The structural origin of broken chemical order in GeSe2 glass

Raman scattering, 119 Sn Mössbauer spectroscopy and temperature-modulated differential scanning calorimetry experiments have been performed on (Ge 0.99 Sn 0.01 ) x Se 1−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 GeSe 2 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 dT g /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 Ge 2 (Se 1/2 ) 6 -bearing nanophase that is formed separately from the Ge(Se 1/2 ) 4 -tetrahedra-bearing backbone of the glasses.