HIGH-TEMPERATURE CALORIMETRIC STUDY OF GLASS-FORMING FLUOROZIRCONATES

Using high temperature calorimetric techniques, enthalpies of mixing o f melts, enthalpies of formation of glasses and enthalpies of formatio n of crystals have been measured in binary systems ZrF4-BaF2, ZrF4-NaF , ZrF4-LaF3 and selected compositions in the system ZrF4-BaF2-LaF3-AlF 3-NaF. The system ZrF4-LaF3 shows essentially zero enthalpies of mixin g of melts. In the other systems, then are strongly exothermic enthalp ies of mixing. In the binary systems ZrF4-BaF2 and ZrF4-NaF, the entha lpy of mixing of melts and the enthalpy of formation of crystals show minima at the same compositions, Ba2ZrF8 and Na3ZrF7, respectively, in dicating the stoichiometric complexes ZrF84- and ZrF73-. These results are discussed in relation to energetics, structure, speciation and ac id-base chemistry and suggest the following: ZrF4, LaF3 and AlF3 are a nionic complex-formers, while BaF2 and NaF are ionized in the molten s tate. The coordination number of the zirconium fluoride anionic comple x varies with the amount of alkali or alkaline earth fluoride. A model is proposed in which glass-forming melts are dominated by ZrF5- speci es and lie near deep eutectics. Therefore, glass formation in fluorozi rconates involves a substantial structural change, which is dominated by short-range atomic arrangements, from the melt with predominantly Z rF5- to the glass where zirconium is 7 or 8 coordinated by fluorine. ( C) 1997 Elsevier Science B.V.