- Email: [email protected]
Magnetic and crystal structure of Th–Fe–Sn intermetallics: ThFe0.22Sn2 and Th4Fe13Sn5
Intermetallics 8 (2000) 273±277
Magnetic and crystal structure of Th±Fe±Sn intermetallics: ThFe0.22Sn2 and Th4Fe13Sn5 O. Mozea, P. Manfrinettib,*, F. Canepab, A. Palenzonab, M.L. Fornasinic, J.R. Rodriguez-Carvajald a
Istituto Nazionale di Fisica della Materia (INFM), and Dipartimento di Fisica, UniversitaÁ di Modena e Reggio Emilia, Via G. Campi 213/A, 41100, Modena, Italy b Istituto Nazionale di Fisica della Materia (INFM), and Dipartimento di Chimica e Chimica Industriale, UniversitaÁ di Genova, Via Dodecaneso 31, 16146, Genova, Italy c Dipartimento di Chimica e Chimica Industriale, UniversitaÁ di Genova, Via Dodecaneso 31, 16146, Genova, Italy d Laboratoire LeÂon Brillouin, CEA-CNRS, Centre d'Etudes de Saclay, 91191 Gif Sur Yvette Cedex, France Received 14 April 1999; accepted 18 October 1999
Abstract The crystal and magnetic structures of two new ternary phases in the Th±Fe±Sn system: ThFe0.22Sn2 and Th4Fe13Sn5 have been investigated by high-resolution neutron powder diraction. The ThFe0.22Sn2 phase crystallizes in an orthorhombic crystal structure, space group Cmcm, whilst Th4Fe13Sn5 crystallizes in the tetragonal space group P4/mbm. In agreement with bulk magnetization measurements, the compound ThFe0.22Sn2 does not order magnetically in the temperature range investigated, while Th4Fe13Sn5 orders below 375 K, with the easy magnetization direction of the Fe moments aligned along the tetragonal c-axis. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Intermetallics, miscellaneous; B. Magnetic properties; D. Site occupancy; F. Diraction; G. Magnetic applications
1. Introduction The crystal structures of two newly identi®ed phases in the ternary Th±Fe±Sn system, ThFe0.22Sn2 and Th4Fe13Sn5, have been recently determined by single crystal X-ray diraction [1]. The phase ThFe0.22Sn2 was found to crystallize in an orthorhombic crystal structure (space group Cmcm) which is a defective structure of the CeNiSi2 type [2]. The phase Th4Fe13Sn5, which crystallizes in the tetragonal space group P4/mbm, was found to display a structure of a new type, formed by two different segments stacked alternatively along the tetragonal c-axis. The ®rst segment, with composition Th2Fe26, is similar to that of the Ce(Mn,Ni)11 structure (space group P4/mbm) [3], whilst the other segment, with composition Th6Sn10, corresponds to a half-cell of the type B3Cr5 (space group I4/mcm) [4]. Electrical resistivity, dierential scanning calorimetry and magnetization measurements reported for Th4Fe13Sn5 indicate * Corresponding author.
that a ferromagnetic-paramagnetic phase transition occurs at approximately 375 K, whereas no such magnetic transition is reported for the ThFe0.22Sn2 phase, even down to low temperatures. This paper reports a high-resolution neutron powder diraction investigation of these two compounds and in particular, the magnetic structure of Th4Fe13Sn5. From these data, the lowtemperature easy magnetization direction and magnitude of the Fe magnetic moments in Th4Fe13Sn5 have been inferred from model calculations of the observed neutron intensities. 2. Experimental details and data analysis 2.1. Sample preparation Samples were prepared from commercially available metals: Th of 99.8 wt% purity from Metal Crystals and Oxides Ltd, UK, Fe (99.998 wt% purity) and Sn (99.999 wt% purity) from Koch-Light Laboratories, UK. Turnings of Th (prepared under Ar) were mixed with Fe
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00109-0
274
O. Moze et al. / Intermetallics 8 (2000) 273±277
2.2. Neutron diraction
diractometer G4.1, located at the Orphee reactor, Laboratoire LeÂon Brillouin, CEA, Saclay, France. An incident neutron wavelength of 234.186 pm was selected from the (111) re¯ection of a Ge monochromator. The following neutron scattering lengths were used: bTh=1.031 10ÿ12 cm, bFe=0.954 10ÿ12 cm and bSn=0.6225 10ÿ12 cm [5]. Measurements were carried out at 300, 150 and 1.5 K for ThFe0.22Sn2, and at 300 and 1.5 K for Th4Fe13Sn5. In each case, approximately 10 g of sample were used.
