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

Abstract

Single crystals of ScAlOC were obtained by reacting Sc(2)O(3) and C in an Al melt at 1600 degrees C. The crystal structure (space group R3m, Z = 6, a = 3.2599(7) A, c = 30.116(9) A, 190 refl., 15 param., R(1)(F) = 0.0212, wR(2)(F(2)) = 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 AlC(3)O tetrahedra as well as ScO(6) and ScC(6) 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 degrees C; in inert atmosphere ScAlOC is stable up to 1300 degrees C at least.