Drastic Differences between the Local and the Average Structures of Sr2MSbO5.5 (M = Ca, Sr, Ba) Oxygen-Deficient Double Perovskites

New examples of non-cooperative octahedral tilting in a double perovskite: phase transitions in K3GaF6

The crystal structures of three polymorphs of K₃GaF₆ and the transition temperatures between these phases are reported for the first time. Synchrotron powder diffraction data clearly show that at 300 K α-K₃GaF₆ crystallizes in space group I4₁/a with lattice parameters of a = 19.1124 (3) Å, c = 34.4165 (6) Å, and Z = 80. The structure is based on the double perovskite but with two fifths of the GaF₆ octahedra rotated by ∼45°. This phase remains stable until ∼460 K, above which it undergoes a transition to I4/m with lattice parameters of a = 13.6088 (4) Å, c = 8.6764 (3) Å, and Z = 10 at 485 K. β-K₃GaF₆ has a similar structure but with only one fifth of the GaF₆ rotated by ∼45°. Above ∼510 K, the cubic Fm3m δ-K₃GaF₆ structure is stabilized, with a lattice parameter of a = 8.6649 (1) Å at 550 K. The F atoms have highly anisotropic displacement parameters which suggest dynamic octahedral tilting is occurring. This work expands the fairly small group of double perovskite compounds which display non-cooperative patterns of octahedral tilting.

Multi-scale structural analysis of the A-site and oxygen deficient perovskite Sr11Mo4O23

The long range average crystal structure, as well as the short and medium range structural features, of the A-site deficient and oxygen deficient perovskite Sr₁₁Mo₄O₂₃ have been determined. Rietveld refinement of synchrotron X-ray and neutron powder diffraction data show that this compound is cubic with space group Fd3[combining macron]m and a lattice parameter of a = 16.4108 Å. These findings contradict earlier reports of a tetragonal crystal structure. Sr₁₁Mo₄O₂₃ appears to be isostructural with Ba₁₁W₄O₂₃, except that the disordered coordination environment around one of the Mo sites seems to be a mixture of octahedral and square pyramidal instead of octahedral and tetrahedral. The short and medium range structural features have been inspected using the neutron pair distribution function (PDF). Short range correlations between the oxygen polyhedra surrounding the Mo(2) atom exist to avoid short O-O contacts. A model has been constructed which contains such correlations and is verified by reverse Monte Carlo (RMC) modeling of the PDF. The RMC refinements also give the distribution of inter-atomic distances in this compound which reveals how the various atomic positions are correlated and over what length scales. These results are important for understanding the ionic conduction pathways.

Identifying the local structural units in La0.5Ba0.5MnO2.5 and BaY0.25Fe0.75O2.5 through the neutron pair distribution function

Neutron pair distribution function data are used to investigate the local structures of two oxygen deficient perovskites with simple cubic crystal structures. La_0.5Ba_0.5MnO_2.5 is found to have alternating layers of MnO₆ octahedra and MnO₄ tetrahedra, making it similar to other members of the La_1-xBa_xMnO_2.5 series with smaller x values which have brownmillerite crystal structures. Our fitting results suggest that La_0.5Ba_0.5MnO_2.5 is not locally similar to any one brownmillerite structure type but instead has a variety of intra-layer and inter-layer relationships. We propose that this could be due to short range segregation of the much differently sized La³⁺ and Ba²⁺ cations, which creates significantly different inter-layer distances. BaY_0.25Fe_0.75O_2.5 is found not to have a brownmillerite-type local structure but rather seems to consist of structural units which are similar to Ba₃YFe₂O_7.5, an earlier member of the Ba_nYFe_n-1O_2.5n series. The PDF analysis shows that there are never neighboring Y atoms and that the O vacancies lie exclusively between Fe atoms, such that Y is always octahedrally coordinated. The PDF also suggests that there are more tetrahedral Fe than expected, which could be due to the presence of terminal O atoms in Fe centered dimeric units, similar to what is found in Ba₃YFe₂O_7.5.

Oxygen Insertion Reactions within the One-Dimensional Channels of Phases Related to FeSb2O4

The structure of the mineral schafarzikite, FeSb₂O₄, has one-dimensional channels with walls comprising Sb³⁺ cations; the channels are separated by edge-linked FeO₆ octahedra that form infinite chains parallel to the channels. Although this structure provides interest with respect to the magnetic and electrical properties associated with the chains and the possibility of chemistry that could occur within the channels, materials in this structural class have received very little attention. Here we show, for the first time, that heating selected phases in oxygen-rich atmospheres can result in relatively large oxygen uptakes (up to ∼2% by mass) at low temperatures (ca. 350 °C) while retaining the parent structure. Using a variety of structural and spectroscopic techniques, it is shown that oxygen is inserted into the channels to provide a structure with the potential to show high one-dimensional oxide ion conductivity. This is the first report of oxygen-excess phases derived from this structure. The oxygen insertion is accompanied not only by oxidation of Fe²⁺ to Fe³⁺ within the octahedral chains but also Sb³⁺ to Sb⁵⁺ in the channel walls. The formation of a defect cluster comprising one 5-coordinate Sb⁵⁺ ion (which is very rare in an oxide environment), two interstitial O^2- ions, and two 4-coordinate Sb³⁺ ions is suggested and is consistent with all experimental observations. To the best of our knowledge, this is the first example of an oxidation process where the local energetics of the product dictate that simultaneous oxidation of two different cations must occur. This reaction, together with a wide range of cation substitutions that are possible on the transition metal sites, presents opportunities to explore the schafarzikite structure more extensively for a range of catalytic and electrocatalytic applications.