High pressure synthesis of rhenium nitride pernitride ReN2 with crystal structure unusual for transition metal dinitrides and high values of hardness and bulk modulus attracted significant attention to this system. We investigate the thermodynamic and electronic properties of the P21/c phase of ReN2 and compare them with two other polytypes, C2/m and P4/mbm phases suggested in the literature. Our calculations of the formation enthalpy at zero temperature show that the former phase is the most stable of the three up to pressure p=170 GPa, followed by the stabilization of the P4/mbm phase at higher pressure. The theoretical prediction is confirmed by diamond anvil cell synthesis of the P4/mbm ReN2 at ≈175 GPa. Considering the effects of finite temperature in the quasi-harmonic approximation at p=100 GPa we demonstrate that the P21/c phase has the lowest free energy of formation at least up to 1000 K. Our analysis of the pressure dependence of the electronic structure of the rhenium nitride pernitride shows a presence of two electronic topological transitions around 18 GPa, when the Fermi surface changes its topology due to an appearance of a new electron pocket at the high-symmetry Y2 point of the Brillouin zone while the disruption of a neck takes place slightly off from the Γ-A line.
@article{arxiv.2109.09422,
title = {Thermodynamic and electronic properties of ReN$_2$ polymorphs at high-pressure},
author = {Ferenc Tasnádi and Florian Bock and Alena Ponomareva and Maxim Bykov and Saiana Khandarkhaeva and Leonid Dubrovinsky and Igor A. Abrikosov},
journal= {arXiv preprint arXiv:2109.09422},
year = {2021}
}