Theoretical investigation of novel binary nitrides M2N3 (M=Ta, Nb, V) as new high-pressure superconductors

Abstract

Superconductivity at room temperature is a hundred year old problem of condensed matter physics. Since the recent discovery of SH3 (200~GPa) and LaH10 (300~GPa), conventional superconductors under pressure play a major role in solid state physics. These BCS-superconductors can be described by theoretical tools and can be more and more synthesized experimentally with strenuous efforts. These classes of materials are especially interesting for they display outstanding parameters influencing the critical temperature. The extreme pressures they are synthesized in lead to completely new materials that would otherwise not spontaneously form in ambient pressures.\cite{main2} The following work is investigating possible superconducting properties of novel metal-nitrides that can be obtained via high-temperature and high-pressure synthesis techniques. They can be recovered at ambient temperatures with interesting parameters, making them versatile for industrial use. A focus is put on binary transition metal nitrides with the nitrogen to metal ratio 3:2, while a close look is taken on Group 5 metals. This material class does not only display superconductivity but also interesting mechanical properties. For all compounds, structural, electronic and vibrational properties are predicted and they are found to be in good agreement with literature. Firstly, the previously synthesized eta-Ta2N3 compound is investigated and its experimentally found critical temperature of ~3K is theoretically confirmed in this work. Similar superconductive parameter are found for the same material at 26.065 GPa. We compare the calculation of superconductive parameters from the software Abinit, which is used to obtain all results, with previous results from Quantum Espresso for the tetragonal Ta2N3 and it is found that the superconductive parameters, especially the logarithmic frequency, are generally underestimated with Abinit. It is proposed to calculate the critical temperature with the help of the more accurately calculated Debye-temperature. The tetragonal Ta2N3 is predicted to be a very low critical temperature superconductor, while it still might display no superconductivity at all. Furthermore, the orthorhombic Nb2N3 is investigated further, its electronic and vibrational parameters are found and it is predicted to have a very high electron phonon coupling that could potentially lead to a superconductivity at 30K. The last Group 5 metal nitride in question is the V2N3. Previously, the instability of the orthorhombic V2N3 was found, which is confirmed by us through calculating the phonon dispersion and finding various imaginary frequencies. Its stable trigonal form is investigated, its electronic structure and vibrational properties are discussed in the following. Superconductivity is either not present in this compound or it can be found at very low critical temperatures.