Bonds, bands, and bandgaps in tetrahedrally bonded ternary compounds: the role of group V Lone Pairs

TL;DR

This paper investigates the electronic structure and bandgap origins of I3-V-VI4 tetrahedral compounds, focusing on the role of group V lone pairs, using ab initio density functional calculations.

Contribution

It provides new insights into the valence state of V atoms and the influence of lone pairs on bandgap formation in these thermoelectric materials.

Findings

V atoms may not be strictly pentavalent.

V lone pairs significantly influence bandgap creation.

States near band edges have characteristic orbital contributions.

Abstract

An interesting class of tetrahedrally coordinated ternary compounds have attracted considerable interest because of their potential as good thermoelectrics. These compounds, denoted as I$_3$-V-VI$_4$, contain three monovalent-I (Cu, Ag), one nominally pentavalent-V (P, As, Sb, Bi), and four hexavalent-VI (S, Se, Te) atoms; and can be visualized as ternary derivatives of the II-VI zincblende or wurtzite semiconductors, obtained by starting from four unit cells of (II-VI) and replacing four type II atoms by three type I and one type V atoms. In trying to understand their electronic structures and transport properties, some fundamental questions arise: whether V atoms are indeed pentavalent and if not how do these compounds become semiconductors, what is the role of V lone pair electrons in the origin of band gaps, and what are the general characteristics of states near the valence band maxima and the conduction band minima. We answer some of these questions using ab initio calculations (density functional methods with both local and nonlocal exchange-correlation potential).