  In the present study, the structural, electronic, optical and thermoelectric
properties of two isostructural chalcogenide materials, NaInS2 and NaInSe2 with
hexagonal symmetry (R-3m) have been studied using the first principles method.
A very good agreement has been found between our results with the available
experimental and theoretical ones. The studied materials are semiconducting in
nature as confirmed from the electronic band structure and optical
properties.The strong hybridizations among s orbitals of Na, In and Se atoms
push the bottom of the conduction band downward resulting in a narrower band
gap of NaInSe2 compared to that of NaInS2 compound. Different optical
(dielectric function, photoconductivity, absorption coefficient, reflectivity,
refractive index and loss function) and thermoelectric (Seebeck coefficient,
electrical conductivity, power factor and thermal conductivity) properties of
NaInX2 (X = S, Se) have been studied in detail for the first time. It is found
that all these properties are significantly anisotropic due to the strongly
layered structure of NaInX2 (X = S, Se). Strong optical absorption with sharp
peaks is found in the far visible to mid ultraviolet (UV) regions while the
reflectivity is low in the UV region for both the compounds. Such features
indicate feasibility of applications in optoelectronic sector.The calculated
thermoelectric power factors at 1000 K for NaInS2 and NaInSe2 along a-axis are
found to be 151.5 micro Watt /cmK2 and 154 micro Watt/cmK2, respectively and
the corresponding ZT values are ~0.70. The obtained thermal conductivity along
a-axis for both compounds is high (~22 W/mK).This suggests that the reduction
of such high thermal conductivity is important to achieve higher ZT values of
the NaInX2(X = S, Se) compounds.

Materials Science