Hansjuergen Mattausch

Ln13Br18B3 (Ln=Gd, Tb)—A Compound Containing a Combination of Discrete and Condensed Clusters†

Metal-rich ternary halides of rare-earth metals form a large group of compounds,[1] the diverse structures of which can be understood within the concept of condensed clusters.[2a] As a rule, the cluster units of the valence-electron-poor rare-earth metals need stabilization by interstitial (endohedral) atoms.[2b] While several types of clusters are observed, octahedral units of metal atoms condensed through edges dominate. The structural systematics range from compounds with discrete clusters to those with chains and layers to those with threedimensional (3D) networks, as illustrated with some arbitrary examples of the particularly rich chemistry of boride and carbide halides.[3] The structure of Tb7X12B (X1⁄4 halogen) contains discrete Tb6BX12 clusters.[1c] The compound Tb10Br15B2, which contains a Tb10B2X18-type cluster built from two octahedral units fused through a common edge,[3a] represents the first condensation step towards the formation of infinite chains as in Tb4X5B. Four Tb6B octahedra are linked to form discrete clusters with a Tb16B4 core in Tb16Br23B4, and this cluster type is the building block of double chains as found in the carbide halide Gd6Br7C2. Fusing an infinite number of chains results in a layer as, for example, in Gd2X2C. Last but not least, a 3D network is present in the structure of Gd3X3B. Here we report on the structure of a reduced rare-earth metal halide that is the first to contain both discrete clusters and chains. This combination is found in the compounds Ln13Br18B3 (Ln1⁄4Gd, Tb), which can be prepared from mixtures of LnBr3, Ln, and B at approximately 1000 8C[4] and which form pillar-shaped, dark golden crystals. X-ray diffraction patterns of single crystals of these compounds are characterized by sharp Bragg reflections besides pronounced diffuse scattering arranged in the form of rods along the c* direction (Figure 1a). This indicates a disordered stacking of ordered layers. The average structure of the compound was solved on the basis of the intensities of Bragg reflections only, and the refinement converged to R values below 0.03. It is described in the orthorhombic space group Immm.[5] Figure 2a depicts the structure of Ln13Br18B3 in a projection along [100]; the disorder is not obvious because it only affects the site occupation factors of some of the atoms. The structure consists of a honeycomb packing of wellordered, single chains of trans edge-sharing Ln6 octahedra, which are centered by B atoms and surrounded by Br atoms (Figure 2b). The channels between these single chains are filled by double chains (Figure 2a). In spite of the very convincing agreement between observed and calculated Bragg intensities, the average structure does not allow a reasonable chemical interpretation as some of the atomic positions are partially occupied as mentioned above and/or characterized by highly anisotropic displacement parameters. The disorder problem could be solved by means of highresolution transmission electron microscopy (HRTEM).[6] A HRTEM image taken along the [110] zone axis (Figure 3) clearly shows the nature of the disorder. First, the double chain resolves into a regular stacking of Ln10B2 units formed by edge-sharing Ln6B octahedra and surrounded by Br atoms (Figure 2c). The image simulation image based on this model is in good agreement with the observed image (cf. the inset in Figure 3). Second, the stacks of well-ordered clusters are shifted relative to each other in an irregular way. This statistical arrangement is the origin of the diffuse scattering shown in Figure 1. A calculated diffraction pattern based on perfectly ordered clusters within layers parallel (001) and assuming stochastic disorder along [001] in the stacking of such layers in a suitable supercell mimics the intensity distribution of the diffuse scattering very well (Figure 1b). The real structure of Ln13Br18B3 (Ln1⁄4Gd, Tb) corresponds to the earlier mentioned honeycomb packing of single chains of condensed Ln6B units surrounded by Br as in the M6X12 cluster. The composition of one chain is Ln4BBr6. Owing to linkage through i±i contacts, that is contacts between anions of COMMUNICATIONS

La6Br10Fe: a La6Fe octahedron with a mixed M6X12/M6X8 type environment.

The title compound was synthesized from La, LaBr3, and Fe under Ar atmosphere at 800 degrees C. It crystallizes in space group P4(1) (No. 76) with lattice constants a = 8.255(1) A and c = 30.033(6) A. The structure features an isolated Fe-centered La 6 octahedron with all corners, 9 of its 12 edges, and 3 of its 8 triangular faces coordinated, bridged, or capped by Br atoms. The La6Fe octahedron is significantly distorted, and the La coordination by Br atoms deviates from the common close-packing arrangements found in other reduced rare earth metal halides. Band structure, bonding, and physical properties of the compound have been investigated.

