Electronic properties of tin dichalcogenides (SnS2 and SnSe2)

Using Phillips and Van Vechten's theory the homopolar gap (E_h), ionic gap (E_c) and Penn gap (E_p) have been derived for the layered compound semiconductors SnS₂ and SnSe₂. The values of E_p so derived have been found to be in agreement with the values obtained from the Grimes-Cowley modification of the Penn model, and also with the reflectance spectra of the respective materials. Using the above data the Phillips ionicity has been calculated and the octahedral coordination of SnS₂ and SnSe₂ has been discussed on the basis of the calculated value of ionicity. The bond electronic polarizability has been evaluated using Chemla's approach and the values so obtained have been found in agreement with the values obtained from the Clausius-Mossottirelation. The Varshni formula has been shown to explain fairly well the temperature dependence of energy gap and the constants involved have been derived. The calculated isobaric temperature gradient of E_g($\text{?E}{}_{\mathrm{g}}\text{?T)}{}_{\mathrm{p}}$] has been found to be in accordance with the reported values, and the electron lattice interaction has been evaluated.