MIXED-ALKALI EFFECT IN THE SYSTEM R(2)O-PBO-P2O5

The addition of a second alkaline oxide in glasses of different compos itions gives the appearance of maxima and minima in those properties t hat depend on the mobility of the modifier ions. In order to study the mixed alkali effect im phosphate glass, samples were prepared with th e composition (25-x) R(2)O.xR(2)'O.20PbO. 55P(2)O(5) (R,R'=Li, Na, K) and their thermal, electrical and chemical properties were analysed as a function of the oxide ratio. The thermal expansion coefficient, alp ha, shows slight negative variations with respect to linearity when it is represented as a function of the ratio of alkaline oxides for the three pairs of studies oxides. The size of the divergences and the oxi de ratio at which the maximum deviation appears depend on the nature o f the modifier oxides. Glasses containing lithium oxide and potassium oxide have a maximum electrical resistance for an equimolar oxide rati o. For this maximum, the divergence of the resistance with respect to additive is close to two factors of ten. The hydrolysis resistance of glasses with two alkaline oxides deviates slightly from linearity. The intensity of derivation does not follow the differences between radii of the present alkali ions. These variations might be explained from the phosphate glass structure and its hydrolysis mechanism.