Thermal transformation of quaternary compounds in NaF–CaF2–AlF3 system

Details of quaternary compounds formation in the system NaF–CaF2–AlF3 are specified. To achieve this aim, the samples of phases NaCaAlF6 and Na2Ca3Al2F14 have been obtained by high-temperature solid-phase synthesis. Their thermal behavior when heated up to 800 °C has been studied using the methods of high-temperature X-ray diffraction (XRD) and thermal analysis (TA). The system under consideration can be regarded as a quasibinary section CaF2–NaAlF4, where at T=745–750 °C invariant equilibrium is implemented with the phases CaF2–NaCaAlF6–Na2Ca3Al2F14–(liquid melt)–(NaAlF4). The peculiarity of the equilibrium is NaAlF4 metastability at normal pressure. Below the equilibrium temperature the quaternary phase Na2Ca3Al2F14 is stable and NaCaAlF6 above this temperature. The phase NaCaAlF6 fixed by rapid quenching from high temperatures and when heated up to 640 °C decomposes, yielding Na2Ca3Al2F14. Further heating in vacuum at temperature up to 740 °C results in decomposition of Na2Ca3Al2F14 into CaF2 and Na3AlF6. The expected reverse transformation of Na2Ca3Al2F14 into NaCaAlF6 has not been observed under experimental conditions. Transformations in bulk samples reveal direct and reverse transformation of quaternary phases.