Phonon and elastic instabilities in MoC and MoN

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.