RARE METALS, Vol. 27, No. 6, Dec 2008, p. 623

Corresponding author: HE Wei E-mail: elsawh@126.com

Crystal structure and Rietveld refinement of the new ternary compound Al14Nd5Si HE Wei, ZHANG Jiliang, ZENG Lingmin, YU Meihua, and XIA Xiuwen Key Laboratory of Nonferrous Metal Materials and New Processing Technology of the Ministry of Education of China, Guangxi University, Nanning 530004, China Institute of Materials Science, College of Physics Science and Technology, Guangxi University, Nanning 530004, China

Received 18 November 2007; received in revised form 29 January 2008; accepted 21 February 2008

Abstract

The new ternary compound Al14Nd5Si has been studied by means of the X-ray powder diffraction technique and the Rietveld method. The ternary compound Al14Nd5Si has a hexagonal Ni3Sn-type structure with space group P63/mmc (No.194), the lattice parameters are a = 0.64470 (2) nm and c = 0.45926 (1) nm. The Smith and Snyder figure of merit for the index, FN, is F30 = 97.8 (30). The X-ray diffraction data indicated that the crystal structure of the compound Al14Nd5Si has been successfully refined by the Rietveld method. The R-factors of Riet-veld refinement are Rp = 0.088 and Rwp = 0.120, respectively. The thermal dependence of magnetization for the compound was measured by a vibrating sample magnetometer. The experimentally determined magnetic effective paramagnetic moment is eff = 3.60 B per Nd atom. The paramagnetic Currie temperature p = 33.7 K was also obtained from the Currie-Weiss law.

Keywords: crystallography; magnetic properties; Reitveld method; rare earth compound

1. Introduction It is well known that rare earth (RE) alloys are paid in-

creasing attention to because of their excellent properties. It has been reported that there are four ternary compounds in the Nd-Al-Si system, which are Al5Nd4Si3, Al58Nd33Si9 [1], RAl0.5 xSi0.5+x, and RAlSi2 [2]. Long et al. have also studied the phase relations in the ternary Nd-Al-Si system and have reported that there are three ternary compounds existing at 500 C, which are NdAl1.25Si0.75, NdAl1.75Si0.25, and NdAl2Si2 [3-4].

In their investigation, the compound NdAl1.5Si0.5 was not found. However, in the authors’ recent studies on the RE-Al-Si system, new ternary compounds Al14RE5Si (RE = Nd, Gd, Dy) were found [5-6]. In this work, the new ternary compound, Al14Nd5Si, was synthesized and the crystal structure was refined by means of the Rietveld method from X-ray powder diffraction data. The thermal dependence of the magnetization for the compound was measured by a vi-brating sample magnetometer.

2. Experimental The alloy Al14Nd5Si was prepared by arc melting under

pure argon from initial materials of at least 99.9 wt.% purity. The sample was turned over and remelted three times to en-sure good homogeneity. The loss of the alloy was less than 0.5%. No chemical analysis was needed. The alloy button was enclosed in an evacuated quartz tube and annealed at 900 C for 30 d and then quenched into liquid nitrogen.

The alloy was brittle. The ingot was ground in an agate mortar and pestle to a particle size of less than 10 m. The powder diffraction data were collected with a Rigaku D/max 2500V diffractometer using Cu K radiation ( = 0.154060 nm). The scan range was 15 to 110 (2 ) with a step size of 0.02 and a count time of 2 s per step. The X-ray powder diffraction data indicated that the crystal structural refine-ment of the compound Al14Nd5Si was performed by the Rietveld method. The magnetization measurement for the sample was implemented by a vibration sample magne-tometer (VSM, Lakeshore 7410).

3. Results and discussion

All X-ray diffraction patterns were indexed successfully on the basis of a hexagonal structure by using Jade5.0 pro-gram [7] with the lattice parameters a = 0.64416 (2) nm and

624 RARE METALS, Vol. 27, No. 6, Dec 2008

c = 0.45894 (2) nm. The Smith and Snyder figure of merit [8] for the index, FN, is F30 = 97.8 (30). The reflection condi-tions yield three possible space groups: P63/mmc (No. 194),

62P c (No. 190), and P63mc (No. 186), for the structure of the compound Al14Nd5Si. The composition of the samples, intensities of reflections and calculated lattice parameters proved that the compound Al14Nd5Si is isostructural with Al14Y5Si (with hexagonal Ni3Sn-type structure, space group P63/mmc, No. 194) [1]. Hence the space group P63/mmc, No. 194 was chosen for the hexagonal structure of Al14Nd5Si.

