Thermally induced changes in the oxidation state of cerium(IV)

Cesium hexachlorocerate(IV), Cs₂CeCl₆ (I) and sodium pentakis(carbonato)cerate(IV), Na₆Ce(CO₃)₅·12H₂O (II) have been investigated in air by simultaneous TG/DTA, FTIR and XRD in order to follow the oxidation state of cerium during their thermal treatment. The thermal decomposition of the hexachloro compound (I) is accompanied by a double change in the oxidation state of cerium. First, in an inner reduction-oxidation reaction, chlorine is evolved and a Cs₂CeCl₅ phase is obtained. The immediately starting oxidation of this Ce(III) species caused various phase transitions in the CeCl₃-CsCl system formed. The presence of Cs₃CeCl₆ above 400°C can also be assumed and finally this phase also oxidizes into CeO₂ with the formation of CsCl as by-product. In the case of the pentacarbonato complex (II), no Ce(III) species were detected. The final products of its decomposition were CeO₂ and Na₂CO₃.