Influence of Se doping in recently synthesized NaInS2-xSex solid solutions for potential thermo-mechanical applications studied via first-principles method

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.