First-principles study of cubic Bi pyrochlores

We examined a range of cubic Bi2B2O6O' (B = Ti, Ru, Rh, Ir, Os, and Pt) pyrochlores with density functional theory (DFT) calculations and report the structural parameters along with the electronic structure of the bismuth pyrochlores in their ideal cubic, defect-free structure. We also examined the role of cation displacements within the cubic structure and find that only Bi2Ti2O7 shows a substantial increase in favorability with Bi cation displacements. For Bi2Ti2O7, we find an average displacement of 0.38 +/- 0.02 angstrom for the Bi cation and an energy change of 0.146 +/- 0.001 eV/Bi atom. The cation displacement in Bi2Ti2O7 follows the spin-ice rules reported in complex Bi-based pyrochlores, where two-long and two-short bonds are found in each tetrahedron of Bi4O'. Examination of the electronic structure shows the main driving force in the displacement is the extent of Bi-O' interactions in Bi2Ti2O7 and metallic bismuth pyrochlores. In Bi2Ti2O7, we observe more overlap of Bi s and p states with O 2p states, similar to the reported electronic structure of Bi2Sn2O7, PbO, and SnO, which leads to the asymmetric electronic structure around the Bi cation. This asymmetric electron structure is associated with the displacement of cations in Bi2Ti2O7. Our DFT results match the general understanding from experimental studies but underestimate the displacement in Bi2Ru2O7.