The effect of trivalent cation on the properties and structure of the glasses in the system Na₂O-M₂O₃-SiO₂ (M: B, Al, Sc, Cr, Fe, Ga, As, Y, In, Sb, La, Bi) was investigated. Three series of glasses, 30Na₂O⋅5M₂O₃⋅60SiO₂, 30Na₂O⋅10M₂O₃⋅60SiO₂ and 21Na₂O⋅9M₂O₃⋅60SiO₂ were prepared, and their properties such as density, refractive index, thermal expansion, DTA, TG, infrared spectrum and Vickers microhardness were measured. The glasses containing Cr₂O₃ or As₂O₃ couldn't be prepared owing to the phase separation of the components or the volatilization of As₂O₃, respectively.
Thermal expansion coefficient factor, α_M, is calculated from the observed values by Takahashi's equation, α=∑p_Mα_M, where α is the expansion coefficient, α_M is the expansion coefficient factor for MO_x, and p_M is the molar fraction of MO_x. From the curve plotting α_M against the field strength of trivalent cation, it is shown that these cations can be classified into two groups, the first group containing B³⁺, Al³⁺, Ga³⁺ and Fe³⁺ ions, and the second containing Sc³⁺, Y³⁺, In³⁺ and La³⁺ ions. The expansion coefficient of the glass decreases by addition of the second-group-cation, because its α_M is negative or positively small in value owing to the high field strength of the cation.
Vickers microhardness of the glass containing the second-group-cation is larger than that containing the first-group-one.
The above results clearly indicate that the first-group-cations are NWF, and the second-group-ones NWM. The introduction of the second-group-cation, however, tends to strengthen the glass structure because of its high field strength in spite of producing nonbridging oxygens in the network. Sb³⁺ and Bi³⁺ ions belong to neither the first group nor the second one owing to its intermediate effect on the glass properties.