On the crystal structure of Sc2MB6 (M = Rh, Ir) compounds

Journal of Alloys and Compounds 396 (2005) 240–242

On the crystal structure of Sc2MB6 (M = Rh, Ir) compounds P.S. Salamakhaa , C. Rizzolib , L.P. Salamakhac , O.L. Sologuba,d,∗ , A. Gonc¸alvesa , M. Almeidaa a

Departamento de Qu`ımica, Instituto Tecnol`ogico e Nuclear, P-2686-953 Sacav`em, Portugal Dipartimento di Chimica GIAF, Universit`a di Parma, Viale delle Scienze, 43100 Parma, Italy Department of Physics of Metals, Faculty of Physics, L’viv National University, Kyryla i Mefodiya str. 6, 79005 L’viv, Ukraine d Research Centre of Low Temperature Studies, L’viv National University, Dragomanova str. 50, 79005 L’viv, Ukraine b

c

Received 18 December 2004; accepted 4 January 2005 Available online 12 February 2005

Abstract Ternary samples Sc2 MB6 and ScMB4 (M = Rh, Ir) were synthesized and studied by X-ray powder and single crystal diffraction. The crystal ˚ b = 11.2105(10) A, ˚ c = 3.4833(3) A, ˚ structure of the ternary boride Sc2 RhB6 , Y2 ReB6 structure type, Pbam space group, a = 8.9144(8) A, ˚ 3 , ρ = 4.917 g cm−3 , µ = 8.216 mm−1 was refined to R = 0.0193, wR2 = 0.0518 from single crystal X-ray diffraction Z = 4, V = 348.10(5) A data (Bruker SMART 100 CCD diffractometer, 573 reflections with I > 2σ(Io ). The lattice parameters for the isotypic Sc2 IrB6 compound are ˚ b = 11.211(19) A, ˚ c = 3.48367(13) A ˚ (X-ray powder diffraction, Image Plate Huber G 670 camera). No evidence of formation a = 8.9149(18) A, of the ternary ScMB4 (M = Rh, Ir) compounds with YReB4 structure type was obtained. © 2005 Elsevier B.V. All rights reserved. Keywords: Crystal structure; X-ray powder diffraction; X-ray single crystal diffraction; Ternary borides

1. Introduction

2. Experimental details

Literature data on Sc(Zr,Hf)-Ir-B illustrated the similarities in component interaction in these systems due to formation of ternary borides with composition 1:3:1, 1:3:2 and 1:3:4 [1–4]. On the other hand, the existence in Sc-transition metal-B systems the borides with Y2 ReB6 and YCrB4 structure types which are typical for ternary Ln-M-B systems [5,6] are the evidence of the tendency of Sc to behave like a heavy lanthanide metal. Up to now, the ternary systems containing scandium, transition metal and boron have been studied insufficiently in the whole concentration regions. Phase diagrams were only constructed for the Sc-(W, Re, Fe, Co, Ni)B systems [5,7]. No information is available on the Sc-Rh-B system, except for the formation of ScRh3 Bx solid solution with AuCu3 structure type [8]. In the present paper we report on the result of our investigations of the Sc2 Rh(Ir)B6 and ScRh(Ir)B4 samples using X-ray powder and single crystal diffraction.

2.1. Sample preparation

∗

Corresponding author. E-mail address: [email protected] (O.L. Sologub).

0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.01.010

The alloys with ScMB4 and Sc2 MB6 (M: Rh, Ir) compositions and a total weight of 0.5 g each, were synthesized by arc melting proper amounts of the constituent elements under high purity argon on a water cooled copper hearth. The melting procedure was repeated three times in order to ensure a better homogeneity. The weight losses were less than 1%. The resulting ingots were then sealed under vacuum into silica tubes and annealed for 20 days at 800 ◦ C, followed by quenching in cold water. 2.2. Characterization by X-ray powder diffraction The as-cast and annealed samples were examined by Xray powder diffraction. Diffracted X-ray intensities were collected from the powdered samples at room temperature with a Image Plate Huber G 670 camera (20 ≤ 2θ ≤ 100◦ , step size 0.005◦ , exposure time 24 h) with monochromic Cu K␣1 ˚ radiation. Phase identification, automatic in(λ = 1.54056 A)

