Oxide ion conductors, used in separation membranes, electrolysers and fuel cells, are typically three dimensional isotropic materials, providing fast ion diffusion pathways. These are typically oxides that have been aliovalently substituted to enhance the concentration of mobile defects, most notably oxygen vacancies. One alternative strategy is to consider materials with anisotropic conduction pathways, and with excess oxygen, accomodated as interstitials. Based on this strategy we have recently investigated a series of oxides, including CeTaO4.17, CeNbO4+d (d = 0, 0.08, 0.25) and developed from this our interest in the structurally related La(Nb,W)O4+d compositions.

Each of these oxidised Ce based phases are known to adopt either a commensurate of incommensurate modulated structure, depending on the level of excess oxygen accommodated [1,2], but from a device perspective performed poorly as the Ce³⁺/Ce⁴⁺ ratio introduced undesriable electronic conductivity. In an effort to maintain the modulated structure(s), suppress electronic charge transport and enhance oxygen transport, we have targeted the LaNb_1-xW_xO_4+d sereis of materials. Our studies have developed the solid solution series phase chemistry and from application of X-ray, neutron and electron diffraction techniques, identified a sequence of modulated monoclinic and tetragonal phases. We have probed the ion transport of a select number of these phases, proving their capability as oxide ion conductors. We highlight the local structrure and variation in the coordination environments that facilitate the fast ion transport, offering routes to optimise and develop new functional oxides.

The collective motion of electrons has always been a fascinating topic in condensed matter physics. In charge density wave (CDW) systems, transport measurements were the first to provide a clear signature of the collective motion of condensed electrons. A non-linear conductivity is observed above a threshold current IT and is attributed to depinning of the CDW on impurities. An excess current then arises as well as a broad band noise and current oscillations. Although the electron density modulation involved in CDWs is very small, x-ray diffraction provides information about the structure of the CDW as it is associated with a periodic lattice distortion. We will show here how state-of-the art x-ray diffraction techniques - coherent and nanoprobe XRD - can reveal the different steps of CDW deformations, from pinning to sliding, in systems of increasing dimensions.

The structures of the rare earth metal polychalcogenides REX_2–δ (RE = La-Nd, Sm; Gd-Lu; X = S, Se, Te; 0 ≤ δ ≤ 0.2) attracted some attention due to their distorted square planar chalcogenide layer and the motives observed within these layers. All structures share a common structural motif of an alternating stacking of puckered [REX] and planar [X] layers (Figure 1a) and are closely related to the ZrSSi structure (space group P4/nmm), which is regarded as their common aristotype [1]. For electronic reasons, the planar [X] layer shows distortions from a perfect square net, forming dianions X₂^2– for the non-deficient REX₂. By reducing the chalcogenide content vacancies are observed within the planar layer, resulting in different superstructures for the REX_2–δ compounds depending on the vacancy concentration. For the sulfides and selenides this results in additional X^2– anions along vacancies to maintain a charge balanced layer. The tellurides, however, show different ordering patterns in the planar [Te] layer for the non-deficient RETe₂ compounds, but also a tendency to form larger anionic fragments for the deficient RETe_2–δ compounds, as seen for the commensurate structure of GdTe_1.8, e.g. [2].

LaTe_1.94 and LaTe_1.82 are two examples of different incommensurate crystal structures for RETe_2–δ compounds, separated by the number of vacancies in the planar [Te] layer [3, 4]. Both compounds share an average tetragonal unit cell with a ≈ 4.50 Å and c ≈ 9.17 Å, based on the structure of their aristotype (Figure 1a). The major difference of these compounds are their respective q vectors, which are compatible with tetragonal symmetry for LaTe_1.94, but indicate a loss of the fourfold rotational axis for LaTe_1.82, ending up in an orthorhombic superspace group. The [Te] layer of LaTe_1.94 is mainly composed of single vacancies (point defects), isolated Te^2– anions and Te₂^2– anions. LaTe_1.82 is more Te deficient and features adjacent vacancies in addition to Te₃^4– anions, to compensate for the missing charges (Figure 1b). To evaluate the formation of possible larger anionic fragments, like a bent Te₃^2– anion and the influence of additional vacancies to the structure, DFT based ELI-D real space analysis of approximant structures were performed (Figure 1c).

Figure 1. a) Average structure of LaTe_1.82; b) section of the modulated [Te] layer of LaTe_1.82; c) orthoslices of ELI-D of the Te layer of LaTe_1.82 with isocontour lines based on a commensurate approximant.

[1] Doert, T. & Müller, C. J. (2016). Reference Module in Chemistry, Molecular Sciences and Chemical Engineering, Elsevier.

[2] Poddig, H., Donath, T., Gebauer, P., Finzel, K., Kohout, M., Wu, Y., Schmidt, P. & Doert, T. (2018). Z. Anorg. Allg. Chem. 644, 1886–1896.

[3] Poddig, H., Finzel, K., Doert, T. (2020) Acta Crystallogr. Sect. C 76, 530–540.

[4] Poddig, H., Doert, T. (2020), Acta Crystallogr. Sect. B, 76, 1092–1099.

Ni-Mn-Ga-based Heusler alloys are broadly studied for their magnetic shape memory (MSM) functionality originating from coupling between ferroelastic and ferromagnetic orders. The ferroelastic order is established after martensitic transformation. Formed ferroelastic domains with different orientation are separated by twin boundaries. In modulated phases, these boundaries are extremely mobile and can be manipulated by magnetic field. Thanks to these, the single crystals of five-layered modulated 10M martensite of Ni-Mn-Ga exhibits magnetically induced reorientation (MIR) of ferroelastic (twin) domains in a moderate field of the order of 0.1 T [1, 2]. This results in 6 % magnetic field induced strain (MFIS) down to liquid helium temperature [3]. Such unique behaviour makes the 10M martensite a perfect candidate for applications in actuators, sensors and energy harvesters.

