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Neutron diffraction study of the magnetic refrigerant Mn1.1Fe0.9P0.76Ge0.24

Published online by Cambridge University Press:  06 March 2012

L. J. Liu ,
D. M. Liu ,
Q. Z. Huang ,
T. L. Zhang ,
L. Zhang ,
M. Yue ,
J. W. Lynn  and
J. X. Zhang
L. J. Liu
Affiliation:
Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, China
D. M. Liu*
Affiliation:
Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, China
Q. Z. Huang
Affiliation:
NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
T. L. Zhang
Affiliation:
Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, China
L. Zhang
Affiliation:
Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, China
M. Yue
Affiliation:
College of Material Science and Engineering, Beijing University of Technology, Beijing 100124, China
J. W. Lynn
Affiliation:
NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
J. X. Zhang
Affiliation:
College of Material Science and Engineering, Beijing University of Technology, Beijing 100124, China
*
a)Author to whom correspondence should be addressed. Electronic mail: dmliu@bjut.edu.cn
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Abstract

Polycrystalline Mn1.1Fe0.9P0.76Ge0.24 was prepared by mechanical alloying and spark plasma sintering. Neutron diffraction was used to determine the structure of the title compound, and its nuclear and magnetic structures were refined using the Rietveld method. It is found that temperature and magnetic field can induce phase transition between the paramagnetic and the ferromagnetic phases in the material, and the paramagnetic and ferromagnetic phases have very distinct crystal structures and different Ge contents.

Keywords

neutron diffractioncrystal structuremagnetic refrigerant
Type
Technical Articles
Information
Powder Diffraction , Volume 25 , Supplement S1 , September 2010 , pp. S25 - S27
Copyright
Copyright © Cambridge University Press 2010

I. INTRODUCTION

Magnetic refrigeration discovered by Warburg in 1881 is based on the magnetocaloric effect (MCE). Tegus et al. (Reference Tegus, Brück, Buschow and de Boer2002) discovered the hexagonal MnFe(PAs) compound for room-temperature applications, which has a first-order phase transition and exhibits a huge MCE. However, there are some problems to use MnFe(PAs) as refrigerant because As is a toxic element. Recently, it was found that by replacing As with Ge and Si in MnFe(PAs), a large MCE can also be achieved near room temperature (Tegus et al., Reference Tegus, Fuquan, Dagula, Zhang, Brück, Si, de Boer and Buschow2005; Brück et al., Reference Brück, Tegus, Zhang, Li, de Boer and Buschow2004; Cam Thanh et al., Reference Cam Thanh, Brück, Tegus, Klaasse, Gortenmulder and Buschow2006; Dagula et al., Reference Dagula, Tegus, Li, Song, Brück, Cam Thanh, de Boer and Buschow2006; Ou et al., Reference Ou, Wang, Lin, Tegus, Brück and Buschow2006). Moreover, the Curie temperature can be adjusted by changing the content of Ge or Si. Liu et al. (Reference Liu, Yue, Zhang, McQueen, Lynn, Wang, Chen, Li, Cava, Liu, Altounian and Huang2009) reported that the compound Mn1.1Fe0.9P0.8Ge0.2 has a value of MCE of 75 J/kg K. However, the Curie temperature of Mn1.1Fe0.9P0.8Ge0.2 was reported by Yue et al. (Reference Yue, Li, Wang, Liu, Zhang and Liu2009) to be below room temperature at about 253 K. In this study, a Mn1.1Fe0.9P0.76Ge0.24 sample was prepared by mechanical alloying and spark plasma sintering (SPS) to enhance the Curie temperature to the room-temperature range. The crystal structure of the title compound was investigated by neutron diffraction.

