Polymorphism in bismuth stannate: A first-principles study

From the wide range of Sn ^IV-based pyrochlores, Bi ₂Sn ₂O ₇ stands out as a material that deviates from the standard cubic-lattice symmetry. At low temperatures, Bi ₂Sn ₂O ₇ adopts a distorted monoclinic √2 × √2 × 2 expansion of the pyrochlore structure (a-phase) and only favors the cubic lattice (γ-phase) above 900 K. In this study, we calculate and examine the electronic structure of both the α- and γ-phases of Bi ₂Sn ₂O ₇ and compare them to the results of two regular pyrochlore materials, La ₂Sn ₂O ₇ and Y ₂Sn ₂O ₇. Our analysis highlights the importance of covalent interactions between the electronic states of the metal with O 2p in Bi ₂Sn ₂O ₇, which are not present in the other oxides. The formation of an asymmetric electron density on Bi is observed as the driving force behind the distorted geometry favored by Bi ₂Sn ₂O ₇.