Stability of Sb-Te layered structures: First-principles study

Abstract

Using an effective one-dimensional cluster expansion in combination with first-principles electronic structure calculations we have studied the energetics and electronic properties of Sb-Te layered systems. For a Te concentration between 0 and 60 at. % an almost continuous series of metastable structures is obtained consisting of consecutive Sb bilayers next to consecutive Sb 2 Te 3 units, with the general formula (Sb 2 ) n (Sb 2 Te 3 ) m (n,m = 1,2, . . .). Between 60 and 100 at. % no stable structures are found. We account explicitly for the weak van der Waals bonding between Sb bilayers and Sb 2 Te 3 units by using a recently developed functional, which strongly improves the interlayer bonding distances. At T = 0 K, no evidence is found for the existence of two separate single-phase regions δ and γ and a two-phase region δ + γ . Metastable compounds with a Te concentration between 0 and 40 at. % are semimetallic, whereas compounds with a Te concentration between 50 and 60 at. % are semiconducting. Compounds with an odd number of Sb layers are metallic and have a much higher formation energy than those with an even number of consecutive Sb layers, thereby favoring the formation of Sb bilayers.

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

“…In contrast to our previous work [19] where we used three-site clusters, we have shown in the present work that a CE solely based on pair interactions yield a converged CE. For details on this CE, see Ref.…”

“…The general composition can therefore be written as (A 2 ) n (A 2 Q 3 ) m , in line with the existing experimental [11][12][13]15,42] and computational data [17,19]. This does not exclude, however, that other layered structures with higher formation energy are observed at higher temperatures [16].…”

“…II, fast convergence of the CE for the Sb-Te system can be achieved by considering a ternary CE where Sb atoms are given a different occupation variable depending on whether they are part of an even or odd number of adjacent Sb layers. In this way, the CE method takes into account the favoring of Sb bilayer formation [19]. Since Bi and Se are expected to behave in a similar way to Sb and Te, respectively, a ternary CE was used for the Bi-Te and Bi-Se alloys (Sec.…”

“…[19], we showed that for the binary Sb-Te system, a converged binary CE of the formation energy cannot be realized with a dichotomous occupation variable P i since no discrimination is made between an odd or an even number of adjacent Sb layers. The energetic difference between structures with an even or odd succession of Sb layers is a consequence of the strong structural relaxations accompanying a Peierls transition leading to the formation of Sb bilayers.…”

“…Since thermoelectric materials are often based on solid solutions [1,20], superstructures [21], or nanostructures [22] of Sb 2 Te 3 and Bi 2 Te 3 or of Bi and Sb [23,24], we will now look for the stable layered phases within the ternary Bi-Sb-Te system for an arbitrary composition by extending the methods used in Ref. [19]. The present method provides insight into the actual ground and metastable states, though it does not yield complete information on the finite temperature stability.…”