The structural origin of broken chemical order in GeSe2

Raman scattering, Sn-119 Mossbauer spectroscopy and temperature-modulated differential scanning calorimetry experiments have been performed on (Ge0.99Sn0.01)(x)Se1-x glasses in the 0.30 < x < 0.36 range. Both Raman and Mossbauer 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 dT(g)/dx of T-g increase slows down markedly at (i) x greater than or equal to 0.31(1), and the rate actually reverses sign (ii) at x greater than or equal to 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.