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

Abstract

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:
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)
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
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

Citation Formats

Jeitschko, Wolfgang, Gerdes, Martin H., Witte, Anne M., and Rodewald, Ute Ch.. Subcell Structure and Two Different Superstructures of the Rare Earth Metal Silicide Carbides Y. United States: N. p., 2001. Web. doi:10.1006/jssc.2000.8917.
Jeitschko, Wolfgang, Gerdes, Martin H., Witte, Anne M., & Rodewald, Ute Ch.. Subcell Structure and Two Different Superstructures of the Rare Earth Metal Silicide Carbides Y. United States. doi:10.1006/jssc.2000.8917.
Jeitschko, Wolfgang, Gerdes, Martin H., Witte, Anne M., and Rodewald, Ute Ch.. Mon . "Subcell Structure and Two Different Superstructures of the Rare Earth Metal Silicide Carbides Y". United States. doi:10.1006/jssc.2000.8917.
@article{osti_784253,
title = {Subcell Structure and Two Different Superstructures of the Rare Earth Metal Silicide Carbides Y},
author = {Jeitschko, Wolfgang and Gerdes, Martin H. and Witte, Anne M. and Rodewald, Ute Ch.},
abstractNote = {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, 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.},
doi = {10.1006/jssc.2000.8917},
journal = {Journal of Solid State Chemistry},
number = 1,
volume = 156,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2001},
month = {Mon Jan 01 00:00:00 EST 2001}
}
  • The 12 title compounds have been prepared by arc-melting cold-pressed pellets of the elemental components and subsequent annealing. They crystallize with an orthorhombic structure and with cell dimensions varying between a = 403.9(1) pm, b = 1688.4(2) pm, and c = 450.6(1) pm for La{sub 3}Si{sub 2}C{sub 2} and a = 379.6(1) pm, b = 1532.8(2) pm, and c = 414.5(1) pm for Tm{sub 3}Si{sub 2}C{sub 2}. The magnetic properties of these compounds were determined with a SQUID magnetometer between 2 and 300 K with magnetic flux densities up to 5.5 T. Y{sub 3}Si{sub 2}C{sub 2} is a Pauli paramagnet.more » The cerium atoms in Ce{sub 3}Si{sub 2}C{sub 2} are trivalent; at low temperatures this compound is ferro- or ferrimagnetic with an ordering temperature of 10({+-}3)K. Pr{sub 3}Si{sub 2}C{sub 2} and Nd{sub 3}Si{sub 2}C{sub 2} are ferromagnetic (T{sub c} = 25({+-}3) and 30({+-}3) K, respectively), whereas the silicide carbides R{sub 3}Si{sub 2}C{sub 2} with R = Sm and Gd-Tm are antiferromagnetic. Ho{sub 3}Si{sub 2}C{sub 2}, Er{sub 3}Si{sub 2}C{sub 2}, and Tm{sub 3}Si{sub 2}C{sub 2} show metamagnetic transitions. The highest ordering temperature occurs for Gd{sub 3}Si{sub 2}C{sub 2} with a Neel temperature T{sub N} = 50({+-}1) K. The electrical conductivities of several compounds were determined between 5 and 300 K. They indicate metallic behavior, and in several cases they reflect the magnetic ordering temperatures.« less
  • The ternary rare earth boride carbides R{sub 2}B{sub 4}C (R=Tb, Dy, Ho, Er) have been synthesized by reacting the elements at temperatures between 1800 and 2000K. The crystal structure of Dy{sub 2}B{sub 4}C has been determined from single-crystal X-ray diffraction data. It crystallizes in a new structure type in the orthorhombic space group Immm (a=3.2772(6) A, b=6.567(2) A, c=7.542(1) A, Z=2, R1=0.035 (wR{sub 2}=0.10) for 224 reflections with I{sub o}>2{sigma}(I{sub o})). Boron atoms form infinite chains of fused B{sub 6} rings in [100] joined with carbon atoms into planar, two-dimensional networks which alternate with planar sheets of rare earth metalmore » atoms. The electronic structure of Dy{sub 2}B{sub 4}C was also analyzed using the tight-binding extended Hueckel method. - Graphical abstract: Dy{sub 2}B{sub 4}C crystallizes a new structure type where planar 6{sup 3}-Dy metal atom layers alternate with planar non-metal layers consisting of ribbons of fused B{sub 6} hexagons bridged by carbon atoms. Isostructural analogues with Tb, Ho and Er have also been characterized.« less
  • The ternary rare-earth boride carbides R{sub 15}B{sub 4}C{sub 14} (R=Y, Gd-Lu) were prepared from the elements by arc-melting followed by annealing in silica tubes at 1270 K for 1 month. The crystal structures of Tb{sub 15}B{sub 4}C{sub 14} and Er{sub 15}B{sub 4}C{sub 14} were determined from single crystal X-ray diffraction data. They crystallize in a new structure type in space group P4/mnc (Tb{sub 15}B{sub 4}C{sub 14}: a=8.1251(5) A, c=15.861(1) A, Z=2, R{sub 1}=0.041 (wR{sub 2}=0.088) for 1023 reflections with I{sub o}>2{sigma}(I{sub o}); Er{sub 15}B{sub 4}C{sub 14}: a=7.932(1) A, c=15.685(2) A, Z=2, R{sub 1}=0.037 (wR{sub 2}=0.094) for 1022 reflections with I{submore » o}>2{sigma}(I{sub o})). The crystal structure contains discrete carbon atoms and bent CBC units in octahedra and distorted