Beryllium fluoride

Beryllium fluoride is the inorganic compound with the formula BeF2. This white solid is the principal precursor for the manufacture of beryllium metal. Its structure resembles that of quartz, but BeF2 is highly soluble in water. Beryllium fluoride has distinctive optical properties. In the form of fluoroberyllate glass it has the lowest refractive index for a solid at room temperature of 1.275. Its dispersive power is the lowest for a solid at 0.0093, and the nonlinear coefficient is also the lowest at 2 × 10−14. The structure of solid BeF2 resembles that of cristobalite. Be2+ centers are four coordinate and tetrahedral and the fluoride centers are two-coordinate. The Be-F bond lengths are about 1.54 Å. Analogous to SiO2, BeF2 can also adopt a number of related structures. An analogy also exists between BeF2 and AlF3: both adopt extended structures at mild temperature. Gaseous beryllium fluoride adopts a linear structure, with a Be-F distance of 143 pm. BeF2 reaches a vapor pressure of 10 Pa at 686 °C, 100 Pa at 767 °C, 1 kPa at 869 °C, 10 kPa at 999 °C, and 100 kPa at 1172 °C. Molecular in the gaseous state is isoelectronic to carbon dioxide. 'Molecules' of liquid beryllium fluoride have a fluctuating tetrahedral structure. Additionally, the density of liquid BeF2 decreases near its freezing point, as Be2+ and F− ions begin to coordinate more strongly with one another, leading to the expansion of voids between formula units. The processing of beryllium ores generates impure Be(OH)2. This material reacts with ammonium bifluoride to give ammonium tetrafluoroberyllate: Be(OH)2 + 2 (NH4)HF2 → (NH4)2BeF4 + 2 H2O Tetrafluoroberyllate is a robust ion, which allows its purification by precipitation of various impurities as their hydroxides. Heating purified (NH4)2BeF4 gives the desired product: (NH4)2BeF4 → 2 NH3 + 2 HF + BeF2 In general the reactivity of BeF2 ions with fluoride are quite analogous to the reactions of SiO2 with oxides.