Thermolysis of Cs₉<i>M</i>O₄ (<i>M</i> = In, Sc): Synthesis, Crystal Structure, Chemical Bonding, and Reactivity of the Subvalent Oxidometalates Cs₇<i>M</i>O₄ (<i>M</i> = In, Sc)

The cesium-poor alkali-metal suboxidometalates Cs₇<i>M</i>O₄ (<i>M</i> = In, Sc) were prepared from the respective cesium-rich suboxidometalates Cs₉<i>M</i>O₄ by thermolysis in a dynamic vacuum at temperatures below 150 °C. They crystallize in a new structure type, comprising isolated tetrahedral oxidometalate anions [<i>M</i>O₄]^5- immersed in a matrix of metallic cesium atoms. This structural separation into alternating ionic and metallic building units is typical for subvalent compounds. Cs₇InO₄ and Cs₇ScO₄ are isotypic and form mixed crystals Cs₇(In_1-<i>x</i>Sc_<i>x</i>)O₄ with statistic distribution of In and Sc atoms. All compounds were characterized by single crystal and powder X-ray diffraction. Calculations of the electronic structure with DFT methods corroborate the coexistence of anionic entities within a metallic matrix and give evidence of the subvalent nature of cesium atoms. Overall metallic behavior is a consequence of the equal volume fractions of metallic and ionic regions, allowing for percolation of the conduction electrons throughout the crystal volume. Differential thermoanalysis was employed to gain insight into the chemical conversion processes during the thermolysis reaction. The thermolytic decomposition of Cs₉<i>M</i>O₄ is a multistep process with, in some instances, reversible amorphization and recrystallization steps. Finally, the formation of ionic compounds is observed. The full decomposition of Cs₉ScO₄ yields the two new oxidoscandates Cs₁₄Sc₄O₁₃ and Cs₁₇Sc₅O₁₅ with tetrahedral [ScO₄] units connected to chain fragments and the indium compound yields cesium oxidoindates with low-condensed [InO₄] building units. The cesium-poor suboxidometalates Cs₇<i>M</i>O₄ themselves can serve as starting materials for the synthesis of further, new suboxidometalates, as they show a unique reactivity toward metals.