Thermal transformation of quaternary compounds in NaF–CaF 2–AlF 3 system

Abstract

Details of quaternary compounds formation in the system NaF–CaF 2–AlF 3 are specified. To achieve this aim, the samples of phases NaCaAlF 6 and Na 2Ca 3Al 2F 14 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 CaF 2–NaAlF 4, where at T=745–750 °C invariant equilibrium is implemented with the phases CaF 2–NaCaAlF 6–Na 2Ca 3Al 2F 14–(liquid melt)–(NaAlF 4). The peculiarity of the equilibrium is NaAlF 4 metastability at normal pressure. Below the equilibrium temperature the quaternary phase Na 2Ca 3Al 2F 14 is stable and NaCaAlF 6 above this temperature. The phase NaCaAlF 6 fixed by rapid quenching from high temperatures and when heated up to 640 °C decomposes, yielding Na 2Ca 3Al 2F 14. Further heating in vacuum at temperature up to 740 °C results in decomposition of Na 2Ca 3Al 2F 14 into CaF 2 and Na 3AlF 6. The expected reverse transformation of Na 2Ca 3Al 2F 14 into NaCaAlF 6 has not been observed under experimental conditions. Transformations in bulk samples reveal direct and reverse transformation of quaternary phases. Synopsis Thermal transformation of the quaternary compounds in system (NaF–CaF 2–AlF 3) was investigated using high-temperature X-ray diffraction (XRD) and thermal analysis (TA). In the system the invariant equilibrium is implemented with the phases CaF 2–NaCaAlF 6–Na 2Ca 3Al 2F 14–(liquid melt)–(NaAlF 4) at T=745–750 °C.