Thermochemistry and structure of the liquid Na 2O-GeO 2 system

Abstract

The integral molar enthalpy of mixing of the liquid mixture Na 2O-GeO 2 was measured in the composition range from 15 to 100 mol. % GeO 2 at 1400 K by the drop-mixing method using a differential high-temperature integrated heat-flux calorimeter. The partial molar enthalpies of mixing of the component oxides were obtained by the method of intercepts. The enthalpies of mixing are exothermic at all compositions. Near x GeO 2 ≈ 0.33, the integral enthalpy shows a minimum value (Δ m H= −134 kJ mol −1), and the partial enthalpies display a sharp dependence on composition. The strong energetic asymmetry of the considered system is attributed to the energy associated with the breakdown of the network structure of liquid GeO 2. The relationship between these structural changes and the thermochemical properties of the liquid mixture is discussed, employing a simple electrostatic model and using the information reported on the structures of sodium germanate glasses and melts in the literature.