Rich structural phase diagram and thermoelectric properties of layered tellurides Mo1-xNbxTe2

TL;DR

This study explores how Nb doping influences the structural phases and thermoelectric properties of layered MoTe2, revealing a complex phase diagram with new polytypes and insights into electronic structure modifications.

Contribution

It provides a detailed phase diagram of Mo1-xNbxTe2, identifies a new rhombohedral phase, and links Nb doping to changes in electronic structure and thermoelectric behavior.

Findings

Identification of four polytypes in Mo1-xNbxTe2.

Discovery of a high-temperature metastable rhombohedral phase.

Nb acts as a hole dopant affecting the electronic structure.

Abstract

MoTe2 is a rare transition-metal ditelluride having two kinds of layered polytypes, hexagonal structure with trigonal prismatic Mo coordination and monoclinic structure with octahedral Mo coordination. The monoclinic distortion in the latter is caused by anisotropic metal-metal bonding. In this work, we have examined the Nb doping effect on both polytypes of MoTe2 and clarified a structural phase diagram for Mo1-xNbxTe2 containing four kinds of polytypes. A rhombohedral polytype crystallizing in polar space group has been newly identified as a high-temperature metastable phase at slightly Nb-rich composition. Considering the results of thermoelectric measurements and the first principles calculations, the Nb ion seemingly acts as a hole dopant in the rigid band scheme. On the other hand, the significant interlayer contraction upon the Nb doping, associated with the Te p-p hybridization, is confirmed especially for the monoclinic phase, which implies a shift of the p-band energy level. The origin of the metal-metal bonding in the monoclinic structure is discussed in terms of the d electron counting and the Te p-p hybridization.