Twinning and intergrowth of rare earth boride carbides

Twins and intergrown crystals of tetragonal rare earth boride carbides, especially those with the La(5)B(2)C(6) structure type, have been investigated by high-resolution electron microscopy and X-ray diffraction. The structure of the twin interface has been determined. It provides an explanation for coherently intergrown domains of different structure. The Sc(3)C(4) structure type is remarkable because it is frequently intergrown with La(5)B(2)C(6)-type phases. It provides, for instance, a model for the intergrowth of other types, e.g. Gd(4)B(3)C(4) and Gd(5)B(2)C(5). The presence of metal-atom square nets in different orientations in the structures accounts for a number of intergrowth phenomena. The possibilities and limitations of X-ray structure determinations are discussed with respect to actual examples.

Boron-Carbon Order and Symmetry Control: Single-Crystal X-Ray Study of SmB2C2

The title compound was prepared from the elements by arc-melting followed by annealing in silica tubes at 1270 K for one week. The crystal structure was investigated by means of X-ray single crystal diffraction: space group P4/mbm, a = 5.366(1), c = 3.690(1) Å , Z = 2, R1 = 0.010, wR2 = 0.022 for 245 unique reflections with Io > 2σ (Io) and 12 refined parameters. Group-subgroup relationships of MB6 and MB2C2 structure models are discussed

The Starting Members of the Series Pr4n+2(C2)nBr5n+5 (n = 1, 2, 3)

The compounds Pr6(C2)Br10, Pr10(C2)2Br15 and Pr14(C2)3Br20 were prepared from PrBr3 and the appropriate amounts of Pr and C and characterized by X-ray structure analyses of single crystals. All three compounds crystallize in space group P1 with lattice parameters a = 7.571(2), b = 9.004(2), c = 9.062(2) Å ,α = 108.57(3), β = 97.77(3), γ = 106.28(3)◦ for Pr6(C2)Br10; a = 9.098(2), b = 10.127(2), c = 10.965(2) A° , α = 70.38(3), β = 66.31(3), γ = 70.84(3)◦ for Pr10(C2)2Br15; a = 9.054(2), b = 10.935(2), c = 13.352(3) Å , α = 86.27(3), β = 72.57(3), γ = 66.88(3)◦ for Pr14(C2)3Br20. They are members of a general series Ln4n+2(C2)nBr5n+5 and isostructural with the corresponding iodides known for Ln = La, Ce, Pr. Pr6(C2)Br10 was further characterized via transmission electron microscopy techniques Graphical Abstract The Starting Members of the Series Pr4n+2(C2)nBr5n+5 (n = 1, 2, 3)

Crystal Structure of the Terbium Borocarbide Tb2B2C3

The title compound was prepared from the elements by arc-melting followed by annealing in silica tubes at 1270 K for one week. The crystal structure was investigated by means of single-crystal X-ray diffraction: space group Cmmm, a = 3.412(1), b = 13.699(8), c = 3.669(1) Å, V = 171.5(1) Å3, Z = 2, R1 = 0.032; wR2 = 0.087 for 123 unique reflections with Io ≥ 2σ (Io) and 11 refined parameters.

La3Cl3BC – Structure, Bonding and Electrical Conductivity

A new rare earth carbide boride halide, La3Cl3BC, has been prepared by heating a mixture of stoichiometric quantities of LaCl3, La, B and C at 1050 °C for 10 days. La3Cl3BC (La3Br3BC type) crystallizes in the monoclinic system with space group P21/m (No. 11), a = 8.2040(16), b = 3.8824(8), c=11.328(2)Å , β =100.82(3)°. In the structure, monocapped trigonal prisms containing B-C units are condensed into chains along the b direction, and the chains are further linked by Cl atoms in the a and c directions. The condensation results in a polymeric anion 1∞[BC] with a spine of B atoms in a trigonal prismatic coordination by La, and the C atoms attached in a square pyramidal coordination. The B-B and B-C distances are 2.16 and 1.63 Å , respectively. La3Cl3BC is metallic. The EH calculation shows that the distribution of valence electrons can be formulated as (La3+)3(Cl−)3(BC)5− · e−.

Are One-Dimensional Structural Features Essential for Superconductivity at 90K ?

The crystal structures of the novel high-Tc oxocuprates1,2,3,4 have certain features in common, but also exhibit essential differences. La1.8M0.2CuO4(M=Ba,Sr; Tc ≈ 30 to 40K) contains copper in an average oxidation state +2.2. The coordination octahedron around Cu is tetragonally elongated (d(Cu-O)=190 pm (4x) and 240 pm (2x)). Taking the drastic differences of the Cu-O distances into account, the structure contains CuO2 layers formed from corner-sharing CuO4 squares according to ∞ 2 [CuO4/2] which are sandwiched between rigid slabs of composition ∞ 2 [(La,M)2O2]. Obviously, the layers are only weakly coupled to the surrounding slabs.