Physical and chemical threshold behavior in chalcogenide networks: 119Sn Mössbauer spectroscopy of Ge(Sn)–As–Se glasses

Abstract

Lamb–Mössbauer factors can serve as sensitive probes of vibrational network dynamics and, as such, are suited to examine the rigidity percolation concept in covalent networks. This is shown by detailed temperature dependent Mössbauer spectroscopic data obtained on ¹¹⁹Sn dopants in the ternary Ge–As–Se glass system. Significant increases of Lamb–Mössbauer factors and effective Mössbauer lattice temperatures are observed, when the average coordination number 〈r〉 reaches the critical value of 2.4 bonds per atom, as predicted by percolation theory. Detailed inspection of three compositional series reveals, however, that this universality is superimposed by a distinct chemical threshold behavior, related to a sudden change in the chemical bond distribution in the selenium-poor region of the composition diagram. Trends observed for both the ¹¹⁹Sn Lamb–Mössbauer factors and isomer shifts indicate that this "‘chemical effect’' diminishes network connectivity, resulting in the conversion of Sn(IV)Se_4/2 units to Sn(II)Se_3/2^- groups. In the Ge–Sn–Se system, where data could be obtained at glass compositions well above 〈r〉=2.67, no evidence was found for the previous suggestion that this average coordination number corresponds to a percolation threshold associated with a transition from two- to three-dimensional connectivity.