bicapped square antiprisms, respectively. In both structures the same type of disorder exists. One R atom position needs to be refined as split atom position with a ratio 9:1 indicative of a 10% substitution of the neighboring C{sup 4-} by C{sub 2}{sup 4-}. The actual composition has then to be described as R{sub 15}B{sub 4}C{sub 14.2}. The isoelectronic substitution does not change the electron partition of R{sub 15}B{sub 4}C{sub 14} which can be written as (R{sup 3+}){sub 15}(C{sup 4-}){sub 6}(CBC{sup 5-}){sub 4{center_dot}}e{sup -}. The electronic structure was studied with the extended Hueckel method. The investigated compounds Tb{sub 15}B{sub 4}C{sub 14}, Dy{sub 15}B{sub 4}C{sub 14} and Er{sub 15}B{sub 4}C{sub 14} are hard ferromagnets with Curie temperatures T{sub C}=145, 120 and 50 K, respectively. The coercive field B{sub C}=3.15 T for Dy{sub 15}B{sub 4}C{sub 14} is quite remarkable. - Graphical abstract: The ternary rare earth boride carbides R{sub 15}B{sub 4}C{sub 14} (R=Y, Gd-Lu) were prepared from the elements by arc-melting followed by annealing in silica tubes at 1270 K for 1 month. Tb{sub 15}B{sub 4}C{sub 14} is a new member of the rare-earth metal boride carbide series in which the finite quasi-molecular CBC entities as well as isolated C atoms are embedded in the voids of the metal atom matrix. The structure of Tb{sub 15}B{sub 4}C{sub 14} contains two types of slabs: one slab contains finite bent CBC units and isolated carbon atoms whereas another is formed only from octahedral coordinated single carbon atoms. The electronic structure for the idealized composition corresponds to an electron partitioning according to (Tb{sup 3+}){sub 15}(C{sup 4-}){sub 6}(CBC{sup 5-}){sub 4{center_dot}}e{sup -} giving rise to a single electron per formula for Tb-Tb framework bonding. The magnetism of the ternary rare earth boride carbides R{sub 15}B{sub 4}C{sub 14} (R=Tb, Dy, Er) is characterized by the onset of ferromagnetic order below T<150 K.« less
  • A combination of electron, synchrotron X-ray and neutron powder diffraction reveals a new orthorhombic structure type within the Sr-doped rare earth perovskite cobaltates Ln{sub 1-x}Sr{sub x}CoO{sub 3-{delta}} (Ln=Y{sup 3+}, Dy{sup 3+}, Ho{sup 3+}, Er{sup 3+}, Tm{sup 3+}and Yb{sup 3+}). Electron diffraction shows a C-centred cell based on a 2{radical}2a{sub p}x4a{sub p}x4{radical}2a{sub p} superstructure of the basic perovskite unit. Not all of these very weak satellite reflections are evident in the synchrotron X-ray and neutron powder diffraction data and the average structure of each member of this series could only be refined based on Cmma symmetry and a 2{radical}2a{sub p}x4a{sub p}x2{radical}2a{submore » p} cell. The nature of structural and magnetic ordering in these phases relies on both oxygen vacancy and cation distribution. A small range of solid solution exists where this orthorhombic structure type is observed, centred roughly around the compositions Ln{sub 0.2}Sr{sub 0.8}CoO{sub 3-{delta}}. In the case of Yb{sup 3+} the pure orthorhombic phase was only observed for 0.850{<=}x{<=}0.875. Tetragonal (I4/mmm; 2a{sub p}x2a{sub p}x4a{sub p}) superstructures were observed for compositions having higher or lower Sr-doping levels, or for compounds with rare earth ions larger than Dy{sup 3+}. These orthorhombic phases show mixed valence (3+/4+) cobalt oxidation states between 3.2+ and 3.3+. DC magnetic susceptibility measurements show an additional magnetic transition for these orthorhombic phases compared to the associated tetragonal compounds with critical temperatures > 330 K. - Graphical abstract: A study using electron, high-resolution synchrotron X-ray and neutron powder diffraction reveals a new family of orthorhombic perovskite superstructures (Ln{sub 0.2}Sr{sub 0.8}CoO{sub 3-{delta}}), that shows A-site (Ln{sup 3+}/Sr{sup 2+}) cation ordering as well as oxygen vacancy ordering.« less
  • The carbides R{sub 2}ReC{sub 2} (R = Y, Ce-Nd, Sm, Gd-Tm, Lu) were prepared by arc-melting of cold-pressed pellets of the elemental components. They crystallize with the orthorhombic space group Pnma and Z = 4 formula units per cell. The lattice constants for Pr{sub 2}ReC{sub 2} are: a = 655.69(6) pm, b = 509.46(5) pm, c = 984.42(8) pm. The crystal structure was determined from quantitative evaluations of Guinier powder data for Y{sub 2}ReC{sub 2} and Pr{sub 2}ReC{sub 2}, which refined to conventional residuals of R = 0.088 and R = 0.072, respectively. The structure of the isotypic carbide Er{submore » 2}ReC{sub 2} was determined from single-crystal diffractometer data: R = 0.032 for 842 structure factors and 26 variable parameters. The positions of the metal atoms correspond to those of the Co{sub 2}Si-branch of the anti-PbCl{sub 2} structure. The carbon atoms are isolated from each other and occupy two different octahedral voids (4R + 2Re, 5R + 1Re).« less