Thermal expansion behaviour in the oxygen deficient perovskites Sr2BSbO5.5 (B=Ca, Sr, Ba). Competing effects of water and oxygen ordering

Neutron diffractions studies reveal the presence of oxygen disorder in the oxygen deficient perovskites Sr2BSbO5.5 (B=Ca, Sr, Ba). Synchrotron X-ray studies demonstrate that these oxides have a double perovskite-type structure with the cell size increasing as the size of the B cation increases from 8.2114(2) Å for B=Ca to 8.4408(1) Å for B=Ba. It is postulated that a combination of local clustering of the anions and vacancies together with water–water and water–host hydrogen bonds plays a role in defining the volume of the encapsulated water clusters and that changes in the local structure upon heating result in anomalous thermal expansion observed in variable temperature diffraction measurements.