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.
Highlights
The high-pressure diamond anvil cell (DAC) experiment is a successful approach to establish a wide variety of physical conditions for synthesizing materials [1,2,3]
Considering the effects of finite temperature in the quasiharmonic 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
The work by Bykov et al [19] has shown that the ReN2 compound in the monoclinic P21/c phase discovered in a DAC experiment can be synthesized in a larger amount in a large-volume press at lower pressure
Summary
The high-pressure diamond anvil cell (DAC) experiment is a successful approach to establish a wide variety of physical conditions for synthesizing materials [1,2,3]. The work by Bykov et al [19] has shown that the ReN2 compound in the monoclinic P21/c phase discovered in a DAC experiment can be synthesized in a larger amount in a large-volume press at lower pressure This compound has a crystal structure that is unusual for transition metal dinitrides MN2. At higher pressures the calculations predict the stabilization of the P4/mbm phase As this phase was not reported in earlier experiments, we carry out the high-pressure synthesis of ReN2 in a diamond anvil cell at ≈175 GPa. The theoretical prediction is verified by a characterization of the synthesized sample, which confirms the stabilization of the P4/mbm ReN2. We calculate electronic properties of the P21/c phase and show the presence of two electronic topological transitions at ≈18 GPa
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