Neutron powder diraction measurements were carried out on the high resolution multi-detector powder
3. Results and discussion
and Sn ®lings and then cold pressed in a steel die. These pellets were then melted under pure Ar in a highfrequency induction furnace on a water-cooled copper boat. Remelting was repeated three times. The resulting ingots were then anealed at 1223 K for 7 days. X-ray powder diractograms were collected by a Guinier-Stoe camera with a Cu Ka tube. The samples were also examined by metallographic and microprobe analysis.
Table 1 Re®ned atomic co-ordinates, lattice and thermal parameters for ThFe0.22Sn2 at 1.5 Ka Atom
Position
x
y
z
B (AÊ2)
Th Fe Sn1 Sn2
4c 4c 4c 4c
0 0 0 0
0.3965(2) 0.1926(2) 0.0603(4) 0.7504(4)
1/4 1/4 1/4 1/4
0.10 0.10 0.10 0.10
Occupancy 0.19(3)
a=446.485(60) pm, b =1701.448(20) pm, c=440.227(9) pm a
Rwp=6.7%; Rexp=2.4%; 2=7.8%.
Data analysis of the neutron powder diraction results was carried out using the FULLPROF Rietveld code for re®nement of crystal and magnetic structures [6,7]. 3.1. ThFe0.22Sn2 The structural parameters obtained from a previous single crystal X-ray diraction determination of the crystal structure were used as trial starting parameters for re®nement of the room temperature data for
Fig. 1. Observed and calculated neutron diraction pattern for ThFe0.22Sn2 measured at 1.5 K. The tick marks, from top to bottom, refer to calculated peak positions for (a) the main phase, ThFe0.22Sn2; (b), an alpha-Fe impurity and (c) cubic ThO2.
O. Moze et al. / Intermetallics 8 (2000) 273±277
ThFe0.22Sn2. These place Th at site 4c (0, 0.3969, 1/4), Fe at 4c (0, 0.197, 1/4) with an occupancy of 0.22 and Sn atoms at sites 4c (0, 0.0600, 1/4) and 4c (0, 0.7498, 1/ 4). With these starting parameters, the re®nement converged immediately. Inspection of the observed and re®ned powder diractograms showed the presence of additional impurity phases. These were identi®ed as alpha-Fe and cubic ThO2. A multi-phase re®nement was then performed with inclusion of these two materials as secondary and tertiary phases. The low-temperature re®nement results, together with R factors, are reported in Table 1; the observed and calculated diraction pattern for ThFe0.22Sn2 is showed in Fig. 1. The atomic positions and lattice parameters are in excellent accord with results obtained from X-ray single crystal diraction. There is a small discrepancy between the X-ray and neutron diraction determinations of the occupancy of Fe in the 4c defective site, but given the rather large
275
neutron scattering length of Fe, the value determined by neutron diraction can be considered as highly reliable. A schematic of the crystal structure of ThFe0.22Sn2 is displayed in Fig. 2. 3.2. Th4Fe13Sn5 Starting parameters for re®nement of the crystal structure were those obtained from a previous X-ray single crystal diraction study [1]. As for ThFe0.22Sn2, inspection of the re®ned room temperature diraction pattern revealed the presence of at least two impurity phases, alpha-Fe and ThO2. These were included in the overall re®nement of the crystal structure. The results from re®nement, with respective R factors values, are reported in Table 2: they are in excellent accord with the single crystal diraction structure determination. The crystal structure is displayed in Fig. 3. The low-temperature diraction pattern for this compound revealed the presence of some very intense