Phonon and Elastic Instabilities in MoC and MoN

TL;DR

This paper investigates the elastic and phonon instabilities in MoC and MoN, revealing persistent elastic instability in MoN under pressure and phonon instabilities in MoC, with implications for their stability and superconductivity potential.

Contribution

It provides new insights into the stability of MoC and MoN in the B1 structure, highlighting phonon and elastic instabilities and systematic electronic structure trends.

Findings

Elastic instability in MoN persists at high pressures.

MoC shows positive elastic constants but phonon instabilities.

Electronic structure trends vary with transition metal carbo-nitrides.

Abstract

We present several results related to the instability of MoC and MoN in the B1 (sodium chloride) structure. These compounds were proposed as potential superconductors with moderately high transition temperatures. We show that the elastic instability in B1-structure MoN, demonstrated several years ago, persists at elevated pressures, thus offering little hope of stabilizing this material without chemical doping. For MoC, another material for which stoichiometric fabrication in the B1-structure has not proven possible, we find that all of the cubic elastic constants are positive, indicating elastic stability. Instead, we find X-point phonon instabilities in MoC (and in MoN as well), further illustrating the rich behavior of carbo-nitride materials. We also present additional electronic structure results for several transition metal (Zr, Nb and Mo) carbo-nitride systems and discuss systematic trends in the properties of these materials. Deviations from strict electron counting dependencies are apparent.