Polymorphism in bismuth stannate: A first-principles study

From the wide range of Sn-IV-based pyrochlores, Bi2Sn2O7 stands out as a material that deviates from the standard cubic-lattice symmetry. At low temperatures, Bi2Sn2O7 adopts a distorted monoclinic root 2 x root 2 x 2 expansion of the pyrochlore structure (cc-phase) and only favors the cubic lattice (gamma-phase) above 900 K. In this study, we calculate and examine the electronic structure of both the alpha- and gamma-phases of Bi2Sn2O7 and compare them to the results of two regular pyrochlore materials, La2Sn2O7 and Y2Sn2O7. Our analysis highlights the importance of covalent interactions between the electronic states of the metal with O 2p in Bi2Sn2O7, 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 Bi2Sn2O7.