Thermodynamic ground states of platinum metal nitrides

TL;DR

This study uses density functional theory to identify new stable crystal structures of platinum group metal nitrides, revealing their thermodynamic stability at ambient and high pressures, and comparing different computational approaches.

Contribution

It introduces two novel simple tetragonal crystal structures for platinum metal nitrides with lower formation enthalpies, expanding understanding of their stability landscape.

Findings

New stable tetragonal structures for Rh, Pd, Ir, and Pt nitrides.

PtN2 tetragonal structures are stable up to 17 GPa.

Different DFT approximations predict varying stabilities.

Abstract

The thermodynamic stabilities of various phases of the nitrides of the platinum metal elements are systematically studied using density functional theory. It is shown that for the nitrides of Rh, Pd, Ir and Pt two new crystal structures, in which the metal ions occupy simple tetragonal lattice sites, have lower formation enthalpies at ambient conditions than any previously proposed structures. The region of stability with respect to those structures extends to 17 GPa for PtN2. Calculations show that the PtN2 simple tetragonal structures at this pressure are thermodynamically stable also with respect to phase separation. The fact that the local density and generalized gradient approximations predict different values of the absolute formation enthalpies as well different relative stabilities between simple tetragonal and the pyrite or marcasite structures are further discussed.