  In the present work, the structural and hitherto unexplored thermal and
mechanical properties of NaInS2-xSex (x = 0, 0.5, 1.0, 1.5 and 2.0) compounds
have been studied using the density functional theory. Besides, the elastic
anisotropy indices and hardnesses of NaInS2-xSex have been investigated as Se
content is varied. The mechanical stability of all the compounds under study
has been confirmed. The ratio of shear to bulk modulus (G/B) is low suggesting
that the NaInS2-xSex (x = 0.5 and 1.5) compounds exhibit damage tolerant
(ductility) properties while rest of the compositions are brittle in nature.
The predicted hardness (H) values are also influenced with the Se content in
the following order: H (NaInSSe) > H (NaInS2) > H (NaInSe2) > H (NaInS1.5Se0.5)
> H (NaInS0.5Se1.5). All the anisotropic indices under study indicate that
NaInS2-xSex compounds are anisotropic in nature. The Mulliken bond population
analysis suggests that the degree of covalency of In-S/Se bonds decreases when
S is substituted by Se. The origin of low Debye temperature ({\Theta}D) and low
minimum thermal conductivity (Kmin) have been successfully explained by
considering the mean atomic weight (M/n) and average bond strength of the
compounds. Temperature dependence of heat capacities (Cv, Cp) and linear
thermal expansion coefficient ({\alpha} ) are also estimated using the
quasi-harmonic Debye model and discussed. The low values of Kmin, {\Theta}D and
{\alpha} and damage tolerant behavior clearly indicate that the NaInS2-xSex (x
= 0.5 and 1.5) compounds can be used as promising thermal barrier coating
materials for high temperature applications.
