Structural Dynamics and Thermal Transport in Bismuth Chalcogenide Alloys

We present a detailed study of the structural dynamics, energetic and dynamical stability, and thermal transport of the bismuth chalcogenides Bi 2 S 3, Bi 2 Se 3, and Bi 2 Te 3 and their alloys. The active Bi lone pairs lead to competition between orthorhombic Pnma and rhombohedral R 3̅ m phases, with the latter favored by the heavier chalcogens, while the reported nonambient Bi 2 Se 3 and Bi 2 Te 3 phases show phonon instabilities under ambient conditions. The Pnma structure has intrinsically weaker chemical bonding and stronger phonon anharmonicity than the R 3̅ m phase, resulting in lower lattice thermal conductivity. A thermodynamic model of Bi 2 (Se 1– x S x ) 3 indicates that the R 3̅ m structure is energetically favored only at low S content, but the stability window may be extended with lower formation temperatures. R 3̅ m Bi 2 (Se 1– x Te x ) 3 is a nonideal solid solution due to a strong preference for the Se and Te atoms to occupy the interior and exterior sites, respectively, in the constituent quintuple layers. Strain-field fluctuations from chemical bonding inhomogeneities are shown to play an important role in the heat transport in the alloys, and chalcogen disorder is found to be an important factor in the lower thermal conductivity of Bi 2 SeTe 2 compared to Bi 2 Te 3 . The microscopic insight from this study provides a new theoretical perspective on bismuth chalcogenides and their alloys to inform ongoing research on the thermoelectric performance of these and related systems.