The effects of substitution of Na₂S to Na₂O on the physical properties, such as the density, elastic moduli and thermal expansivity, of germanate glasses in the systems Na₂OGeO₂ and Na₂S-GeO₂ were studied in order to clarify the behavior of sulphur introduced in large quantities into glass structure. The density of the glasses was determined by Archmedes method with a sensitive microbalance and kerosene. The elastic moduli were calculated from the sound velocities of the glasses measured by a pulse superposition method. The thermal expansivity was determined by using a dilatometer and the transformation temperature was obtained from the thermal expansion curve. The results are summarized as follows:
The density and bulk modulus of the glasses in both systems increase with increasing alkali content at first, but start to decrease when alkali content becomes more than 17-18mol% in Na₂O-GeO₂ glasses and 6-7mol% in Na₂S-GeO₂ glasses. The density and bulk modulus of a glass containing Na₂S are considerably lower than those of a glass containing Na₂O when both glasses have the same alkali content. The glasses with Na₂S have higher thermal expansivity and lower transition temperature than those with Na₂O.
The increase in molar volume of these glasses, resulting from the substitution of S to O, was calculated from the density and composition of glasses and compared with the value calculated from the difference in ionic radius or covalent bond length between O and S.

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