II. EXPERIMENTAL

The starting materials for the polycrystalline sample used in this work, pure Mn (>99.99%), Fe powders (>99.99%), red P (>99.3%), and Ge chips (>99.9999%), were submitted to ball milling, which was carried out under argon atmosphere for 1.5 h in a high-energy pulverisette 4 mill. Then, the milled powders were collected into a graphite mold and consolidated into a ∅ 20 mm×5 mm wafer at 1203 K under 30 MPa by SPS.

Neutron powder-diffraction (NPD) data were collected at the NIST Center for Neutron Research on a high-resolution powder neutron diffractometer (BT1) with monochromatic neutrons of wavelength 1.5403 Å produced by a Cu (311) monochromator. Data of the nuclear and magnetic structures were refined by the Rietveld method using the General Structure Analysis System (GSAS).

A TA Instrument differential scanning calorimetry (DSC) Q200 was used for calorimetric measurements. Nitrogen gas with a flow rate of 50 ml/min was purged into the cell. The sample with about 30 mg was investigated by heating at the rate of 1 K/min between 190 and 320 K.

III. RESULTS AND DISCUSSION

Neutron diffraction analysis of the Mn1.1Fe0.9P0.76Ge0.24 compound reveals that the compound is single phase and paramagnetic (PM) at higher temperatures and ferromagnetic (FM) at lower temperatures, and in between the system goes

Figure 1. (Color online) Refined NPD patterns for Mn1.1Fe0.9P0.76Ge0.24 sample collected at room temperature (upper panel) and 267 K (lower panel).

through a strongly first-order phase transition. The transition can be induced by either temperature or applied magnetic field. Both phases have the same symmetry space group (P 62m).

Figure 1 shows the neutron powder-diffraction patterns for the Mn1.1Fe0.9P0.76Ge0.24 sample measured at room temperature (upper panel) and 267 K (lower panel). The upper panel displays a fit with only the nuclear structure model of the PM phase. The lower panel depicts a fit by including the PM phase as well as a ferromagnetic phase with its moments in the a-b plane. The refined data are given in Table I.

The crystal structure of the FM phase is shown in Figure 2. It has the Fe2P-type hexagonal structure with space group P 62m, and the symmetry of the magnetic structure is P11m′. The Mn atoms are coplanar with the P/Ge(1) atoms, and the Fe/Mn atoms are coplanar with P/Ge(2). The Mn atoms are surrounded by four P/Ge(2) atoms located on above and below and by one apical P/Ge(1) atom on the same layer, forming a pyramid. The Fe/Mn site is coordinated by two P/Ge(2) atoms located on the same layer and two P/Ge(1) atoms in the layers above and below, forming a tetrahedron. The tetrahedra

TABLE I. Results on Rietveld-refinement analysis of Mn1.1Fe0.9P0.76Ge0.24 NPD patterns measured at RT and 267 K (space group P 62m and z=3).

1 The Wyckoff positions for space group (P 62m) at the 3f site are (x,0,0), (0,x,0), and (−x,−x,0), at the 3g site are (x,0,1/2), (0,x,1/2) and (−x,−x,1/2), at the 2c site are (1/3,2/3,0) and (2/3,1/3,0), and at the 1b site is (0,0,1/2).

share edges with each other and share corners with the nearest pyramids. The intraplane transition metals form a triangular configuration.

At room temperature the sample is in the PM phase and there is no magnetic order. The Rietveld-refinement results show that the sample contains a small amount of impurity phases (4.54% MnO and Fe3Mn4Ge6). The Mn/Fe ratio is

Figure 2. (Color online) Crystal structure of Mn1.1Fe0.9P0.76Ge0.24.

1.058/0.942, very close to the nominal value of Mn/Fe=1.1/0.9. As shown in Table I, the 3g site has 97.9% Mn and only 2.1% Fe atoms, while the 3f site has 92.1% Fe and 7.9% Mn atoms. The Ge atoms prefer the 2c site (24.9% Ge) to the 1b site (2.4% Ge).

