New structure type of ternary intermetallic borides CePt2B

Journal of Alloys and Compounds 307 (2000) 40–44

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New structure type of ternary intermetallic borides CePt 2 B ¨ c , M. Potel c , M. Almeida b , C. Godart a , * O. Sologub a , P. Salamakha a,b , H. Noel a

CNRS-UPR209, Groupe des Laboratoires de Thiais, 2 – 8 rue Henri Dunant, 94320 Thiais, France b ` ` ` , Portugal , Instituto Tecnologico e Nuclear, P-2686 -953 Sacavem Departamento de Quımica c ´ , Universite´ de Rennes 1, CNRS-UMR 6511, Laboratoire de Chimie du Solide et Inorganique Moleculaire ´ ´ Avenue du General Leclerc — Campus de Beaulieu, 35042 Rennes, France Received 16 February 2000; accepted 3 March 2000

Abstract ˚ c 5 7.8860(9) A, ˚ Z 5 3, The crystal structure of a new ternary boride CePt 2 B, space group P62 22 (N180), a 5 5.4898(5) A, ˚ r 5 13.097 g cm 23 , m 5 117.63 mm 21 was refined to R 5 0.0384, wR2 5 0.0926 from single-crystal X-ray diffraction V 5 205.83(1) A, data. Rietveld refinement of the X-ray powder data collected from the CePt 2 B sample annealed at 1070 K confirmed the structural model ˚ c 5 7.8830(2) A, ˚ R F 5 0.0497, R B 5 0.0668 from X-ray powder diffraction). This obtained by single-crystal refinement (a 5 5.4811(3) A, is the first representative of a new structure type of intermetallic compound. Preliminary magnetic measurements show the sample to be nonsuperconducting at 2 K with possible magnetic ordering below 7 K.  2000 Elsevier Science S.A. All rights reserved. Keywords: Rare earth compounds; Crystal structure; X-ray diffraction; Magnetization

1. Introduction Only few data are reported on ternary intermetallic cerium-based platinum borides. Recently, we presented the crystal structure for the as-cast CePt 2 B 22x ternary com¨ pound refined from X-ray single-crystal data [1]. Sullow et al. reported the X-ray powder diffraction data and magnetism of the compound CePt 4 B [2]. In this paper we present the results of crystal structure investigations for the compound CePt 2 B, which is the first representative of a new structure type of intermetallic compound, as well as of preliminary magnetic measurements.

2. Experimental The ternary sample Ce 25 Pt 50 B 25 was synthesized by arc melting of the constituent elements under a high-purity argon atmosphere on a water-cooled copper hearth. In order to ensure homogeneity, the arc-melted button was turned over and remelted several times The sample was

wrapped in tantalum foil, sealed in an evacuated silica tube and annealed at 1070 K for 20 days and then cooled to room temperature by submerging the tube in water. For X-ray single-crystal data collection, a single crystal was glued on top of a glass fiber and mounted on the goniometer head. The X-ray diffraction data set was collected on a Nonius Kappa CCD diffractometer. The orientation matrix and the unit cell parameters were determined from the first 10 measured frames of the data using the program Denzo [3]. Further details are listed in Table 1. For Rietveld refinement, the X-ray powder intensities were recorded on a Bruker D8 diffractometer from a flat rotating sample using Cu Ka radiation in the range 158 # 2u # 1208 with a step width of 0.028 and a constant counting time of 12 s per step. Magnetic measurements were performed using a SQUID magnetometer.

3. Results and discussion

3.1. X-ray single-crystal refinement *Corresponding author. Tel.: 133-1-4978-1247; fax: 133-1-49781203. E-mail address: [email protected] (C. Godart)

The space group extinctions led to the possible space groups P62 , P64 , P64 22 and P62 22, of which the group P62 22 was found to be correct during structure refinement.

0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )00825-2

O. Sologub et al. / Journal of Alloys and Compounds 307 (2000) 40 – 44

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Table 1 Parameters for the single-crystal X-ray data collections Compound Space group

CePt 2 B P62 22

Lattice parameters ( A˚ ) a c a, c (from Rietveld refinement)

5.4898(5) 7.8860(9) 5.4811(3), 7.8830(2)

˚ 3) Cell volume (A Calculated density (g cm 23 ) Linear absorption coefficient (mm 21 ) 2umax Number of measured reflections Number of unique reflections Number of reflections with I0 . 4s (I0 ) Number of refined parameters Reciprocal space R, wR2 Goodness of fit Structure solution program

