Drastic Differences between the Local and the Average Structures of Sr₂MSbO_5.5 (M = Ca, Sr, Ba) Oxygen-Deficient Double Perovskites

Abstract

For many disordered materials, knowing their average crystal structure is insufficient for explaining and predicting their macroscopic properties. It has been found that a description of the short-range atomic arrangements is needed to understand such materials. In order to understand the conduction pathways in ionic conductors which have random distributions of vacancies it is imperative to know the local structures which are present. In this study the local structures of three oxygen-deficient double perovskites, Sr(2)MSbO(5.5) (M = Ca, Sr, Ba), have been investigated by neutron pair distribution function analysis. The ions in these compounds are all found to have local coordination environments which are radically different than those given by their average structures. While there is no long-range ordering of the oxygen vacancies in these compounds, a considerable amount of short-range order does exist. The conditions which drive the short-range ordering are discussed as are the possible mechanisms for achieving it. It is proposed that the SbO(5) polyhedra form distorted trigonal bipyramids by moving oxygen atoms into interstitial positions. In the M = Sr compound 45° rotations of SbO(6) octahedra are also present, which add additional oxygen atoms into the interstitial sites. Large displacements of the Ca(2+), Sr(2+), and Ba(2+) cations are also present, the directions of which are correlated with the occupancies of the interstitial oxygen sites. Reverse Monte Carlo modeling of the pair distribution function data has provided the actual bond length distributions for the cations.

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

“…A number of recent studies have highlighted how the actual coordination environments in oxygen deficient perovskites can be quite different from that implied by the average long range model. [4][5][6][7][8][9][10] Cryolite related materials are a class of double perovskites with an A 3 BX 6 formula such that one of the B-sites is occupied by the same element as the A-site (A 2 ABX 6 ). Certain cryolite compositions which have small tolerance factors and a very large size difference between the A and B cations have been found to have highly complex crystal structures.…”

“…[11][12][13][14][15] There are also oxygen deficient cryolites and related double perovskite compositions, such as Sr 3 SbO 5.5 , which have simple cubic crystal structures but locally have a large rearrangement of the atoms around the oxygen vacancy such that the actual coordination environments of the cations are similar to those seem in stoichiometric cryolites. 5 There exists a small class of materials which have cryolite-like formulas but with small concentrations of vacancies on the A-sites and anion sites. A while ago it was realized that the cryolite related compound Ba 3 WO 6 does not actually form with this composition but instead always contains some A-site and oxygen vacancies such that the true formula is Ba 11 W 4 O 23 .…”

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

“…Very recently Rb 3 AlF 6 was reported to have the same structure as -K 3 AlF 6 at room temperature (Rakhmatullin et al, 2020). There are also a few oxide and oxyfluoride compounds which have structures based on non-cooperative octahedral tilting (Bramnik et al, 2000;King et al, 2010King et al, , 2012King et al, , 2017Stö ger et al, 2010;Fry & Woodward, 2013).…”

Low‐energy Sr2MSbO5.5 (M = Ca and Sr) structures show significant distortions near oxygen vacancies

“…[ 9–11 ] However, some oxygen‐deficient perovskite systems do show significant rearrangements around oxygen vacancies. [ 12–15 ] A neutron pair distribution analysis [ 12 ] study of the double perovskite Sr 2 MSbO 5.5 (M = Ca and Sr) found that multiple structures where oxygen vacancies drastically altered the local geometry of the system could explain the same neutron‐scattering measurements.…”