Kinetics of crystal growth of germanium disulfide in Ge0.38S0.62 chalcogenide glass

The crystal growth kinetics of GeS2 in Ge0.38S0.62 glass has been studied by Differential Scanning Calorimetry (DSC) and microsopy. The linear crystal growth kinetics of both high temperature α-GeS2 and low temperature β-GeS2 polymorphs has been observed over a relatively broad range of temperatures, i.e. 420 < T < 494 °C that correspond to viscosity of supercooled melt: 3 × 109 > η > 8 × 105 Pa s. It seems that 2D nucleated growth is the most probable mechanism of crystallization for high temperature α-GeS2 under these conditions. However, there are significant deviations for this model for the crystallization of low-temperature β-GeS2. This might indicate some changes in crystal-melt interfacial energy or break down of Stokes–Einstein relation in that particular case. At temperatures below 500 °C the temperature range of directly observed crystal growth overlaps with isothermal DSC measurements. In this case overall crystallization kinetics can be described by the Johnson–Mehl–Avrami (JMA) nucleation-growth model for kinetic exponent n ≅ 4. The value of activation energy of nucleation estimated from these experiments EN = 434 kJ mol−1 is comparable with the activation energy of viscous flow in supercooled Ge0.38S0.62 melt (Eη = 478 kJ mol−1). A more complex eutectic crystallization involving both GeS2 and GeS phases has been observed at higher temperatures. This process is probably associated with secondary nucleation and cannot be described by a simple JMA model.