P.S. Salamakha et al. / Journal of Alloys and Compounds 396 (2005) 240–242

241

Table 1 Parameters for the single crystal X-ray data collection and refinement Compound Space group Z Diffractometer Wavelength

Sc2 RhB6 Pbam 4 Bruker SMART 100 CCD ˚ 0.71073 A

˚ Lattice parameters (A) a b c

8.9144(8) 11.2105(10) 3.4833(3)

˚ 3) Cell volume, (A Calculated density, (g cm−3 ) Linear absorption coefficient (mm−1 ) θ range for data collection (◦ ) Index ranges Number of measured reflections Number of unique reflections Number of reflections with I > 2σ(Io ) Number of refined parameters Final R indices (I > 2σ(Io )) R indices (all data) Goodness of fit Highest/lowest residual electron density (e A−3 )

348.10(5) 4.917 8.216 2.92–30.44 −12 ≤ h ≤ 12, −15 ≤k ≤ 15, −4 ≤ l ≤ 4 4647 581 (Rint = 0.0281) 573 56 R1 = 0.0193, wR2 = 0.0518 R1 = 0.0197, wR2 = 0.0519 1.179 1.158/−0.679

dexing and lattice parameter refinements were accomplished using the WinPlotr [9], Powder Cell [10], TREOR [11] and DICVOL [12] programs. 2.3. Single crystal X-ray diffraction A single crystal suitable for the X-ray measurements was isolated from the surface of the annealed at 800 ◦ C Sc2 RhB6 sample, glued on the top of a glass fiber and mounted onto the goniometer head. The X-ray diffraction data were obtained using a Bruker SMART 100 CCD diffractometer (graphite ˚ The data monochromatized Mo K␣ radiation, λ = 0.71073 A). ◦ set was recorded at 20 C in the ␻-scan mode. A total of 4848 frames were collected with a ϕ of 1.5◦ and an exposure time of 30 s. Data reduction was carried out using the SAINT suite of programs [13]. The intensities were corrected for absorption with the assistance of the program SADABS [13]. Further details of single crystal data collection and structural refinement are listed in Table 1.

Fig. 1. Projection of the crystal structure of the Sc2 RhB6 compound on XY plane.

˚ c = 3.4833(3) A ˚ (432 reflections used for b = 11.2105(10) A, the unit cell determination). For the analyses of the X-ray single crystal data and structure refinement, the WinGX 1.64 program package [14] was used. The structure was solved with the aid of the SHELXS-97 [15] program in the space group Pbam [16] using the Patterson method, which resulted in the positions of Sc and Rh atoms. Difference Fourier synthesis enabled us to localize the position of the boron atoms. The structure was refined by a full-matrix least square program using atomic scattering factors provided by the program package SHELXL-97 [15]. The weighting schemes included a term, which accounted for the counting statistics, and the parameter correcting for isotropic secondary extinction was optimized. The anisotropic displacement parameters for all atoms were refined. The atomic coordinates were standardized using the program Structure Tidy [17]. The crystal structure of the Sc2 RhB6 compound is shown in Fig. 1. The final residuals are given in Table 1. The atomic coordinates and isotropic thermal parameters are listed in Table 2. Selected interatomic distances in the structure of Sc2 RhB6 are given in Table 3. Typically for borides with high concentration of boron, the interatomic distances dB B are slightly Table 2 Atomic coordinates and thermal parameters for the Sc2 RhB6 compound Atom

Wyckoff position

x

y

z

Ueq. × 102 ˚ 2) (A

Sc1 Sc2 Rh B1 B2 B3 B4 B5 B6

4(g) 4(g) 4(g) 4(h) 4(h) 4(h) 4(h) 4(h) 4(h)

0.44446(8) 0.81941(8) 0.14063(3) 0.2964(5) 0.4800(5) 0.2920(4) 0.1316(4) 0.1025(5) 0.0535(5)

0.12638(6) 0.08563(6) 0.18023(3) 0.2381(4) 0.2873(4) 0.0781(4) 0.3193(4) 0.4724(4) 0.0639(4)

0 0 0 1/2 1/2 1/2 1/2 1/2 1/2

0.372(16) 0.480(16) 0.614(13) 0.54(7) 0.56(7) 0.59(7) 0.45(8) 0.55(7) 0.55(7)

