Rubidium fluoride (RbF) is the fluoride salt of rubidium. It is a cubic crystal with rock-salt structure.
There are several methods for synthesising rubidium fluoride. One involves reacting rubidium hydroxide with hydrofluoric acid:[1]
RbOH + HF → RbF + H2OAnother method is to neutralize rubidium carbonate with hydrofluoric acid:
Rb2CO3 + 2HF → 2RbF + H2O + CO2Another possible method is to react rubidium hydroxide with ammonium fluoride:
RbOH + NH4F → RbF + H2O + NH3The least used method due to expense of rubidium metal is to react it directly with fluorine gas, as rubidium reacts violently with halogens:
2Rb + F2 → 2RbF
Rubidium fluoride is a white crystalline substance with a cubic crystal structure that looks very similar to common salt (NaCl). The crystals belong to the space group Fm3m (space group no. 225) with the lattice parameter a = 565 pm and four formula units per unit cell. The refractive index of the crystals is nD = 1.398.[2] Rubidium fluoride colors a flame (Bunsen burner flame) purple or magenta red (spectral analysis).
Rubidium fluoride forms two different hydrates, a sesquihydrate with the stoichiometric composition 2RbF·3H2O and a third hydrate with the composition 3RbF·H2O.[3]
In addition to simple rubidium fluoride, an acidic rubidium fluoride with the molecular formula HRbF2 is also known,[4] which can be produced by reacting rubidium fluoride and hydrogen fluoride. The compounds H2RbF3 and H3RbF4 were also synthesized.[5]
The solubility in acetone is 0.0036 g/kg at 18 °C and 0.0039 g/kg at 37 °C.[6]
The standard enthalpy of formation of rubidium fluoride is ΔfH0298 = −552.2 kJ mol−1,[7] the standard free enthalpy of formation ΔG0298 = −520.4 kJ mol−1, and the standard molar entropy S0298 = 113.9 J K −1 ·mol−1. The enthalpy of solution of rubidium fluoride was determined to be −24.28 kJ/mol.[8]