Rubidium oxide

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Rubidium oxide

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Template:Longitem	Rb2O
Molar mass	186.94 g/moL
Appearance	Yellow solid
Density	4 g/cm3
Melting point	Template:Chembox CalcTemperatures
Template:Longitem	+1527.0·10−6 cm3/mol
Template:Longitem	Antifluorite (cubic), cF12
Template:Longitem	Fm3m, No. 225
Template:Longitem	Tetrahedral (Rb+); cubic (O2−)
NFPA 704 (fire diamond)	Template:NFPA 704 diamond
Flash point	Template:Chembox CalcTemperatures
Template:Longitem	Rubidium sulfide Rubidium selenide Rubidium telluride Rubidium polonide
Template:Longitem	Lithium oxide Sodium oxide Potassium oxide Caesium oxide
Template:Longitem	Rubidium suboxide Rubidium peroxide Rubidium sesquioxide Rubidium superoxide Rubidium ozonide
Template:Longitem	Rubidium hydroxide

Rubidium oxide is the chemical compound with the formula Template:Chem/link. Rubidium oxide is highly reactive towards water, and therefore it would not be expected to occur naturally. The rubidium content in minerals is often calculated and quoted in terms of Template:Chem/link. In reality, the rubidium is typically present as a component of (actually, an impurity in) silicate or aluminosilicate. A major source of rubidium is lepidolite, Template:Chem/link, wherein Rb sometimes replaces K.

Template:Chem/link is a yellow colored solid. The related species Template:Chem/link, and Template:Chem/link are colorless, pale-yellow, and orange, respectively.

The alkali metal oxides Template:Chem/link crystallise in the antifluorite structure. In the antifluorite motif, the positions of the anions and cations are reversed relative to their positions in CaF₂, with rubidium ions 4-coordinate (tetrahedral) and oxide ions 8-coordinate (cubic).

Properties

Like other alkali metal oxides, Rb₂O is a strong base. Thus, Rb₂O reacts exothermically with water to form rubidium hydroxide.

Rb₂O + H₂O → 2 RbOH

So reactive is Rb₂O toward water that it is considered hygroscopic. Upon heating, Rb₂O reacts with hydrogen to rubidium hydroxide and rubidium hydride:

Rb₂O + H₂ → RbOH + RbH

Synthesis

For laboratory use, RbOH is usually used in place of the oxide. RbOH can be purchased for ca. US$5/g (2006). The hydroxide is more useful, less reactive toward atmospheric moisture, and less expensive than the oxide.

As for most alkali metal oxides, the best synthesis of Rb₂O does not entail oxidation of the metal but reduction of the anhydrous nitrate:

10 Rb + 2 RbNO₃ → 6 Rb₂O + N₂

Typical for alkali metal hydroxides, RbOH cannot be dehydrated to the oxide. Instead, the hydroxide can be decomposed to the oxide (by reduction of the hydrogen ion) using Rb metal:

2 Rb + 2 RbOH → 2 Rb₂O + H₂

Metallic Rb reacts with O₂, as indicated by its tendency to rapidly tarnish in air. The tarnishing process is relatively colorful as it proceeds via bronze-colored Rb₆O and copper-colored Rb₉O₂. The suboxides of rubidium that have been characterized by X-ray crystallography include Rb₉O₂ and Rb₆O, as well as the mixed Cs-Rb suboxides Cs₁₁O₃Rb_n (n = 1, 2, 3).

The final product of oxygenation of Rb is principally RbO₂, rubidium superoxide:

Rb + O₂ → RbO₂

This superoxide can then be reduced to Rb₂O using excess rubidium metal:

3 Rb + RbO₂ → 2 Rb₂O