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.

Graphical abstractSelected area electron diffraction (SAED) patterns showed highly ordered diffraction maxima with characteristic superlattice weak diffraction spots of the pyrochlore structure for (a) Ca0.67Y1.33Zr1.33Ta0.33O7 (C2YZT2) and bright diffraction maxima arranged in a ring pattern of the fluorite structure for (b) Ca0.297Y1.71Zr1.71Ta0.29O7 (CY6Z6T).Figure optionsDownload full-size imageDownload as PowerPoint slide