Spatial distribution of rare-earth ions and GaS₄ tetrahedra in chalcogenide glasses studied via laser spectroscopy and ab initio molecular dynamics simulation

The spatial distribution of Nd³⁺ ions and GaS₄ tetrahedral units in Nd-doped Ge-As-Ga-S glasses has been studied by laser spectroscopy and ab initio molecular dynamics (MD) simulations. A sharp increase in Nd³⁺ fluorescence intensities and lifetimes was observed with increasing Ga content, and attributed to the formation of tightly bound Nd ³⁺ clusters in Ga-free glasses and the subsequent dissolution of such clusters upon Ga doping. A large modification in Nd³⁺ sites was also identified from low-temperature site-selective excitation spectra, suggesting preferential spatial correlations between Nd³⁺ and GaS₄ tetrahedra even at low Ga-doping levels. MD simulations of these materials in the liquid state showed a tendency for Ga cluster formation as well as spatial correlations between Nd and Ga atoms consistent with the experimental results. On the basis of this result, a comprehensive structural model for Nd- and Ga-doped sulfide glasses is proposed.