(695d) Hopping Mechanisms in Dielectric Relaxation of Pyrochlores From First Principles Calculations

There is considerable interest in pyrochlore systems (A₂B₂O₇), with the Fd-3m (No. 227) space group, for use in high-permittivity dielectrics, capacitors, and high-frequency filter applications.  The properties of these materials can be tuned through substitutions on the A and B cation sites, resulting in an extensive parameter space.  Better understanding of the role of the local atomic structure and dynamics on the macroscopic properties will enable rational design within the vast number of possibilities.  For example, the dielectric relaxation in (Bi_1.5Zn_0.5)(Nb_1.5Zn_0.5)O₇ was linked to local atomic hopping events and it has been proposed that a highly polarizable cation (Bi), chemical substitution, atomic displacement, and the resulting fractional occupancy of equivalent sites, are necessary for dielectric relaxation in pyrochlores.  This work focused on de-convoluting these three proposed features to better understand the nature of the dielectric relaxation in pyrochlores by comparing quantum mechanical calculations on Bi_1.5ZnNb_1.5O₇, Ca_1.5Ti_1.5NbO₇, and Bi₂Ti₂O₇.  Bi₂Ti₂O₇ is an ideal compound to de-convolute these features since we have shown based on density functional theory (DFT) calculations that Bi₂Ti₂O₇ is expected to have ionic displacements in addition to the highly polarizable A-site cation without chemical substitution of multi-valence cations.  Ca_1.5Ti_1.5NbO₇ also isolates the chemical substitution and atomic displacement without the highly polarizable cation.  We will present DFT results on the atomic displacement patterns, atomic hopping events, and the static dielectric permittivity in all three systems to understand the effects of each feature on the observation of dielectric relaxation.  Where possible, these predictions are compared with available experimental results.