ENTHALPIES OF FORMATION OF VITREOUS AND CRYSTALLINE PYROPHOSPHATES OF(LITHIUM-OXIDE OR SODIUM-OXIDE PLUS ZINC-OXIDE PLUS PHOSPHORUS PENTOXIDE)

The glasses and crystals of {xM(4)P(2)O(7) + (1 - X)Zn2P2O7} (M = Li,N a) as well as glasses of the xLi(4)P(2)O(7) .(0.3 - x)Na4P2O7 . 0.7Zn( 2)P(2)O(7) composition were studied by both solution calorimetry at th e temperature T = 298.15 K and high temperature calorimetry (solution calorimetry at T = 973 K, transposed and drop solution calorimetry, an d d.s.c.). The binary pyrophosphate glasses are formed from oxides wit h high exothermic effects. The molar enthalpy of formation for glasses depends on the nature of alkali cation and correlates with its electr onegativity (exothermic effect of formation decreases from Na to Li). A practically linear composition dependence of enthalpy of formation f or the glasses does not contradict the assumption that the structures of the extreme series members are realized in their structure. Inasmuc h 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 Tg values. The lowest values are characteristic of th e glasses with the mole ratio n(Na)/{n(Na) + n(Li)} similar or equal t o 0.7. (C) 1996 Academic Press Limited