Crystal structure and magnetism of YPt2B

Journal of Alloys and Compounds 438 (2007) 62–65

Crystal structure and magnetism of YPt2B M. Dias a , O.L. Sologub a,b,∗ , L.C.J. Pereira a , A.P. Gonc¸alves a a

b

Departamento de Qu´ımica, Instituto Tecnol´ogico e Nuclear/CFMC-UL, P-2686-953 Sacav´em, Portugal Research Centre of Low Temperature Studies, L’viv National University, Dragomanova Str. 50, 79005 L’viv, Ukraine Received 5 June 2006; accepted 16 August 2006 Available online 2 October 2006

Abstract The new ternary YPt2 B compound was synthesized by arc-melting, followed by annealing at 1370 K. The structure was refined down to R = 0.0408, wR2 = 0.0692 from single crystal X-ray diffraction data (CAD-4 diffractometer, Mo K␣, 269 reflections with I > 2σ(Io )) and to RF = 0.0539, RB = 0.0970 from X-ray powder diffraction for 77 reflections. YPt2 B was found to crystallize with a CePt2 B structure type, P62 22 space group (no. ˚ c = 7.8875(2) A, ˚ Z = 3, V = 191.715(8) A ˚ 3 , ρ = 12.752 g cm−3 ). Magnetization and AC-susceptibility measurements indicate 180) (a = 5.2977(1) A, that there is no magnetic or superconducting transition in YPt2 B down to 2 K. © 2006 Elsevier B.V. All rights reserved. Keywords: Rare earth intermetallics; Crystal structure of intermetallics; Diffraction

1. Introduction The discovery of superconductivity in quaternary Y–Pd(Pt)– B–C samples [1] has attracted the interest of many researchers for these systems. However, the ternary Y–Pd(Pt)–B phase diagrams were not explored, and the crystal structures of some binary compounds were not yet determined [2]. In the frame of our general work on the study of ternary systems with f-elements and boron, a systematic investigation of the Y–Pd(Pt)–B phase diagrams was recently undertaken. Up to the beginning of the present work no systematical investigation was reported for these two phase diagrams. Not so far we have presented the structure and magnetism of the new Y2 Pd14 B5 compound [3]. In related systems, a new structure type of rare earth plat˚ inum borides CePt2 B (space group P62 22, a = 5.4898(5) A, ˚ c = 7.8860(9) A, Z = 3) was also recently observed from X-ray single crystal and powder diffraction of an annealed at 1070 K sample [4]. Other RPt2 B (R = La, Pr, Nd) compounds were later found to be isotypic with CePt2 B [5]. No data on formation of compounds with this structure in heavy rare earth platinum boron systems were reported up to now.

∗

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

0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.08.044

In this paper we present detailed results on the crystal structure and magnetic investigations of a new ternary YPt2 B compound. 2. Experimental details Sample with YPt2 B composition and a total weight of 0.5 g, was synthesized by arc-melting the 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 found to be less than 1%. Part of the material was sealed under vacuum into silica tube and annealed for 14 days at 1370 K, followed by quenching in cold water. The as-cast and annealed samples were examined by X-ray powder diffraction at room temperature using a Philips X’Pert diffractometer (Cu K␣ radiation, 2θ range 15–115◦ , step width of 0.02◦ , a constant counting time of 10 s/step). Phase identification, automatic indexing and lattice parameters refinement were accomplished using the WinPLOTR [6], PowderCell [7], TREOR [8] and DICVOL [9] programs. A single crystal suitable for the X-ray measurements was isolated from the annealed at 1370 K sample, glued on the top of a glass fiber and mounted onto a goniometer head. X-ray single crystal diffraction data were obtained using a four circle diffractometer Enraf-Nonius CAD-4 with graphite monochromatized Mo ˚ The data set was recorded at room temperature K␣ radiation (λ = 0.71073 A). in a ω–2θ scan mode. The intensities of the 1317 measured reflections (with 10◦ ≤ 2θ ≤ 70◦ ) were corrected for absorption by an empirical method based on Ψ scans [10], and for Lorentz and polarization effects. Further details of single crystal data collection and structural refinement are listed in Table 1. Magnetization and AC-susceptibility measurements were carried out on polycrystalline samples in the 1.7–300 K temperature range, using a multipurpose characterization system, MagLab 2000 (Oxford Instruments). The