The ferroelastic microstructure represents a challenge for proper determination of martensite phase structure. Due to the modulated nature together with complex hierarchical twinning (compound and type I and II a/c twins; and non-conventional twins) [4, 5], the structure of the 10M martensite has not yet been completely solved. There is even an ongoing discussion about the nature of the modulation where two main concepts are considered: i) general crystallographic wave modulation approach, and ii) nanotwinning. As the structural modulation seems to be the critical factor for the extremely high twin boundary mobility [5], the problem is pressing.

Using the X-ray and neutron diffraction, we investigated on the character and temperature evolution of 10M martensite phase. We found transition from commensurate to incommensurate 10M modulated structure in Ni₅₀Mn₂₇Ga₂₂Fe₁ single crystal [6]. The modulation vector gradually increases upon cooling from commensurate q = (2/5) g₁₁₀, where g₁₁₀ is the reciprocal lattice vector, to incommensurate with q up to pseudo-commensurate q = (3/7) g₁₁₀. Further cooling results in transition to 14M with q = 2/7 g₁₁₀. Upon heating, reverse changes of the commensurate-incommensurate transition are observed with a thermal hysteresis of ≈ 60 K. We detected the same hysteretic behaviour in the electrical resistivity and the effective elastic modulus. Scanning electron microscopy showed that the changes are accompanied by the refinement of the a/b laminate.

Furthermore, we observed continuous modulation changes within the 10M martensite of wide range of Ni-Mn-Ga(-Fe) compositions that undergo the Austenite → 10M → 14M martensite transition sequence. Based on these observations, we suggest that the commensurate state is a metastable form of 10M martensite. Upon cooling, this phase evolves through nanotwinning into a more irregular and more stable incommensurate structure, further supported by recent high-resolution TEM observation [7].

[1] Ullakko, K., Huang, J. K., Kantner, C. & Handley, R. C. O. (1996) Appl. Phys. Lett. 69, 1966–8.

[2] Kellis, D., Smith, A., Ullakko, K. & Müllner, P. (2012) J. Cryst. Growth 359, 64-68.

[3] Heczko, O., Kopecký, V., Sozinov, A. & Straka, L. (2014) Appl. Phys. Lett. 103, 198-211.

[4] Straka, L., et al. (2011) Acta Mater. 59, 7450–63.

[5] Seiner, H., Straka, L. & Heczko, O. (2013) J. Mech. Phys. Solids 64, 072405.

[6] Veřtát, P., et al. (2021) J. Phys.: Condens. Matter, accepted, https://doi.org/10.1088/1361-648X/abfb8f

[7] Ge, Y. et al., "Transitions between austenite and martensite structures in Ni₅₀Mn₂₅Ga₂₀Fe₅ thin foil", available at: http://dx.doi.org/10.2139/ssrn.3813433

This work was supported by Operational Programme Research, Development and Education financed by the European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports, project number SOLID21 CZ.02.1.01/0.0/0.0/16_019/0000760. P.V. thanks for the support by the Grant Agency of the Czech Technical University in Prague, grant number SGS19/190/OHK4/3T/14. We acknowledge the Institut Laue-Langevin and the project LTT20014 financed by the Ministry of Education, Youth and Sports, Czech Republic, for the provision of neutron radiation facilities.

The tungsten bronzes are low-dimensional transition metal oxides of great interest for their electronic instabilities. They show exotic physical properties such as superconductivity and charge density wave (CDW) phases. An important subfamily is (PO₂)₄(WO₃)_2m, which is interesting for its optical/magnetic behaviours, where the band filling and CDW phases coupled in different way with the lattice. These properties can be tuned by m, the thickness of the perovskite-like WO₆ –octahedra block¹.

To understand the electronic instabilities, correlated to the nesting properties of the Fermi surface and the consequent CDW phases, we used the combination of two techniques: diffuse scattering (DS) and inelastic x-ray scattering (IXS). This allows rapid identification of the nature of diffuse features in the patterns and the study of the lattice dynamics. Three different members are chosen in order to show the evolution of the behaviour in the family. In this context, we will focus on the lattice dynamics and framework instability. The first member, m=2, presents a quasi-1D instability given by the WO₃-octahedra zig-zag chains, which are isolated by the phosphates. A CDW phase is found, T_C=270K, and it is linked to a rigid-body motion. Different behaviour can be found in the members m=6 and 8, where the instability is found in the WO₃ slabs, realised as correlated displacements of tungsten atoms along the octahedral 4-fold axis direction. The three members show different diffuse patterns, figure 1. The results are linked to the lattice dynamics behaviour, which present a Kohn anomaly above the transition temperature, however as predicted from the diffuse results, has a different Q- and temperature-dependence in each member.

[1] P. Roussel et al., Acta Cryst. B 57 (2001) 603-632

The novel aperiodic titanate Ba₁₀Y₆Ti₄O₂₇ has a thermal conductivity that equals the lowest reported for an oxide at room temperature. All of the atomic sites are described by crenel function occupancy modulations. The resulting localisation of lattice vibrations suppresses phonon transport of heat. Thus Ba₁₀Y₆Ti₄O₂₇ represensts a new lead material for low thermal conductivity oxides, the possibility of using the structural description to slect other new leads will be explored.