Structure-Function Relationships in Pyrochlore Oxides

Coherent off-centering of the A-site cation can be frustrated on the rigid pyrochlore lattice, preventing the ferroelectric-paraelectric phase transition in majority of pyrochlore oxide materials. One of the few ferroelectric pyrochlores is Cd2Nb2-xTaxO7. While it has been studied for some time, the low temperature structures are still not well understood, hindering the understanding of ferroelectric phase transitions and long–range ordering of distortions in pyrochlores. In this work synchrotron X-ray scattering data was used to characterize the low temperature ferroelectric structures of Cd2Nb2-xTaxO7 (x = 0, 1, 2) and better understand the structural changes accompanying the ferroelectric to paraelectric phase transitions. For the x= 0 material (Cd2Nb2O7), it was found that second order Jahn-Teller distortions, stemming from Nb(V), caused overall correlated displacements in both the NbO6 and Cd2O’ networks. Another pyrochlore of interest is Bi2Ti2O7, which exhibits local polar distortions, but lacks long-range ferroelectricity due to frustration of the A-site cations. Herein we attempted to relieve the frustration of Bi lone pairs through partial substitution of smaller cations with no lone pairs into the A-site. The solid solutions studied in this thesis are Bi2-xYxTi2O7 (x = 0.5, 1, 1.5, 2) and Bi2-xZnxTi2-xNbxO7 (x = 0.25, 0.75, 1). The average structure of all samples at 100K and 300K was characterized using synchrotron X-ray diffraction experiments. In addition, local structure at 300K of Bi2-xYxTi2O7 was investigated using pair distribution function (PDF) analysis. Analysis of Bi2-xYxTi2O7 solid solution revealed that increasing Bi content increases atomic motion in the A–site and local disorder in the 1 to 10 angstrom range. Even though yttrium substitution increased structural order, the frustration of Bi lone pairs was not relieved and a ferroelectric phase transition was not observed at 100K or 300K. Zinc substituted samples formed multiple pyrochlore phases, which were non-ferroelectric at the studied temperatures