Vacancy ordering and superstructure formation in dry and hydrated strontium tantalate perovskites: A TEM perspective

Crystal structures of Sr4(Sr2Ta2)O11 and Sr4(Sr1.92Ta2.08)O11.12, synthesized by solid state reaction technique in dry and hydrated state have been studied mainly using Transmission Electron Microscopy. Due to the lesser ability of X-rays to probe details in oxygen sublattice, the change in crystal symmetry due to ordering of oxygen vacancies could be detected better using Transmission Electron Microscopy. After detailed analysis through TEM, it was observed that no major change occurs in the cation sublattice. The TEM observations are compared with XRD data and discussed. The crystal symmetries and corresponding unit cells of all the perovskites based on the ordering of oxygen vacancies is deduced. Crystal unit cells based on the observations are proposed with ideal atomic coordinates. Finally an attempt is made to explain the water uptake behaviour of these perovskites based on the proposed crystal structure.