A comparative study of the physical properties of layered transition metal nitride halides MNCl (M = Zr, Hf): DFT based insights

Abstract

ZrNCl and HfNCl belong to a class of layered transition metal nitride halides MNCl (M = Zr, Hf). They are from the space group R-3m (No-166) and crystallize in the rhombohedral structure. Both of these materials have shown promising semiconducting behaviors. Recent studies showed their versatility as semiconductors and also as superconductors when intercalated with alkaline metals. This paper explores the mechanical, optical and electronic properties of these two semiconducting crystals in depth. A comparative study between the two materials in their elastic constants, anisotropy measures, electronic density of states and band structures, optical spectra has been performed with first principles density functional theory (DFT) based calculations within the local density approximation (with appropriate U for the energy gap calculations in case of HfNCl). HfNCl is more machinable than ZrNCl and is relatively softer as indicated by the lower Debye temperature. ZrNCl has stronger layering due to which it exhibits brittle nature. HfNCl has a larger band gap. ZrNCl is a better reflector of ultraviolet radiation. On the other hand, HfNCl is a good ultraviolet absorber. Both materials are anisotropic in regards to structure, electronic energy dispersion and optical parameters. Overall, the degree of anisotropy is more prominent in ZrNCl compared to that in HfNCl. Possible sectors for applications of the ZrNCl and HfNCl semiconductors are discussed.