Order–disorder phase transformations in quaternary pyrochlore oxide system: Investigated by X-ray diffraction, transmission electron microscopy and Raman spectroscopic techniques

Order–disorder transformations in a quaternary pyrochlore oxide system, Ca–Y–Zr–Ta–O, were studied by powder X-ray diffraction (XRD) method, transmission electron microscope (TEM) and FT-NIR Raman spectroscopic techniques. The solid solutions in different ratios, 4:1, 2:1, 1:1, 1:2, 1:4, 1:6, of CaTaO3.5 and YZrO3.5 were prepared by the conventional high temperature ceramic route. The XRD results and Rietveld analysis revealed that the crystal structure changed from an ordered pyrochlore structure to a disordered defect fluorite structure as the ratios of the solid solutions of CaTaO3.5 and YZrO3.5 were changed from 4:1 to 1:4. This structural transformation in the present system is attributed to the lowering of the average cation radius ratio, rA/rB as a result of progressive and simultaneous substitution of larger cation Ca2+ for Y3+ at A sites and smaller cation Ta5+ for Zr4+ at B sites. Raman spectroscopy and TEM analysis corroborated the XRD results.