Synthesis, structure, and bonding of the hexagonal form of the oxide carbide ScAlOC

Red single crystals of a hexagonal form of ScAlOC were obtained as a by-product from the synthesis of ScAl3C3 by reacting scandium and carbon in an aluminium melt at 1820 °C. The crystal structure (hP8, space group P63mc, Z = 2, a = 3.24793(3) Å, c = 10.1739(1) Å, 629 refl., 15 param., R1(F) = 0.010, wR2(F2) = 0.023) can directly be derived from the binary nitrides AlN and ScN or the oxide carbides Sc(O,C) and Al(O,C), respectively. ScAlOC-II or h-ScAlOC represents a new structure type with simple closest packing of alternating layers of oxygen and carbon. The stacking sequence is ABAC (=(hc)2) with oxygen in a cubic and carbon in a hexagonal sequence. According to the difference in size Sc occupies octahedral voids between every second layer leading to layers of edge-sharing ScO3C3 octahedra. Aluminium is located in half of the tetrahedral voids. The AlOC3-tetrahedra are connected to layers by common corners of the carbon atoms. h-ScAlOC continues the row of the rare examples of oxide carbides with ordered anion distribution. Band structure calculations by FP-(L)APW methods revealed that ScAlOC is electron precise with a band gap of 1.2 eV. Calculations of charges by the Bader-method reveal values of Sc+1.87, Al+2.33, O−1.52 and C−2.67. Together with the charge densities and the values of the Laplacian this stands for a mainly ionic bonding containing significant covalent contributions, too. Despite the close similarity to rhombohedral ScAlOC (r-ScAlOC) there is a striking difference in colour, as r-ScAlOC is black. This is confirmed and explained by the results of the P-DOS, because the lowest states of the conduction band are determined by Sc-d-states. These are significantly lower in r-ScAlOC with ScC6 and ScO6 octahedra than in h-ScAlOC leading to a calculated band gap of 0.3 eV.

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