Refined diffraction data at 267 K show that 42.09% of the PM phase, 51.63% of the FM phase, and 6.28% impurities (MnO and Fe3Mn4Ge6 mainly) coexist. The unit-cell parameters for the PM phase are as follows: a=b=6.081 80 Å, c=3.446 56 Å, and V=110.40 Å3. For the PM phase, the 3g site has 99.9% Mn and only 0.1% Fe atoms, while the 3f site has 93.2% Fe and 6.8% Mn atoms. The 2c site has 69.4% P and 30.6% Ge atoms, while the 1b site has 93.5% P and 6.5% Ge atoms (see Table I). The actual composition of the PM phase was calculated to be Mn1.067Fe0.933P0.774Ge0.226.

The crystal structure of the FM phase is different from that of the PM phase for the Mn1.1Fe0.9P0.76Ge0.24 sample at 267 K. The crystal-structure parameters of the PM phase are as follows: a=b=6.155 85 Å, c=3.364 04 Å, and V=110.4 Å3. The 3g site has 99.6% Mn and only 0.4% Fe atoms, while the 3f site has 94.3% Fe and 5.7% Mn atoms. The 2c site has 70.3% P and 29.7% Ge atoms, while the 1b site has 85% P and 15% Ge atoms (see Table I). The actual calculated composition of the FM phase is Mn1.053Fe0.947P0.752Ge0.248. Combining the FM and the FM phases, we have found that the actual composition of the whole sample is Mn1.06Fe0.94P0.762Ge0.238, which is very close to the nominal composition of Mn1.1Fe0.9P0.76Ge0.24. It should be noted the FM phase has a higher Ge content than that of the PM phase. The refined data reveal that the ferromagnetic structure has Mn and Fe moments parallel in the a-b plane at 267 K. The ferromagnetic moments are 3.02μB and 1.027μB for the Mn and the Fe atoms, respectively. The a axis of the FM phase is larger, while the c axis is smaller than those of the PM phase. However, the unit-cell volumes for these two phases are practically the same.

Figure 3 shows the continuous-heating DSC curve for Mn1.1Fe0.9P0.76Ge0.24 alloy. An obvious endothermic peak is shown in the graph, which corresponds to the reaction process from the FM phase to the PM phase. It reveals that the transition from the FM phase to the PM phase is a first-order phase transition and the Curie temperature of Mn1.1Fe0.9P0.76Ge0.24 is ∼278 K, indicating that Mn1.1Fe0.9P0.76Ge0.24 is superior to be used as a room-temperature magnetocaloric material compared to the Mn1.1Fe0.9P0.8Ge0.2 compound with a T C of 253 K reported by Yue et al. (Reference Yue, Li, Wang, Liu, Zhang and Liu2009).

IV. CONCLUSION

Polycrystalline Mn1.1Fe0.9P0.76Ge0.24 prepared by mechanical alloying and spark plasma sintering has a T C at about 278K. The neutron powder-diffraction measurements

Figure 3. DSC heat flow.

reveal that the FM and the PM phases have two very distinct crystal structures and different Ge contents after a phase change induced by a temperature change. Compared to the PM phase, the a axis of the FM phase is relatively larger and the c axis is smaller. The volumes of the unit cell for these two phases are practically the same.

ACKNOWLEDGMENT

This work was financially supported by the National Basic Research Program of China (Grant No. 2010CB833100).

References

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Figure 0

Figure 1. (Color online) Refined NPD patterns for Mn1.1Fe0.9P0.76Ge0.24 sample collected at room temperature (upper panel) and 267 K (lower panel).

Figure 1

TABLE I. Results on Rietveld-refinement analysis of Mn1.1Fe0.9P0.76Ge0.24 NPD patterns measured at RT and 267 K (space group P62m and z=3).

Figure 2

Figure 2. (Color online) Crystal structure of Mn1.1Fe0.9P0.76Ge0.24.

Figure 3

Figure 3. DSC heat flow.