Thermodynamic Stability, Thermoelectric, Elastic and Electronic Structure Properties of ScMN2-Type (M = V, Nb, Ta) Phases Studied by ab initio Calculations

Robert Pilemalm,Sergei Simak,Per Eklund,Igor Mosyagin,Leonid Pourovskii

doi:10.3390/condmat4020036

Abstract

ScMN2-type (M = V, Nb, Ta) phases are layered materials that have been experimentally reported for M = Ta and Nb, but they have up to now not been much studied. However, based on the properties of binary ScN and its alloys, it is reasonable to expect these phases to be of relevance in a range of applications, including thermoelectrics. Here, we have used first-principles calculations to study their thermodynamic stability, elastic, thermoelectric and electronic properties. We have used density functional theory to calculate lattice parameters, the mixing enthalpy of formation and electronic density of states as well as the thermoelectric properties and elastic constants (cij), bulk (B), shear (G) and Young’s (E) modulus, which were compared with available experimental data. Our results indicate that the considered systems are thermodynamically and elastically stable and that all are semiconductors with small band gaps. All three materials display anisotropic thermoelectric properties and indicate the possibility to tune these properties by doping. In particular, ScVN2, featuring the largest band gap exhibits a particularly large and strongly doping-sensitive Seebeck coefficient.

Highlights

MAX phases, where M is a transition metal, A is an A-group element and X is carbon and/or nitrogen, comprise a family of more than 70 compounds
In analogy with artificial superlattices, inherently nanolaminated materials like ScTaN2, ScNbN2 and ScVN2 could be of interest for this purpose. Requires determining their structural and electronic properties, e.g., are they metallic or semiconductors? This motivates the present theoretical study of the material properties of the inverse MAX phases, in order to screen their possibility for thermoelectrics
For higher-level energies in the conduction band, the contribution to the density of states (DOS) for all three systems is mainly from the Sc states. These result(sba)re consistent with ScTaN2, where it has previously been shown that the first peak is mainly due to N (2p) states and the peak to the right of the chemical potential is due to Ta (5d) mixed with Sc (3d) states and the peak to the left of it is mainly due to Ta (5d) statesF[i1g1u].re 1

Summary

Introduction

MAX phases, where M is a transition metal, A is an A-group element and X is carbon and/or nitrogen, comprise a family of more than 70 compounds. A related structure to a 211 MAX phase is the ScTaN2- and ScNbN2-type structure. These phases have been observed experimentally [9,10,11], and a basic characterization of structure and some properties has been made. The structure of ScTaN2, ScNbN2 and ScVN2 can generally be described as the ScMN2-type structure [12], which has space group P63/mmc (#194) and comprises of alternating layers of ScN6/3 octrahedra and MN6/3 prisms. Because of this relationship to a MAX phase, we term this structure “inverse MAX phase” (in analogy with, e.g., inverse perovskites). This motivates the present theoretical study of the material properties of the inverse MAX phases, in order to screen their possibility for thermoelectrics Requires determining their structural and electronic properties, e.g., are they metallic or semiconductors? This motivates the present theoretical study of the material properties of the inverse MAX phases, in order to screen their possibility for thermoelectrics

Computational Details

Results and Discussion

Conclusions