205.83(1) 13.097 117.63 80.32 2315 438 405 13 0 # h # 9, 2 9 # k # 8, 2 14 # l # 14 0.0384, 0.0926 1.178 SHELXL-97

The structure was solved with the aid of SHELXS-86 [4] using a Patterson function, which resulted in the positions of the Ce and Pt atoms. Difference Fourier syntheses enabled us to localize the position of the boron atom. The structure was refined by a full-matrix least-squares program using atomic scattering factors provided by the program package SHELXL-97 [5]. The weighting schemes included a term which accounted for the counting statistics, and the parameter correcting for isotropic secondary extinction was optimized. For all atoms the anisotropic displacement parameters were refined. The final residuals are presented in Table 1. The atomic coordinates, which

correspond to their standardized form according to STIDY [6], thermal parameters and interatomic distances are shown in Tables 1–3.

3.2. Rietveld refinement The structure of CePt 2 B was also refined from X-ray powder data collected over the diffraction range 15–1208 from the alloy, annealed at 1070 K, of the proper composition using the atomic positions obtained from singlecrystal refinement as the initial positions. The Rietveld refinement was performed with the aid of the

Table 2 Atomic coordinates and thermal parameters for the compound CePt 2 B a Atom

Wyckoff position

x

y

z

U11 3 10 2 ˚ 2) (A

U22 3 10 2 ˚ 2) (A

U33 3 10 2 ˚ 2) (A

U12 3 10 2 ˚ 2) (A

U13 3 10 2 ˚ 2) (A

Ueq. 3 10 2 ˚ 2) (A

Ce Pt B

3c 6i 3d

0.5 0.15128(6) 0.5

0 0.30256(12) 0

0 0 0.5

1.04(3) 1.10() 0.88(46)

0.95(3) 1.18(2) 1.24(72)

0.86(4) 0.94(2) 1.58(88)

0.0 0.10(1) 0.0

0.48(2) 0.59(1) 0.62(36)

0.96(2) 1.06(2) 1.19(35)

a

U23 5 0.

Table 3 ˚ and coordination numbers of atoms (CN) for the compound CePt 2 B Selected interatomic distances, d (A), Atom

˚ d (A)

Ce–4B –4Pt –4Pt –2Pt

3.0433(2) 3.0990(4) 3.1104(3) 3.3158(6)

CN

14

Atom

˚ d (A)

Pt–2B –Pt –2Pt –2Ce 2Ce Ce

2.1195(3) 2.8769(11) 2.9965(4) 3.0990(4) 3.1104(3) 3.3158(6)

CN

10

Atom

˚ d (A)

CN

B–4Pt –4Ce

2.1195(3) 3.0433(2)

8

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O. Sologub et al. / Journal of Alloys and Compounds 307 (2000) 40 – 44

a pseudo-Voigt profile shape function were used. The background was refined with a polynomial function. As a result, 20 variables were refined to R F 5 0.0497, R B 5 0.0668 and the following refined atomic parameters were obtained: Ce in (3c), x 5 0.5, y 5 0, z 5 0, B 5 2.2(9) 3 ˚ 2 ; Pt in (6i), x 5 0.1505(2), y 5 0.3010(2), z 5 0, 10 22 A ˚ 2 ; B in (3d), x 5 0.5, y 5 0, z 5 0.5, B 5 2.2(9) 3 10 22 A 22 ˚ 2 B 5 3.5(9) 3 10 A , which is in good agreement with the results obtained on the single crystal.

3.3. Atomic coordination

Fig. 1. Projection of the CePt 2 B unit cell on the XY plane (Ce atoms are shown as large white circles; Pt — light gray circles of smaller size; B — small dark gray circles).

FULLPROF98 programme [7]. An experimentally determined Ka 1 / Ka 2 intensity ratio of 0.5, a factor cos u 5 0.7998 for the monochromator polarization correction and

The projection of the CePt 2 B unit cell on the XY plane is presented in Fig. 1. Coordination polyhedra for Ce, B and Pt are shown in Fig. 2. Each cerium atom is surrounded by a polyhedron of 14 atoms, i.e. 10 atoms of Pt and four atoms of B (Fig. 2a). Pt is essentially bound to two B atoms and forms a polyhedron of 10 atoms (Fig. 2b). The first coordination sphere for B is the tetrahedron formed by ˚ This tetrafour platinum atoms (d B – Pt 5 2.1195(3) A).

