Subcell Structure and Two Different Superstructures of the Rare Earth Metal Silicide Carbides Y3Si2C2, Pr3Si2C2, Tb3Si2C2, and Dy3Si2C2

• Published in: Journal of solid state chemistry 2001-01, Vol.156 (1), p.1-9
• Main Authors: Jeitschko, Wolfgang, Gerdes, Martin H., Witte, Anne M., Rodewald, Ute Ch
• Format: Article
• Language: English
• Subjects: Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Hydrides, borides, nitrides, etc., and metalloidal compounds; Inorganic compounds; Physics; Structure of solids and liquids; crystallography; Structure of specific crystalline solids
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1006/jssc.2000.8917

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 (and 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 C2 pairs with split atomic positions. This structure may be considered the thermodynamically stable forms of these compounds at high temperatures. Two different superstructures with doubled a or c axes, respectively, of the subcell have been observed, where the C2 pairs have different orientations. In the structure of Tb3Si2C2 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 Y3Si2C2 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 C2 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 C2 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(C2)−3. A more detailed analysis of the interatomic distances showed 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 somewh