Structures and properties of defect perovskites in the strontium zirconium niobium oxide system

School of Chemistry, University of Sydney
â—‹Siegbert Schmid Ivanka Barisic

The SrxNbO3, 0.7 < x < 1, solid solution with niobium in oxidation states +IV and +V for x < 1, has been reported to adopt the ideal cubic perovskite structure across the whole solid solution field. Despite intensive searching when data were collected on good quality single crystals no additional reflections were detected. This indicates random ordering between strontium and vacancies on the perovskite A sites. Since niobium and zirconium are able to occupy positions in a structure at random, it was expected that a similar solid solution might be formed if niobium +IV is replaced by zirconium +IV. Therefore an investigation was undertaken in the SrO-ZrO2-Nb2O5 system to see whether an analogous solid solution is indeed formed. The composition range of Sr0.7+xZr0.4+2xNb0.6-2xO3 has been investigated in detail (as well as the Hf and Ta analogues) and specimens synthesised with 0.01 < x < 0.30. X-ray and neutron powder diffraction patterns have shown that while there is an underlying pseudo cubic perovskite sub-structure the symmetry for all investigated compositions is lowered to tetragonal or orthorhombic. The low strontium content end member of the solid solution appeared to adopt cubic symmetry (in accordance with the laboratory X-ray and electron diffraction data), however, profile fitting clearly showed that the phase is actually tetragonal. The rest of the solid solution requires symmetry lowering to orthorhombic to fit the patterns satisfactorily. It now appears that there is no complete solid solution from ~0.01 < x < 0.3, but rather a series of smaller solid solutions the extent of which needs to be carefully examined.