Rubidium fluoride

Rubidium fluoride (RbF) is the fluoride salt of rubidium. It is a cubic crystal with rock-salt structure.

Synthesis
There are several methods for synthesising rubidium fluoride. One involves reacting rubidium hydroxide with hydrofluoric acid:
 * RbOH + HF → RbF + H2O

Another method is to neutralize rubidium carbonate with hydrofluoric acid:
 * Rb2CO3 + 2HF → 2RbF + H2O + CO2

Another possible method is to react rubidium hydroxide with ammonium fluoride:
 * RbOH + NH4F → RbF + H2O + NH3

The 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

Properties
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. 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.

In addition to simple rubidium fluoride, an acidic rubidium fluoride with the molecular formula HRbF2 is also known, which can be produced by reacting rubidium fluoride and hydrogen fluoride. The compounds H2RbF3 and H3RbF4 were also synthesized.

The solubility in acetone is 0.0036 g/kg at 18 °C and 0.0039 g/kg at 37 °C.

The standard enthalpy of formation of rubidium fluoride is ΔfH0298 = −552.2 kJ mol−1, 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.