Fig. 2. Coordination polyhedra for the atoms in the CePt 2 B structure (Ce atoms are shown as large white circles; Pt — light gray circles of smaller size; B — small dark gray circles).

O. Sologub et al. / Journal of Alloys and Compounds 307 (2000) 40 – 44

hedron is situated inside a tetrahedron consisting of cerium ˚ Two tetrahedra are arranged atoms (d Ce – B 5 3.0433(2) A). in such a way so as to form a tetragonal antiprism Ce 4 Pt 4 (Fig. 2c). No boron–boron contact was observed. The interatomic distances in CePt 2 B are equal to the ˚ sum of the radii of the elements (d B – Pt 5 2.1195(3) A, ˚ d Ce – Pt 5 3.0990(4)–3.1104(3) A) or slightly larger ˚ d Pt – Pt 5 2.8769(11)–2.9965(4) A). ˚ (d Ce – B 5 3.0433(2) A, These bond lengths correspond to those typical for ternary rare earth borides [8]. The relations between the cell parameters of the three ternary compounds observed in the Ce–Pt–B system (CePt 4 B (1), CePt 2 B 22x (2) and CePt 2 B (3)) can be expressed as a 1 ¯ a 2 œ3 ¯ a 3 , b 1 ¯ b 2 ¯ b 3 , c 1 ¯ c 2 ¯ c 3 . Atoms in all three structures are situated in layers. Contrary to the CePt 2 B 22x structure, where the layers are formed by one type of atom, in the CePt 2 B structure the Ce and Pt atoms are situated in three layers located parallel to the XY plane and constitute four corner Ce 2 Pt 2 nets which are arranged in a way to form tetragonal antiprisms and tetragonal pyramids. The boron atoms are accommodated inside a part of the tetragonal antiprism; other tetragonal antiprisms and tetragonal pyramids remain unfilled. The arrangement of the fragments in the CePt 2 B structure is presented in Fig. 3. The structure belongs to class N9 of intermetallic compounds [9] with a tetragonal antiprism as a coordination polyhedron for the smallest atom.

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Fig. 4. Zero field cooled susceptibility in the low-temperature range in CePt 2 B and CePt 4 B.

field cooled samples the magnetization was measured versus temperature in the range 2–30 K under a field of 10 G. Both samples are nonsuperconducting at 2 K (see Fig. 4). CePt 4 B shows magnetic ordering around 2.5 K in agreement with Ref. [2]. CePt 2 B seems to order below 7 K. Magnetic susceptibility measurements versus temperature under a higher field and magnetization versus field measurements are planned.

Acknowledgements

3.4. Magnetic measurements Preliminary magnetic measurements were performed on CePt 2 B and on CePt 4 B as a reference material. On zero

The work of O.S. was supported by a CNRS–NATO grant. P.S. wishes to thank NATO for a fellowship in Portugal.

Fig. 3. Arrangement of filled and unfilled tetragonal antiprisms and unfilled tetragonal pyramids in the CePt 2 B structure.

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O. Sologub et al. / Journal of Alloys and Compounds 307 (2000) 40 – 44

References [1] P. Salamakha, O. Sologub, C. Godart, J. Alloys Comp. 299 (2000) 189. ¨ [2] S. Sullow, B. Ludoph, G.J. Nieuwenhuys, A.A. Menovsky, J.A. Mydosh, Physica B 223 / 224 (1996) 347. [3] Nonius Kappa CCD Program Package, Nonius, Delft, The Netherlands, 1998. [4] G.M. Sheldrick, SHELXS-86, program for crystal structure de¨ termination, Univ. Gottingen, Germany, 1986. [5] G.M. Sheldrick, SHELXS-97, program for crystal structure refine¨ ment, Univ. Gottingen, Germany, 1997.

´ L. Gelato, B. Chabot, M. Penzo, K. Cenzual, R. [6] E. Parthe, Gladyshevskii, TYPIX. Standardized data and crystal chemical characterization of inorganic structure types, in: 8th Edition, Gmelin Handbook of Inorganic and Organometallic Chemistry, Vol. 1, Springer, Berlin, 1993. [7] J. Rodriguez-Carvajal, FULLPROF, Laboratoire Leon Brillouin (CEA-CNRS), 1998. [8] Yu.B. Kuzma (Ed.), Crystal Chemistry of Borides, Vyshcha Shkola Press, Lvov, 1983, p. 1. [9] P.I. Krypyakevych (Ed.), Structure Types of Intermetallic Compounds, Nauka Press, Moscow, 1977, p. 1.