Enthalpies of formation of vitreous and crystalline pyrophosphates of (lithium oxide or sodium oxide + zinc oxide + phosphorus pentoxide)

Abstract

The glasses and crystals of { xM 4P 2O 7+ (1 − x)Zn 2P 2O 7} (M = Li, Na) as well as glasses of the xLi 4P 2O 7· (0.3 − x)Na 4P 2O 7·0.72Zn 2P 2O 7composition were studied by both solution calorimetry at the temperature T= 298.15 K and high temperature calorimetry (solution calorimetry at T= 973 K, transposed and drop solution calorimetry, and d.s.c.). The binary pyrophosphate glasses depends on the nature of alkali cation and correlates with its electronegativity (exothermic effect of formation decreases from Na to Li). A practically linear composition dependence of enthalpy of formation for the glasses does not contradict the assumption that the structures of the extreme series members are realized in their structure. Inasmusch as the effects of mixing for the ternary pyrophosphate glasses from the series end members ( x= 0 and x= 0.3) are weakly exothermic, the existence of the mixed alkali effect is assumed. The effect causes the decrease of the T gvalues. The lowest values are characteristic of the glasses with the mole ratio n(Na) /{ n(NA) + n(LI)} ≃ 0.7.