Equilibria of SiF4 with SiO2, Be2SiO4, and BeO in molten Li2BeF4

Equilibrium partial pressures of SiF 4 were measured for the reactions 2SiO 2 ( c )+2BeF 2 ( d )⇋SiF 4 ( g )+Be 2 SiO 4 ( c ) (log P siF4 (mm) = [8.790 ‐ 7620/ T ] ±0.06(500°–640°C)) and Be 2 SiO 4 ( c ) +2BeF 2 ( d )⇋SiF 4 ( g ) +4BeO( c )(log P siF4 (mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF 2 was present in solution with LiF as molten Li 2 BeF 4 . The solubility of SiF 4 was low (∼0.04 mol kg ‐1 atm ‐1 ) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved Δ H f and Δ G f values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO 2 and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF 4 generated showed an increasing positive deviation from the expression given for the equilibrium involving Be 2 SiO 4 and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO 2 .