Rubidium selenate is an inorganic compound of rubidium and the selenate ion, with the chemical formula Rb2SeO4. It is a colorless crystalline salt that is readily soluble in water. At room temperature it adopts an orthorhombic β-K2SO4-type structure and at high temperature undergoes a first-order transition to a hexagonal α-K2SO4-type phase.
Preparation Rubidium selenate can be prepared by neutralization of selenic acid with rubidium carbonate, followed by evaporation of the aqueous solution:
Rb2CO3 + H2SeO4 → Rb2SeO4 + CO2↑ + H2O
Single crystals have been grown by slow evaporation of an equimolar aqueous solution of rubidium carbonate and selenic acid at about 311 K.
Structure and phase transitions At room temperature, Rb2SeO4 crystallizes in the orthorhombic crystal system in space group Pnma (No. 62), historically also written in the non-standard setting Pnam. Its lattice parameters are approximately a = 7.961 Å, b = 10.794 Å and c = 6.161 Å, with four formula units per unit cell.
The room-temperature structure is of the β-K2SO4 type. It contains nearly regular SeO42− tetrahedra surrounded by rubidium ions. The arrangement is pseudohexagonal, reflected in the approximate relation between the orthorhombic lattice parameters.
On heating, rubidium selenate undergoes a first-order structural phase transition at about 818–821 K (545–548 °C). The high-temperature form is hexagonal, space group P63/mmc (No. 194), with lattice parameters at 898 K of a = 6.3428 Å and c = 8.5445 Å, and two formula units per unit cell.
The high-temperature phase is isostructural with Tl2SeO4 and belongs to the α-K2SO4 structure type.
Unlike several related A2BX4 compounds, Rb2SeO4 does not undergo a low-temperature incommensurate or ferroelectric transition. Heat-capacity, X-ray and neutron-scattering measurements found no additional structural phase transition down to 2 K, although a slight softening of a phonon branch was observed below about 100 K.
The same calorimetric study observed melting at about 1233 K (960 °C).
Chemical properties In the presence of excess selenic acid, rubidium selenate forms rubidium hydrogen selenate:
Rb2SeO4 + H2SeO4 → 2 RbHSeO4
Under humid reducing conditions at elevated temperature, Rb2SeO4 can partially decompose with formation of rubidium hydroxide. This decomposition produces a marked increase in electrical conductivity; at 317 °C in humidified 9% H2/N2, conductivities around 2 × 10−4 S cm−1 have been reported.
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