Homologous series of layered structures in binary and ternary Bi-Sb-Te-Se systems:Ab initiostudy

Abstract

In order to account explicitly for the existence of long-periodic layered structures and the strong structural relaxations in the most common binary and ternary alloys of the Bi-Sb-Te-Se system, we have developed a one-dimensional cluster expansion (CE) based on first-principles electronic structure calculations, which accounts for the Bi and Sb bilayer formation. Excellent interlayer distances are obtained with a van der Waals density functional. It is shown that a CE solely based on pair interactions is sufficient to provide an accurate description of the ground-state energies of Bi-Sb-Te-Se binary and ternary systems without making the data set of ab initio calculated structures unreasonably large. For the binary alloys ${A}_{1\ensuremath{-}x}{Q}_{x}$ ($A=\text{Sb}$, Bi; $Q=\text{Te}$, Se), a ternary CE yields an almost continuous series of (meta)stable structures consisting of consecutive $A$ bilayers next to consecutive ${A}_{2}{Q}_{3}$ for $0<x<0.6$. For $x>0.6$, the binary alloy segregates into pure $Q$ and ${A}_{2}{Q}_{3}$. The Bi-Sb system is described by a quaternary CE and is found to be an ideal solid solution stabilized by entropic effects at $T\ensuremath{\ne}0$ K but with an ordered structure of alternating Bi and Sb layers for $x=0.5$ at $T=0$ K. A quintuple CE is used for the ternary Bi-Sb-Te system, where stable ternary layered compounds with an arbitrary stacking of ${\text{Sb}}_{2}$${\text{Te}}_{3}$, ${\text{Bi}}_{2}$${\text{Te}}_{3}$, and Te-Bi-Te-Sb-Te quintuple units are found, optionally separated by mixed Bi/Sb bilayers. Electronic properties of the stable compounds were studied taking spin-orbit coupling into account.