Modeling the ¹¹⁹Sn Mössbauer spectra of chalcogenide glasses using density-functional theory calculations

We have used first-principles calculations based on density-functional theory to investigate the ¹¹⁹Sn Mössbauer spectrum of a-Ge_0.99xSn_0.01xSe_1-x and a-Ge_0.99xSn_0.01xS_1-x. Using calculated electric field gradients and contact charge densities, we compute Mössbauer isomer shifts and quadrupole splittings for a number of cluster models incorporating proposed environments for Sn atoms in the glasses. The calculated parameters are in excellent agreement with experimental values for tetrahedrally coordinated Sn atoms and for ionic, threefold-coordinated Sn atoms. Parameters computed for Sn atoms in ethanelike environments, however, do not match experimental values attributed to these sites. We also compute site energies to determine the most energetically favorable sites for Sn atoms in these systems. For the Ge-S system, we find the threefold environments to be favored, while for Ge-Se, the threefold and tetrahedral environments are essentially degenerate.