Thermodynamic and electronic properties of ReN$_2$ polymorphs at high-pressure

TL;DR

This study investigates the thermodynamic and electronic properties of different ReN₂ polymorphs under high pressure, identifying their stability ranges, phase transitions, and electronic topological changes through theoretical calculations and experimental synthesis.

Contribution

It provides the first comprehensive comparison of ReN₂ polymorphs' stability and electronic structure at high pressures, including experimental validation of the P4/mbm phase.

Findings

P2₁/c phase is most stable up to 170 GPa

P4/mbm phase stabilizes above 170 GPa and is experimentally synthesized at 175 GPa

Electronic topological transitions occur around 18 GPa affecting Fermi surface topology

Abstract

High pressure synthesis of rhenium nitride pernitride ReN$_2$ 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 P2$_1$/c phase of ReN$_2$ 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 ReN$_2$ at $\approx$175 GPa. Considering the effects of finite temperature in the quasi-harmonic approximation at p=100 GPa we demonstrate that the P2$_1$/c phase has the lowest free energy of formation at least up to 1000 K. Our analysis of the pressure dependence of the electronic structure of the rhenium nitride pernitride shows a presence of two electronic topological transitions around 18 GPa, when the Fermi surface changes its topology due to an appearance of a new electron pocket at the high-symmetry Y$_2$ point of the Brillouin zone while the disruption of a neck takes place slightly off from the $\Gamma$-A line.