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Title: Subcell Structure and Two Different Superstructures of the Rare Earth Metal Silicide Carbides Y

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 Pr{sub 3}Si{sub 2}C{sub 2}: 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{sub 2} 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 C{sub 2} pairs have different orientations. In the structure of Tb{sub 3}Si{sub 2}C{sub 2} 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 f or 1437 structure factors and 22 variable parameters. Dy{sub 3}Si{sub 2}C{sub 2} 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 Y{sub 3}Si{sub 2}C{sub 2} 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,more » 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 C{sub 2} 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{sub 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{sup +3}){sub 3}(Si{sup {minus}3}){sub 2}(C{sub 2}){sup {minus}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 somewhat higher than 0.5.« less
Authors:
; ; ;
Publication Date:
OSTI Identifier:
784253
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Solid State Chemistry; Journal Volume: 156; Journal Issue: 1; Other Information: PBD: 1 Jan 2001
Research Org:
Anorganisch-Chemisches Institut, Univ. Muenster, Muenster (DE)
Sponsoring Org:
German Research Foundation (DFG); Fonds der Chemischen Industrie; International Centre for Diffraction Data (US)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ATOMS; BONDING; CARBIDES; CHAINS; INTERATOMIC DISTANCES; OXIDATION; RARE EARTHS; SILICIDES; SILICON; SPACE GROUPS; STRUCTURE FACTORS; X-RAY DIFFRACTOMETERS; YTTRIUM SILICIDE; YTTRIUM CARBIDE; PRASEODYMIUM SILICIDE; PRASEODYMIUM CARBIDE; TERBIUM SILICIDE; TERBIUM CARBIDE; X-RAY DIFFRACTION; EXPERIMENTAL DATA; ORTHORHOMBIC LATTICE; LATTICE PARAMETERS; STRUCTURE FUNCTIONS; VALENCE; BOND LENGTHS; ELECTRIC CONDUCTIVITY