Heats of Solution of Mixed Alkali Phosphate Glasses

The heats of solution have been measured for glasses of Li2O-Na2O-P2O5 systems. A twin-microcalorimeter was used. The molar ratios (R) of total alkali oxide to P2O5 in the glasses studied were 1.0 and 1.2. Heats of mixing were thermochemically determined by assuming the mixed alkali glass to behave as a binary solid solution of single alkali glasses. That is, the enthalpy change of the process, x[RLi2O⋅P2O5](gl)+(1-x)[RNa2O⋅P2O5](gl)→R[xLi2O⋅(1-x)Na2O]⋅P2O5(gl), where x stands for atomic fraction of lithium, Li/(Li+Na) and (gl) means “in glassy state”, was obtained by subtracting the observed heat of the mixed glass from the weighted average of heats of solution of single alkali glasses. Enthalpies of mixing for all compositions were negative and showed symmetrical dependence on the atomic fraction, Li/(Li+Na) at about x=0.5. The polarization energy contribution on mixing unlike alkali cations to the enthalpy of mixing, ΔHm, was estimated by application of Lumsden model. The estimated values in the present work were in good agreement with the observed ΔHm, while in the case of borate glasses studied previously the agreement was fairly good only for the glass of relatively high alkali content.