M. Dias et al. / Journal of Alloys and Compounds 438 (2007) 62–65 Table 1 Crystal data and structure refinement for YPt2 B single crystal Space group ˚ Lattice parameters (A) a c ˚ 3) Cell volume (A Formula per unit cell Calculated density (Mg/m3 ) Absorption coefficient (mm−1 ) Data collection Theta range for data collection (◦ ) Data set Number of measured reflection Number of unique reflections Number of reflections with I > 2σ(I0 ) Number of refined parameters R1 , wR2 (I > 2σ(I0 ) R1 , wR2 (all data) Goodness of fit on F2 Extinction coefficient Absolute structure parameter Highest/lowest peaks of electron ˚ 3) density (e/A Refinement method, software

P62 22 (no. 180) 5.296(2) 7.879(5) 191.38(11) 3 12.752 131.529 ˚ CAD-4, Mo K␣, 0.71073 A 4.44–34.87 −8 ≤ h ≤ 8, −8 ≤ k ≤ 8, −12 ≤ l2 ≤ 12 3362 289 269 14 0.0408, 0.0692 0.0460, 0.0711 1.038 0.0030(4) 0.08(28) 3.08/−3.508 Full-matrix least-squares on F2 , WinGX 1.70

magnetization was measured by an extraction technique using fields up to 12 T. Magnetization versus temperature measurements were performed after zero-field cooling (ZFC) and field cooling (FC). Both components of the ACsusceptibility, χ = χ − iχ , were measured in the frequency-range 95–9995 Hz and with an excitation field of 1 Oe.

3. Results and discussion The experimental X-ray powder diffraction patterns for both as-cast and annealed alloys showed obvious similarity with the one simulated for CePt2 B (program PowderCell [7]). Indexing the observed reflections with TREOR 90 [8] and DICVOL [9] programs unambiguously suggested a hexagonal unit cell ˚ c = 7.879(5) A. ˚ The structural refinement with a = 5.296(2) A, of the YPt2 B compound from powder data was performed using the program FULLPROF98 [6]. An experimentally determined K␣2 /K␣1 ratio of 0.5, a factor cos θ = 0.7998 for the monochromator polarization correction and a Pseudo-Voigt profile shape function were used. The background was refined with a polynomial function. Further details on data collection and Rietveld refinement are given in Table 2. The observed, calculated and difference powder patterns for the YPt2 B are shown in Fig. 1. The lattice parameters and the hexagonal symmetry of YPt2 B were confirmed by the automatic indexing and least-square refinement of the 25 well-centered and strong reflections measured from the single crystal in the various regions of the reciprocal space. For the absorption correction and analyses of the X-ray single crystal data and structure refinement was used the WinGX 1.70 program package [10]. The systematic extinctions led to the possible P62 , P64 , P62 22 and P64 22 space groups [11], of which the group P62 22 was found to be correct during

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Table 2 Parameters for X-ray powder diffraction data recording and structural Rietveld refinement of YPt2 B Compound Space group

YPt2 B P62 22 (no. 180)

˚ Cell parameters (A) a c

5.2977(1) 7.8875(2)

˚ 3) Volume (A Radiation Data range, 2θ (◦ ) Step size (◦ ) Number of reflections η (Pseudo-Voigt)

191.715(8) Cu K␣ 15–115 0.02 154/2 1.13(3)

Halfwidth parameters U V W

0.070(4) −0.060(5) 0.0286(1)

˚ 2) Atomic coordinates and isotropic thermal parameters (A Y in 3(c) 1/2 0 0 4.085(3) Pt in 6(i) 0.1528(1) 0.3054(1) 0 3.452(7) B in 3(d) 1/2 0 1/2 3.108(7) Rietveld reliability factors RP RWP χ2 RB RF

0.106 0.137 0.0425 0.0970 0.0539

the structure refinement. The structure was solved with the aid of SHELXS-97 [12] using a Patterson function, which resulted in the positions of the Y and Pt atoms. Difference Fourier syntheses 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 [13]. 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 and are given in Table 3, together with the standardized atomic coordinates (STIDY [14]). The final residuals are given in Table 1. Selected interatomic distances and coordination

Fig. 1. Observed, calculated and differential X-ray powder diffraction profiles for the YPt2 B compound.

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M. Dias et al. / Journal of Alloys and Compounds 438 (2007) 62–65

Table 3 ˚ 2 ) for the YPt2 B compound from X-ray single crystal diffraction Atomic coordinates and thermal parameters (×103 , A Atom

Wyckoff position

x

y

z

Ueq.

