Solubility of calcium and strontium fluorides in a sodium nitrate melt and choosing a crucible material for working with their solution melts

Sodium nitrate is a promising medium for the preparation of nanoparticles of some inorganic fluorides and for studying low-temperature phase equilibria in fluoride systems. In our study, we investigated the possibilities of carrying out longterm (hundreds of hours) experiments with МF2–NaNO3 (М = Ca, Sr). We performed an experimental evaluation of the solubility of calcium (CaF2) and strontium SrF2 fluorides in a melt of sodium nitrate NaNO3 in the temperature range of 320–500 оС. The article demonstrates that for both fluorides it is low, but the solubility of SrF2 is almost an order of magnitude higher than the solubility of CaF2 and is about 1 g/100 g of NaNO3 at 500 оС. The absence of perceptible oxidative processes and the low solubility of СaF2 and SrF2 fluorides in sodium nitrate make it possible to synthesize solid solutions based on them in this medium. The article also considers the possibility of using crucibles made of glazed ceramics, glass-carbon, and aluminium for working with МF2–NaNO3 (М = Ca, Sr) melt solutions. It is shown that glass-carbon and aluminium react with the NaNO3–SrF2 melt solution to form strontium carbonate and several oxide phases, respectively. It is recommended to use glazed ceramics as a crucible material for long-term solution-melt processes. The aluminium crucible showed high resistance to the NaNO3 melt without dissolved fluorides.