Subcell structure and two different superstructures of the rare earth metal silicide carbides Y3Si2C2, Pr3Si2C2, Tb3Si2C2, and Dy3Si2C2

The title compounds crystallize with a very pronounced subcell structure that has been determined from single-crystal X-ray diffractometer data of all four compounds, Only subcell land no superstructure) reflections have been observed for Pr3Si2C2: space group Cmmm, a = 396.7(1) pm, b = 1645.2(3) pm, c = 439.9(1) pm, R = 0.019 for 309 structure factors and 20 variable parameters. In this subcell structure there are C-2 pairs with split atomic positions. This structure may be considered the thermodynamically stable form of these compounds at high temperatures. Two different superstructures with doubled a or c axes, respectively, of the subcell have been observed, where the C-2 pairs have different orientations. In the structure of Tb,Si,Cz the a axis of the subcell is doubled. The resulting superstructure in the standard setting has the space group Pbcm: a = 423.6(1) pm, b = 770.7(1) pm, c = 1570.2(3) pm, R = 0.031 for 1437 structure factors and 22 variable parameters. Dy3Si2C2 has the same superstructure: a = 420.3(1) pm, b = 767.5(1) pm, c = 1561.1(3) pm, R = 0.045, 801 F values, 22 variables. In the structure of Y3S2C2 the c axis of the subcell is doubled, resulting in a body-centered space group with the standard setting Imma: a = 842.6(2) pm, b = 1563.4(2) pm, c = 384.6(1) pm, R = 0.035, 681 F values, 15 variables. In all of these structures the rare earth atoms form two-dimensionally infinite sheets of edge-sharing octahedra that contain the Ct pairs. In between these sheets there are zig-zag chains of silicon atoms with Si-Si distances varying between 246.2(3) and 253.6(3) pm, somewhat longer than the two-electron bonds of 235 pm in elemental silicon, suggesting a bond order of 0.5 for the Si-Si bonds. The C-C distances in the C-2 pairs vary between 127(1) and 131(1) pm, corresponding to a bond order of approximately 2.5. Hence, using oxidation numbers, the compounds may to a first approximation be represented by the formula (R+3)(3)(Si-3)(2)(C-2)(-3). A more detailed analysis of the interatomic distances shows that the shortest R-R distances are comparable with the R-R distances in the structures of the rare earth elements, thus indicating some R-R bonding. Therefore, the oxidation numbers of the rare earth atoms are slightly Lower than +3, in agreement with the metallic conductivity of these compounds. As a consequence, considering the relatively short Si-Si bonds, the absolute value of the oxidation number of the silicon atoms may be lower than 3, resulting in a Si-Si bond order somewhat higher than 0.5. (C) 2001 Academic Press.