U11

U33

U12

Y Pt B

3(c) 6(i) 3(d)

1/2 0.1513(1) 1/2

0 0.3026(1) 0

0 0 1/2

1.94(4) 1.42(2) 2.79(35)

1.26(6) 1.01(2) 1.40(44)

3.18(3) 2.43(1) 2.38(20)

0.95(3) 0.80(1) 0.72(22)

U13 = U23 = 0, U11 = U22. Table 4 ˚ for the atoms in the YPt2 B compound Selected interatomic distances (d, A) Atom

˚ d (A)

CN

Atom

˚ d (A)

CN

Atom

˚ d (A)

CN

Y-4 B Y-4 Pt Y-4 Pt Y-2 Pt

2.9557(9) 2.9898(12) 3.0773(7) 3.1984(14)

14

Pt-2 B Pt-Pt Pt-2 Pt Pt-2 Y Pt-2 Y Pt-1 Y

2.0728(6) 2.7762(17) 2.9706(7) 2.9898(12) 3.0773(7) 3.1984(14)

10

B-4 Pt B-4 Y

2.0728(6) 2.9557(9)

8

numbers for atoms are presented in Table 4. The projection of the YPt2 B unit cell on the XY plane is presented in Fig. 2. The structure belongs to the CePt2 B structure type [4]. The temperature dependence of the magnetization, measured under 0.025 and 5 T magnetic fields both upon warming after ZFC and FC procedures, are shown in Fig. 3. The M(T) dependence shows no signs of any magnetic transition between 300 and 2 K. An overlap between the ZFC and FC curves can also be observed, pointing to a negligible coercive field in this sample. The dependence of magnetization on applied field, M(B) up to 1.5 T, for a polycrystalline sample at different temperatures is shown in Fig. 4. The curves show a behavior characteristic of a ferromagnetic material, with the rapid increase of the magnetization at low fields. This region is followed by a zone where the magnetization grows slowly, that is probably related with the polycrystalline character of the sample and with its magnetic anisotropy. The spontaneous magnetization, obtained by the linear extrapolation of M(B → 0) at each temperature, as a function of the temperature is shown in Fig. 5. The spontaneous magnetization is very small in all the temperature range, taking the

Fig. 2. Projection of the crystal structure of the YPt2 B unit cell on XY plane. Large dark gray circles stay for yttrium atoms, smaller light circles for platinum atoms and smallest dark circles for boron atoms.

Fig. 3. Temperature dependence of the magnetization M(T) for the YPt2 B compound.

Fig. 4. Magnetic field dependence of the magnetization for the YPt2 B compound.

M. Dias et al. / Journal of Alloys and Compounds 438 (2007) 62–65

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shifts can be detected even when the absolute moment is large. Both χ (T) and χ (T) taken at 995 Hz are shown in Fig. 6. No magnetic or superconducting transitions can be seen between 2 and 300 K, confirming the magnetization measurements, which indicate that YPt2 B is not ordered at low temperatures. 4. Conclusions

Fig. 5. Temperature dependence of the spontaneous magnetization.

The crystal structure of the new ternary boride YPt2 B was determined and refined from powder and single crystal X-ray data. This compound crystallizes in the CePt2 B ˚ structure type, P62 22 space group (no. 180) (a = 5.2977(1) A, 3 −3 ˚ ˚ c = 7.8875(2) A, Z = 3, V = 191.715(8) A , ρ = 12.752 g cm , μ = 131.529 mm−1 ). Magnetization and AC-susceptibility measurements indicate that there is no superconducting or magnetic transition in the YPt2 B compound down to 2 K. Acknowledgements The work of O.S. at the Institute of Nuclear Technology, Sacav´em, Portugal, was supported by the FCT grant (project SFRH/BPD/18810/2004). This work was partially supported by POCTI, under contract no. QUI/46066/2002. References

Fig. 6. Temperature dependence of the real, χ (T), and imaginary, χ (T), components of the AC-susceptibility for the YPt2 B compound taken at 995 Hz with an AC amplitude of 1 Oe.

0.0085 ␮B /fu value at 2 K. The low magnetic moment value and the non-magnetic character of all the constituent elements points to the presence of a minor amount of a ferromagnetic impurity, and the inexistence of any magnetic transition between 300 and 2 K indicates a Curie temperature above the room temperature. The presence of this ferromagnetic impurity in our sample does not allow the clear determination of the magnetic state of YPt2 B at low temperatures from the magnetization measurements. AC-susceptibility is a powerful tool to examine the existence of magnetic transitions. Both real (χ ) and imaginary (χ ) components are very sensitive to phase changes and are often used to measure transition temperatures. Besides, since the AC measurement is sensitive to the slope of the magnetization curves as a function of field and not to its absolute value, small magnetic

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