BCHoNi, tetragonal, P4/nmm (No. 129), a =3.5621(5) Å, c = 7.556(2) Å, V = 95.9 Å3, Z = 2, Rgt(F) = 0.030, wRref(F2) = 0.076, T= 300 K

Geupel, S.

Graw, G.

Paufler, P.

Zahn, G.

English

Repositorium für Naturwissenschaften und Technik

Z. Kristallogr. NCS 215 (2000) 4 7 9 ^ 8 0 479 © by Oldenbourg Wissenschaftsverlag, München Refinement of the crystal structure of holmium nickel borocarbide, HoNiBC S. Geupel*11, G. Zahn1, P. Paufler1 and G. Graw11 I Technische Universität Dresden, Institut für Kristallographie und Festkörperphysik, D-01062 Dresden, Germany II Institut für Festkörper- und Werkstofforschung Dresden, Postfach 270016, D-01171 Dresden, Germany Received May 8, 2000, CSD-No. 409498 Abstract BCHoNi, tetragonal, P4/nmm (No. 129), a = 3.5621(5) A, c = 7.556(2) A, V = 95.9 A 3 , Z = 2, Rgt(F) = 0.030, wRlef(F2) = 0.076, T = 300 K. Source of material A coarse-grained sample with nominal composition HoNiBC was prepared from holmium pieces and powders of nickel, boron and carbon, respectively. A stoichiometric mixture of the high-purity elements was pressed to pellets, which have been arc-melted under argon atmosphere on a water-cooled copper heart. Melting of the buttons has been repeated three times in order to improve the sample homogeneity. A special annealing treatment (duration 75 h, temperature up to 1733 K) was performed in a resistance furnace under argon atmosphere. The composition of the annealed sample was determined with electron probe microanalysis applying the WDX mode. A single grain for single-crystal diffractometry was extracted from the polycrystalline aggregate. Experimental details Accurate lattice parameters of HoNiBC were measured at temperatures between 300 K and 100 K using a STOE STADI4 four-circle diffractometer equipped with a CRYOSTREAM cooling system (Oxford Cryosystems). Discussion The title compound is isotypic with LuNiBC [ 1 ]. Along the c axis, inverse PbO-type Ni2B2 layers are separated by double NaCl-type HoC layers, in contrast to the alternate stacking of Ni2B2 and HoC layers in the superconducting quaternary phase HoNi2B2C. This structural modification changes the space group from I4/mmm - edba (HoNi2B2C) to PA/nmm - C3a (HoNiBC). The only known structure refinement on HoNiBC has been per-formed using X-ray powder diffraction data from a sample con-taining unidentified impurity phases [2]. Here we present the first structure refinement using single-crystal diffraction data. Atomic coordinates correspond to the standardized form accord-ing to STRUCTURE TIDY [4], shifting the origin by [0 0 1/2], Compared with LuNiBC, a replacement of the smaller Lu atom by the larger Ho atom shifts the lattice parameters in an opposite manner: the a axis expands, whereas the c axis contracts. Each Ho atom is in a square-planar coordination by four C atoms at 2.520 A and vice versa. The Ni atom is tetrahedrally surrounded by four B atoms, with B-Ni-B bond angles of 106.3° and 116.0° and a Ni—B distance of 2.10 A. The refinement of site occupan-cies results in the composition HoNio.97(i)BC. The analysis of the U\j reveals a preferred displacement of Ho and Ni atoms in the a-b plane. As distinct from HoNi2B2C [3], the linear thermal expan-sion at temperatures between 300 K and 100 K indicates a nearly isotropic behaviour: a a = etc = 1.2 x 10~5 K_ l . Table 1. Data collection and handling. Crystal: metallic lustre, block, size 0.02 x 0.04 x 0.06 mm Wavelength: Mo Ka radiation (0.71073 Â) M: 502.52 cm Diffractometer, scan mode: STOE STADI4, to/9 29max: 74.74° WiWJmeasured, N(hkl)a„¡que: 1136,185 Criterion for /0bs, N(hkl)et: /obs > 2 a(Iobs), 156 N(param)rs:f,aei' 12 Programs: STRUCTURE TIDY [4], SHELX-97 [5], SCHAKAL92 [6] * Correspondence author (e-mail: sandra.geupel@uni-bayreuth.de) ' current address: Universität Bayreuth, Lehrstuhl für Kristallographie, D-95440 Bayreuth, Germany 480 Holmium nickel borocarbide Table 2. Atomic coordinates and displacement parameters (in A2), origin at (2/m). Atom Site x y z t/iSO B 2c 1/4 1/4 0.147(2) 0.007(2) C 2c 1/4 1/4 0.345(2) 0.006(2) Table 3. Atomic coordinates and displacement parameters (in A2), origin at (2/m). Atom Site Occ. x y z U\\ U22 U33 U12 U13 t/23 Ho 2c 1/4 1/4 0.6660(1) 0.0064(3) U11 0.0032(3) 0 0 0 Ni la 0.97(1) 3/4 1/4 . 0 0.0068(6) Un 0.0025(7) 0 0 0 Acknowledgment. Support by the Deutsche Forschungsgemeinschaft is grate-fully acknowledged. References 1. Siegrist, T.; Zandbergen, H. W.; Cava, R. J.; Krajewski, J. J.; Peck Jr., W. F.: The crystal structure of superconducting LUNÌ2B2C and the related phase LuNiBC. Nature 367 (1994) 254-256. 2. El Massalami, M.; Baggio-Saitovich, E.; Sulpice, A.: The magnetic prop-erties of HoNiBC: absence of superconductivity and helical ground-state. J. Alloys. Compd. 228(1995)49-53. 3. Geupel, S.; Zahn, G.; Paufler, P.; Graw, G.: Strukturuntersuchungen an Phasen des Systems Ho-Ni-B-C. Z. Kristallogr. Suppl. 17 (2000) 161. 4. Gelato, L. M.; Parthé, E.: STRUCTURE TIDY - a computer program to standardize crystal structure data. J. Appi. Crystallogr. 20 ( 1987) 139-143. 5. Sheldrick, G.: SHELX-97, program for the solution and refinement of crystal structures. University of Gottingen, Germany 1997. 6. Keller, E.: SCHAKAL92, computer program for the graphic representa-tion of molecular and crystallographic models. University of Freiburg, Germany 1992. 

Refinement of the crystal structure of holmium nickel borocarbide, HoNiBC

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Refinement of the crystal structure of holmium nickel borocarbide, HoNiBC

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