Synthesis, Structure, Bonding, and Properties of the Oxide Carbide ScAlOC, a New Type of Compound

Single crystals of ScAlOC were obtained by reacting Sc2O3 and C in an Al melt at 1600 °C. The crystal structure (space group R3̅m, Z = 6, a = 3.2599(7) Å, c = 30.116(9) Å, 190 refl., 15 param., R1(F) = 0.0212, wR2(F2) = 0.0397) can directly be derived from the binary nitrides AlN and ScN. ScAlOC represents a new type of compound. It is the first oxide carbide of a transition metal with an ordered distribution of the anions. They form a cubic closest packing with alternating double layers of C and O while Al and Sc occupy tetrahedral and octahedral voids, respectively. The resulting polyhedra are AlC3O tetrahedra as well as ScO6 and ScC6 octahedra. According to band structure calculations ScAlOC is electron precise with an indirect band gap of 0.6 eV. Calculations of charges and charge densities reveal that the mainly ionic bonding contains significant covalent contributions, too. The black crystals of ScAlOC are very brittle and show a microhardness of 9.0 GPa. Thermal decomposition on air starts at 650 °C; in inert atmosphere ScAlOC is stable up to 1300 °C at least.