We present a detailed study of the structural dynamics, energetic and dynamical stability, and thermal transport of the bismuth chalcogenides Bi₂S₃, Bi₂Se₃, and Bi₂Te₃ and their alloys. The active Bi lone pairs lead to competition between orthorhombic Pnma and rhombohedral R3̅m phases, with the latter favored by the heavier chalcogens, while the reported nonambient Bi₂Se₃ and Bi₂Te₃ phases show phonon instabilities under ambient conditions. The Pnma structure has intrinsically weaker chemical bonding and stronger phonon anharmonicity than the R3̅m phase, resulting in lower lattice thermal conductivity. A thermodynamic model of Bi₂(Se_1–xS_x)₃ indicates that the R3̅m structure is energetically favored only at low S content, but the stability window may be extended with lower formation temperatures. R3̅m Bi₂(Se_1–xTe_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 Bi₂SeTe₂ compared to Bi₂Te₃. 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.

中文翻译：

---

铋硫族合金的结构动力学和热传递

我们详细研究了铋硫族化物 Bi ₂ S ₃、Bi ₂ Se ₃和 Bi ₂ Te ₃及其合金的结构动力学、能量和动力学稳定性以及热传输。活跃的 Bi 孤对导致正交Pnma和菱形R 3̅ m相之间的竞争，后者受到较重硫属元素的青睐，而报道的非环境 Bi ₂ Se ₃和 Bi ₂ Te ₃相在环境条件下表现出声子不稳定性。该晶Pnma与R 3̅ m相相比，该结构本质上具有更弱的化学键合和更强的声子非谐性，导致较低的晶格热导率。Bi ₂ (Se _{1– x} S _x ) ₃的热力学模型表明，R 3̅ m结构仅在低硫含量时在能量上有利，但稳定窗口可能会随着地层温度的降低而扩大。R 3̅ m Bi ₂ (Se _{1– x} Te _x ) ₃是非理想的固溶体，因为 Se 和 Te 原子强烈偏好分别占据组成五重层中的内部和外部位置。来自化学键合不均匀性的应变场波动显示出在合金的热传输中起重要作用，并且发现硫族元素无序是与 Bi ₂ Te ₃相比Bi ₂ SeTe ₂导热率较低的重要因素。这项研究的微观洞察为铋硫属元素化物及其合金提供了新的理论视角，为正在进行的这些和相关系统的热电性能研究提供信息。