3. Results and discussion Automatic indexing of the 22 reflections of the Xray powder diffraction pattern (2θ max = 55◦ ) collected from the annealed Sc2 RhB6 sample, using the TREOR [11] and DICVOL [12] programs suggested an orthorhom˚ bic unit-cell with lattice parameters a = 8.9149(18) A, ˚ ˚ b = 11.211(19) A, c = 3.48367(13) A. These values were very ˚ close to those obtained from single crystal: a = 8.9144(8) A,

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P.S. Salamakha et al. / Journal of Alloys and Compounds 396 (2005) 240–242

Table 3 ˚ for the atoms in the Sc2 RhB6 Selected interatomic distances (d, (A)) compound Atom

˚ d (A)

Atom

˚ d (A)

Atom

˚ d (A)

Sc1–2 B4 2 B5 2 B5 2 B3 2 B1 2 B2 Rh Rh

2.488(3) 2.488(3) 2.499(3) 2.504(3) 2.519(3) 2.528(3) 2.7751(8) 2.7852(7)

B1–B2 B4 B3 2 Rh 2 Sc1 2 Sc2

1.728(6) 1.728(6) 1.836(6) 2.320(3) 2.519(3) 2.642(3)

B4–B1 B5 B2 2 Rh 2 Sc1 2 Sc2

1.728(6) 1.736(6) 1.804(6) 2.339(3) 2.488(3) 2.640(3)

Sc2–2 B3 2 B4 2 B1 2 B2 2 B6 2 B5

2.609(3) 2.640(3) 2.642(3) 2.667(3) 2.670(3) 2.682(3)

B2–B1 B6 B4 Rh 2 Sc1 2 Sc2

1.728(6) 1.792(6) 1.804(6) 2.283(3) 2.528(3) 2.667(3)

B5–B4 B3 B5 2 Sc1 2 Sc1 2 Sc2

1.736(6) 1.757(6) 1.931(8) 2.488(3) 2.499(3) 2.682(3)

Rh–2 B2 2 B3 2 B6 2 B1 2 B4 Sc1 Sc1

2.283(3) 2.308(3) 2.310(3) 2.320(3) 2.339(3) 2.7751(8) 2.7852(7)

B3–B5 B6 B1 2 Rh 2 Sc1 2 Sc2

1.757(6) 1.776(6) 1.836(6) 2.308(3) 2.504(3) 2.609(3)

B6–B6 B3 B2 2 Rh 2 Sc2 2 Sc2

1.721(8) 1.776(6) 1.792(6) 2.310(3) 2.670(3) 2.729(3)

metal layer arrangements are observed also in the orthorhombic YCrB4 structure (space group Pbam) where boron atoms form 5- and 7-membered rings. Based on this boron-ring formation tendency, the three structure types YCrB4 , Y2 ReB6 , and AlB2 are closely related (Fig. 2). In hexagonal AlB2 structure boron atoms form 6-membered rings. The lattice parameters for the isotypic Sc2 IrB6 compound ˚ b = 11.211(19) A, ˚ c = 3.48367(13) A. ˚ are a = 8.9149(18) A, X-ray phase analyses of the ternary samples ScRhB4 and ScIrB4 showed that these samples contain two phases: RhB, Sc2 RhB6 and IrB1.35 , Sc2 IrB6 , respectively. No evidence of the formation of ternary ScMB4 (M = Rh, Ir) compounds with YReB4 structure type was observed. No visible discrepancies were found in the X-ray powder diffraction patterns of both “as cast” and annealed Sc2 MB6 and ScMB4 (M = Rh, Ir) alloys as well.

Acknowledgement O.S. is grateful to Austrian FWF for the Lise Meitner Grant (Project M635).

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[9]

[10] [11]

Fig. 2. Interconnection between AlB2 ,Y2 ReB6 and YReB4 structure types.

shorter than the sum of atomic radii of the components [18]. The crystal structure of the compound belongs to the Y2 ReB6 structure type. The Y2 ReB6 structure type is characterized by a two-dimensional boron network (composed of 5-, 6-, and 7-membered rings) sandwiched between metal layers. These boron atoms reside in the interstitial sites of trigonal prisms formed by Sc and Rh atoms. Similar boron and

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[18]

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