Polymorphism in Bismuth Stannate: A First-Principles Study

From the wide range of Sn IV -based pyrochlores, Bi 2 Sn 2 O 7 stands out as a material that deviates from the standard cubic-lattice symmetry. At low temperatures, Bi 2 Sn 2 O 7 adopts a distorted monoclinic √2 × √2 × 2 expansion of the pyrochlore structure (α-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 2 Sn 2 O 7 and compare them to the results of two regular pyrochlore materials, La 2 Sn 2 O 7 and Y 2 Sn 2 O 7 . Our analysis highlights the importance of covalent interactions between the electronic states of the metal with O 2p in Bi 2 Sn 2 O 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 2 Sn 2 O 7 .