  ZrNCl and HfNCl belong to a class of layered transition metal nitride halides
MNCl (M= Zr,H). 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 have shown 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 ZrNCl and HfNCl semiconductors are discussed.
