Structure of Glasses and Melts in the Na 2 O·GeO 2 ·B 2 O 3 System

Density and viscosity results are presented for ternary Na 2 O·GeO 2 ·B 2 O 3 melts (∼600° to 1300°C) and glasses containing as much as 35 mole % Na 2 O. Synthetic partial molar volume models indicate a fairly broad stability region for BO 4 tetrahedra in the B 2 O 3 ‐rich melts. Similar models for GeO 2 ‐rich melts reveal a more limited stability region for GeO 6 octahedra. The expansion coefficient contours and viscosity isotherms confirm the volume‐based conclusions for the liquid state. The high‐temperature volume models were used to develop glass volume models that agree to within several percent of experiment. It has been concluded that the melts and glasses possess similar structures. The relatively greater compositional stability of GeO 6 octahedra in the presence of B 2 O 3 (compared to Al 2 O 3 ) can be related to the smaller average number of oxygens around boron (III), at a fixed O/Ge ratio, compared to aluminum (III). Evidence is presented for a slight decrease of the thermal stability of GeO 6 octahedra in the GeO 2 ‐rich melts above about 1000°C.