The Rietveld’s method was employed to analyze the X-ray powder diffraction patterns of the ternary compound Al14Nd5Si. Structural refinement was performed using the DBWS9411 program [9]. The pseudo-Voigt function was used for the simulation of the peak shapes. The DMPLOT plot view program [10] was used to follow the refinement results. The lattice parameters obtained from the Jade5.0 program were taken as the starting values to refine the structural parameters of Al14Nd5Si. The atomic position pa-rameters of Ni3Sn [1] were chosen as the initial atomic posi-tions for structural refinement. A total of 21 parameters, in-cluding the lattice constants, full width at half maximum (FWHM), preferred orientation, atomic parameters, and thermal parameters were refined. The crystal data and Riet-veld structural refinement result are presented in Table 1. The reliability factors, pattern R-factor, Rp, and the weighted pattern R-factor, Rwp, are Rp = 0.088 and Rwp = 0.120, re-spectively. The observed, calculated data and differences of the powder diffraction patterns of Al14Nd5Si are shown in Fig. 1. The present structural refinement supports that, in the hexagonal structure of Al14Nd5Si, the Nd atoms are in the 2c positions, while the Al and Si atoms occupy the 6h positions. The crystal structure of Al14Nd5Si is given in Fig. 2. Fig. 2(a) shows of the crystal structure of Al14Nd5Si. The environ-ments of atoms Nd at 2c positions and atoms M (M = Al, Si) at 6h positions are presented in Figs. 2(b) and 2(c). Table 2 gives the atomic parameters of Al14Nd5Si and the fractional coordinates for atoms Nd at 2c positions and atoms M at 6h positions.

Table 1. Crystal data and Rietveld refinement result of Al14Nd5Si

Space group P63/mmc (No. 194) Cell Parameters / nm a = 0. 64470 (2), c = 0.45926 (1)

Volume of a unit cell / nm3 0.16531 Calculated density / (g cm 3) 4.526

Number of refined parameters 21 FN F30 = 97.8 (30)

Reliability factors (R-factor) Rp = 0.088, Rwp = 0.120

The temperature dependence of the magnetization for the Al14Nd5Si compound was measured in a temperature range from 85 K to 250 K under an applied field of 0.5 T. The re-ciprocal of the molar magnetic susceptibilities versus tem-perature (1/xM-T curve) for the Al14Nd5Si compound is shown in Fig. 3. In the temperature range of 85 to 250 K, the linearity of the curve indicates clearly that the paramagnet-ism of the Al14Nd5Si compound obeys the Curie-Weiss law. The experimentally determined magnetic effective para-magnetic moment is 3.60 B per Nd atom. The magnetic moment agrees well with the theoretical value calculated from a tripositive ground state of Nd, that is, the free ions Nd+3 (3.62 B). The paramagnetic Currie temperature p =

33.7 K was also obtained from the Currie-Weiss law. The negative value of p may suggest some antiferromagnetic correlations at low temperature.

Fig. 1. X-ray powder diffraction patterns of Al14Dy5Si.

4. Conclusion

A new ternary compound Al14Nd5Si crystallizes in a hex-agonal Ni3Sn-type structure with space group P63/mmc (No. 194), the lattice parameters a = 0. 64470 (2) nm and c = 0.45926 (1) nm. The Smith and Snyder figure of merit for the index, FN, is F30 = 97.8(30). The crystal structure of Al14Nd5Si has been successfully refined by the Rietveld method. The R-factors of Rietveld refinement are Rp = 0.088 and Rwp = 0.120, respectively. In the hexagonal structure of Al14Nd5Si, the Nd atoms are in the 2c positions, while the Al and Si atoms occupy the 6h positions. The temperature de-pendence of the magnetization curve for the Al14Nd5Si compound was measured in a temperature range from 85 K to 250 K under an applied field of 0.5 T. The experimentally determined magnetic effective paramagnetic moment is 3.60

B per Nd atom. The paramagnetic Currie temperature p = 33.7 K has also been obtained from the Currie-Weiss law.

He W. et al., Crystal structure and Rietveld refinement of the new ternary compound Al14Nd5Si 625

Fig. 2. Crystal structure of Al14Nd5Si: (a) crystal structure of Al14Nd5Si; (b) environments of Nd atoms at 2c positions; (c) environ-ments of M atoms (M = Al, Si) at 6h positions.

Table 2. Atomic parameters of Al14Nd5Si and fractional coordinates of each atom

Fractional Coordinates Atom Position

x y z Occupancy

1/3 2/3 1/4 1 Gd 2c

1/3 2/3 3/4 1 0. 8564 (4) 0. 7128 (4) 1/4 (0.933Al+0.067Si) 0.2872 (4) 0.1436 (4) 1/4 (0.933Al+0.067Si) 0.8564 (4) 0.1436 (4) 1/4 (0.933Al+0.067Si) 0.1436 (4) 0.2872 (4) 3/4 (0.933Al+0.067Si) 0.7128 (4) 0.8564 (4) 3/4 (0.933Al+0.067Si)

M = Al, Si 6h

0.1436 (4) 0.8564 (4) 3/4 (0.933Al+0.067Si)

Fig. 3. Reciprocal of molar magnetic susceptibility xM versus temperature for Al14Nd5Si. The solid line represents a linear fitting based on the Curie-Weiss law.

Acknowledgment

This work was financially supported by the Natural Sci-ence Foundation of Guangxi Province, China (No. 0575001).

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626 RARE METALS, Vol. 27, No. 6, Dec 2008

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