The structural origin of broken chemical order in GeSe2 glass

Abstract

Abstract Raman scattering, 119Sn Mössbauer spectroscopy and temperature-modulated differential scanning calorimetry experiments have been performed on (Ge0.99 Sn0.01)x Se1−x glasses in the 0.30 < x < 0.36 range. Both Raman and Mössbauer spectroscopies show that Ge‒Ge signatures first appear near x = 0.31(1), and their concentration slowly increases with increasing x to acquire a value of 1.92(30)% at x = 1/3, corresponding to GeSe2 glass. Thereafter (1/3 < x < 0.36) the concentration of these bonds increases precipitously with increasing x. Glass transition temperatures T g(x) reflect the connectivity of the network and are found to increase with increasing x; however, the rate dTg/dx of T g increase slows down markedly at (i) x ≥ 0.31(1), and the rate actually reverses sign (ii) at x ≥ 0.34. Feature (i) coincides with nucleation and (ii) with precipitous growth of Ge‒Ge signatures. These T g trends show that the presence of Ge‒Ge signatures decreases the global connectivity of the glasses. The results unequivocally demonstrate that the Ge‒Ge bonds constitute part of a marginally rigid Ge2(Se1/2)6-bearing nanophase that is formed separately from the Ge(Se1/2)4-tetrahedra-bearing backbone of the glasses.