Investigating The Phase Stability and Thermal Properties of Compositionally Complex Titanate Pyrochlores

In multicomponent ceramics there are varying factors that determine the phase selection and stability such as the configurational entropy of the system or the ionic radii of the constituent cations. The influence of each of the aforementioned factors can vary based on the complexity of the crystal structure of the material. Stability rules based on ionic radius ratios are well established for rare earth pyrochlores (RE₂³⁺B₂⁴⁺O₇). From these rules, members of the titanate pyrochlore family have been synthesized from the Lanthanide series (La to Lu, large to small) that take advantage of the wide range of ionic radii and stable +3 oxidation state across the majority of the series to create a series of samples with different combinations and distributions of cations as well as including cations that are not normally stable as pyrochlores in single component systems based on known stability rules. The trends in the crystal structure were studied through the use of X-Ray diffraction and Rietveld refinement. Thermal properties were investigated using the Transient Plane Source method and dilatometry. Altogether, the impact of the higher entropy configurations and the distribution of constituent cations on pyrochlore radius ratio rules, thermal properties, and other potential crystal-chemical effects will provide deeper understanding of possible design considerations for intrinsic property tuning in these and other multicomponent complex ceramics. In particular, tunable rare earth titanate pyrochlores would have a wide range of applications in various fields such as in magnetic materials because of the spin ice properties, in thermal barrier coatings due to low thermal conductivity and good coefficients of thermal expansion, and in ionic conductors.