Structural origin of the anisotropic and isotropic thermal expansion of K2NiF4-Type LaSrAlO4 and Sr2TiO4

Nonstoichiometry Defects in Double Oxides of the A2BO4-Type

Double oxides with the structure of the Ruddlesden-Popper (R-P) layered perovskite A_n+1B_nO_3n+1 attract attention as materials for various electrochemical devices, selective oxygen-permeable ceramic membranes, and catalytic oxidative reactions. In particular, Sr₂TiO₄ layered perovskite is considered a promising catalyst in the oxidative coupling of methane. Our high-resolution transmission electron microscopy (HRTEM) studies of Sr₂TiO₄ samples synthesized using various methods have shown that their structure often contains planar defects disturbing the periodicity of layer alternation. This is due to the crystal-chemical features of the R-P layered perovskite-like oxides whose structure is formed by n consecutive layers of perovskite (ABO₃)_n in alternating with layers of rock-salt type (AO) in various ways along the c crystallographic direction. Planar defects can arise due to a periodicity violation of the layers alternation that also leads to a violation of the synthesized phase stoichiometry. In the present work, a crystallochemical analysis of the possible structure of planar defects is carried out, structures containing defects are modeled, and the effect of such defects on the X-ray diffraction patterns of oxides of the A₂BO₄ type using Sr₂TiO₄ is established as an example. For the calculations, we used the method of constructing probabilistic models of one-dimensionally disordered structures. For the first time, the features of diffraction were established, and an approach was demonstrated for determining the concentration of layer alternation defects applicable to layered perovskite-like oxides of the A₂BO₄ type of any chemical composition. A relation has been established between the concentration of planar defects and the real chemical composition (nonstoichiometry) of the Sr₂TiO₄ phase. The presence of defects leads to the Ti enrichment of particle volume and, consequently, to the enrichment of the surface with Sr. The latter, in turn, according to the data of a number of authors, can serve as an explanation for the catalytic activity of Sr₂TiO₄ in the oxidative coupling of methane.

Ge-Containing Oxide-Ion Conductors with CaEu2Ge3O10-Type Structure Discovered by the Bond-Valence Method and Experiments

In the present work, we have discovered the first example of a CaEu₂Ge₃O₁₀-type oxide-ion conductor, Ca_1.05Sm_1.95Ge₃O_9.975. The CaEu₂Ge₃O₁₀-type structure was selected by screening 624 Ge-containing materials by the bond-valence-based-energy calculations. CaEu₂Ge₃O₁₀-type CaEu₂Ge₃O₁₀, CaGd₂Ge₃O₁₀, and a new material CaSm₂Ge₃O₁₀ were synthesized. CaSm₂Ge₃O₁₀ showed the highest electrical conductivity among these three materials. Ca_1+xSm_2-xGe₃O_10-x/2 (x = 0.05, 0.1, and 0.2) were also synthesized, and we found that Ca_1.05Sm_1.95Ge₃O_9.975 exhibited the highest conductivity of 1.2 × 10^-5 S cm^-1 at 1373 K. Oxygen transport numbers in Ca_1.05Sm_1.95Ge₃O_9.975 were determined to be 0.64(5) at 1073 K and 0.65(8) at 1123 K, which indicates that the major carrier is the oxide ion. Therefore, CaEu₂Ge₃O₁₀-type Ca_1.05Sm_1.95Ge₃O_9.975 is a new structure family of oxide-ion conductors. The crystal structures of the new materials CaSm₂Ge₃O₁₀ and Ca_1.05Sm_1.95Ge₃O_9.975 were successfully analyzed by the CaEu₂Ge₃O₁₀-type structure (space group P2₁/c) using the single-crystal X-ray diffraction data. The bond-valence-based-energy calculation for the refined crystal structure of Ca_1.05Sm_1.95Ge₃O_9.975 suggested that oxide ions migrate along the [2 0 1], [0 1 0], and [12.88 6.43 1] directions with energy barriers of 0.88, 0.92, and 1.1 eV, respectively, which indicates three-dimensional oxide-ion diffusion in Ca_1.05Sm_1.95Ge₃O_9.975.

Crystal growth and structure of a high temperature polymorph of Sr2TiO4 with tetrahedral Ti-coordination, and transition to the Ruddlesden–Popper tetragonal phase

We have grown single crystals of a new polymorph of Sr2TiO4. It contains titanium in an unusual tetrahedral coordination and transforms to the Ruddlesden–Popper structure with an interesting orientational relationship.

Structural, electrical characterization and oxygen-diffusion paths in LaSrGa1−xMgxO4−δ (x = 0.0–0.2) layered perovskites: an impedance spectroscopy and neutron diffraction study

We present the diffusion pathways in perovskite-like structures. The modules of Gfourir and BondStr of Fullprof are feasible to obtain different paths in first approximation.

Water insertion and combined interstitial-vacancy oxygen conduction in the layered perovskites La1.2Sr0.8−xBaxInO4+δ

H₂O molecules are split within the structure with protons bonded to the axial oxygens of the InO₆ octahedra, and with OH units occupying the interstitial space.

A new structure family of oxide-ion conductors Ca0.8Y2.4Sn0.8O6 discovered by a combined technique of the bond-valence method and experiments

Mg₃TeO₆-type Ca_0.8Y_2.4Sn_0.8O₆ was found as a new structure family of oxide-ion conductors by BVE calculations followed by experiments.

The Flexible Unit Structure Engine (FUSE) for probe structure-based composition prediction

Computationally led materials discovery requires efficient methods to generate either exact or approximate crystal structures that span the composition range of a chosen phase space. Here we present a new tool, the Flexible Unit Structure Engine (FUSE), for the generation of approximate 'probe structures' to predict regions of composition space where compounds can be experimentally realised. We then test FUSE by applying it to 42 compositions in the Y3+-Sr2+-Ti4+-O2- phase field. FUSE correctly identifies all of the target compounds in the regions of stability and identifies the exact crystal structure for 8 out of the 10 compositions.