPa when the glass density reaches crystalline Sb₂Se₃, and then becomes more prominent in the body-center-cubic (BCC) phase above 50 GPa. Upon decompression, superconductivity persists until a sharp density drop where BCC phase transforms back to low density amorphous (LDA). Ab initio simulations reveal that the BCC-like local geometry motifs form in HDA by increasing fractions of short atomic rings, which could simultaneously transform the covalent bonds into “metavalent bonds”, a recent classification of chemical bonding coined in chalcogenide materials. Our results demonstrate that the intermediate amorphous state is responsible for the incipient superconductor prior to normal superconductive behavior.

The past few decades have witnessed mechanochemistry emerging at the forefront of solid-state chemical synthesis, driven by the search of new and cleaner synthetic methodologies. Mechanochemical synthesis utilizes high energy impact phenomenon to initiate chemical reactions. The peak impact pressures, which the individual sample particles experience, vary depending on the type of mill, milling speed, as well as size, shape, and density of the milling components, but can often exceed 10 GPa, while the temperature remains below 100 ^oC. Evolving beyond simply a solvent-free alternative, mechanochemistry offers significant sample quantities (grams) processed over a short period of time (minutes to hours). The design of the mill (e.g. tumbler, oscillatory, planetary) can also control the relative contributions of friction and impact during the milling process. In an effort to introduce mechanochemistry further into geoscience, the current presentation wishes to showcase the successful mechanochemical synthesis of compounds in the Mg-Co olivine solid solution series (e.g. Mg₂SiO₄, MgCoSiO₄, Co₂SiO₄) starting from simple oxide precursors such as MgO, CoO, and SiO₂ utilizing oscillating mill equipped with tungsten carbide (WC) jars/ balls as reaction vessels [1,2]. We further address on the contamination issue of the final synthesized product with debris shaved off from milling media (e.g. stainless steel, WC) and report on a successful development of method for converting WC to a water-soluble form [3]. Lastly, we report our investigations into pressure-induced phase transformation of anatase TiO₂ to rutile TiO₂, quartz-type α-GeO₂ to rutile GeO₂, and cubic Dy₂O₃ to monoclinic Dy₂O₃, processes that require pressure up to 7.7 GPa in a typical diamond anvil cell experiment [4]. Powder X-Ray Diffraction was employed as the main process characterization, with complete Rietveld refinements of the powder patterns of end products, where applicable.

This work was performed with the financial support provided by the Office of Naval Research, Department of Navy’s Historically Black Colleges and Universities/ Minority Institutions, the Materials for Thermal and Chemical Extreme program, grant number FOA N00014-19-S-F004.

[1] Nguyen, P. Q. H, Zhang, D., Rapp, R., Bradley, J. P., Dera, P. (2021). RSC Adv. 11, 20687.

[2] Nguyen, P. Q. H., McKenzie, W., Zhang, D., Xu, J.; Rapp, R., Bradley, J. P., Dera, P. submitted

[3] Nguyen, P. Q. H., McKenzie, W., Zhang, D., Xu, J., Dera, P. submitted

[4] Nguyen, P. Q. H., Dera, P. submitted

Alloys are important because of their superior physical properties and are extensively used in industries. Substitutional alloys are formed by randomly substituting one element by another. The formation of substitutional alloys made from metals are ruled by Hume Rothery rules. These rules say, that an alloy can be formed only if, the difference in atomic size of the solute and solvent is within 15%, the valency of solute and solvent is similar and the difference in electronegativity is small. Very few alloys have been synthesized from non-metallic elements. Well known examples, are the pnictogen chalcogenides (Bi₂Te₃, Sb₂Te₃, Sb₂Se₃, Bi₂Se₃) which form a disordered bcc substitutional alloy at high pressure[1-6].It is interesting to know that this is despite the fact that the atomic radii of Se is 26 % and 28 % smaller than Sb and Bi respectively. In order to understand if the presence of Pb will still lead to the formation of a substitutional alloy we have investigated the high pressure behaviour of this layered topological insulator, PbBi₄Te₇[6] consists of a seven-layer block Te-Bi-Te-Pb-Te-Bi-Te ,where the layers are linked by weak van der Waals forces.

Our x-ray diffraction studies show that PbBi₄Te₇ undergoes a phase transition at 6.15 GPa. However, beyond 10 GPa it transforms to a cubic bcc substitutional alloy despite the presence of Pb. Our ab-initio density functional theory based calculations show that at high pressure, there is a charge transfer from Bi and Pb atom to Te atom which makes the radii of these atoms approximately equal thus favouring the formation of a substitutional alloy. The covalent bonds become weaker with pressure as the ionicity increases. It was also observed that insertion of Pb enhances the charge transfer and thus lowers the pressure at which the substitutional alloy is formed in comparison to the parent compound Bi₂Te₃.

[1] I. Efthimiopoulos, C. Buchan, Y. Wang, Scientific Reports 6, 24246 (2016).

[2] A. Bera, et al. Phys. Rev. Lett. 110, 107401 (2013).

[3] A. Polian et al, Phys. Rev. B 83, 113106 (2011).

[4] S. M. Souza et al, arXiv preprint arXiv:1105.1097 (2011).

[5] D Pal et al, Materials Letters, 302, 130401, 2021

[6] Taichi Okuda et al., (2013). Phys. Rev. Lett. 111, 206803.

The current state-of-the-art synchrotron x-ray techniques combined with diamond anvil cell (DAC) and large volume press (LVP) techniques make phase transition studies one of most active, burgeoning fields in the high pressure community. The structural evolution of material under pressure is the long-term active research subject, which in fact strongly depends on the development of corresponding high pressure and synchrotron technologies. The selected research cases for various types of material, such as metallic glasses, melt, and crystalline materials under high pressure conditions, will be presented in this paper. The topics will include pressure-induced polyamorphization in several typical metallic glass systems, pressure-induced potential liquid-liquid in gallium melt, evaluation on novel characteristic method of fractional dimensionality for non-crystalline cases, pressure-induced phase transition consequence trend in metal dioxides at multiple 100 GPa conditions, etc. The contributions from the first principle calculations and synergetic effort in synchrotron sources will be discussed based on these scientific cases.

The development of many advanced synchrotron X-ray techniques provided great opportunity for the researches under high pressure conditions. Besides the most popular synchrotron X-ray diffraction technique to study the crystalline samples, we could use the high energy X-ray scattering technique combined with the Pair Distribution Function (PDF) method, to study structural evolution of non-crystalline samples in DAC or LVP. In addition, the densities of non-crystalline samples in DAC or LVP could be directly measured by synchrotron X-ray tomographic techniques. With instrumental developments in the collection of sparse scattering signals and to an increased flux and coherence of X-ray beams, X-ray photon correlation spectroscopy (XPCS) has recently emerged as a very powerful technique able to follow the evolution of the dynamics at the atomic length scale in crystalline and amorphous materials. Different from the conventional diffraction and scattering methods, which offer the information of average structures over the diffraction volume in sample, the XPCS could uncover the local order from time domain when the coherent beam size is equal to the illuminated sample volume, and exposure time is shorter than the onset time for the speckle dynamics. So the temporal relaxation procedure on the origin of amorphous state to another amorphous state transition process upon compression could be monitored. XPCS experiments under high pressure conditions were performed at room temperature, and results on selected typical metallic glass systems, will be presented by comparing with temperature effect using same XPCS techniques.