re¯ections at low scattering angles (Fig. 4). In particular, a strong re¯ection occurs at 2y = 11 ; this indexes as an (001) peak. The (001) peak has a large nuclear structure factor and model calculations showed that all of this observed intensity can be accounted for just by the nuclear scattering. The magnitude and orientation of magnetic moments for magnetic structures with a con®gurational symmetry lower than cubic can be determined even for polycrystalline materials [8]. For a tetragonal crystal, the scattered neutron intensity for a powder specimen I(hkl), for a speci®ed re¯ection with Miller indices (hkl), can be most simply written as: " X jnuc F2hkl nuclear I hkl A hkl
X jmag hq2hkl iF2hkl magnetic
#
hkl
Table 2 Re®ned atomic co-ordinates, Th4Fe13Sn5 at 1.5 Ka
lattice,
thermal
parameters
Atom
Position
x
y
z
B (AÊ2)
Th1 Th2 Fe1 Fe2 Fe3 Fe4 Sn1 Sn2
4h 4c 8k 8k 8i 2d 8k 2h
0.6579(1) 0 0.1751(8) 0.6219(7) 0.0687(1) 0 0.1462(9) 0
0.1579(1) 0 0.6751(8) 0.1219(7) 0.2057(9) 1/2 0.6462(9) 0
1/2 0.2243(1) 0.1074(6) 0.1848(6) 0 0 0.3206(8) 1/2
0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10
a=825.51(6) pm, c=1193.77(9) pm Fig. 2. A schematic of the crystal structure of ThFe0.22Sn2.
for
a
Rwp=7.8%; Rexp=3.2%; Rmag=8.6%; w2=5.9%.
276
O. Moze et al. / Intermetallics 8 (2000) 273±277
where A() is an angle-dependent absorption factor, jnuc and F2hkl are multiplicities and structure factors for nuclear re¯ections, with their corresponding counterparts for magnetic re¯ections. The term hq2hkl i in the part containing information about the magnetic structure can be expressed as: 1 hq2hkl i 1 ÿ h2 k2 a 2 sin2 12 c 2 cos2 d2 2 where is the angle between the spin direction and the c-axis, a* and c* are the reciprocal lattice parameters, and d is the d-spacing for the re¯ection, which for a tetragonal crystal is given by; 1 h2 k2 12 2 2 a2 c d
Fig. 3. A schematic of the crystal structure of Th4Fe13Sn5.
It is the hq2hkl i term which allows one to deduce the orientation of the magnetic moments with respect to the unique tetragonal axis, i.e. if is 0 or 90 , or even at an intermediate angle between these. Naturally for a powder specimen, the moment direction in the (a±b) plane cannot be determined. Assuming that the Fe magnetic moments on the four available sites are aligned collinearly, the easy
Fig. 4. Observed and calculated neutron diraction pattern of Th4Fe13Sn5, measured at 1.5 K. The tick marks, from top to bottom, refer to calculated peak positions for (a) the main phase, Th4Fe13Sn5; (b) the magnetic re¯ections for Th4Fe13Sn5, and (c), (d) and (e), respectively, for alphaFe, cubic ThO2 and ThFe0.22Sn2 impurity.
O. Moze et al. / Intermetallics 8 (2000) 273±277
magnetization direction most likely lies along the tetragonal axis of the crystal, since this gives the most consistent agreement with the observed low-temperature intensites. Attempts were made to re®ne the magnitude of the magnetic moments at the four Fe sites, but the presence of impurity phases led to extremely unstable re®nements. A model calculation showed that an Fe moment of approximately 1.5 Bohr magnetons gives a good agreement with the observed intensities of the low angle re¯ections. This is in good accord with the observed magnitude for the Fe moment in a wide range of Th intermetallic compounds. Acknowledgements The authors would like to thank the Italian Ministry of University and Scienti®c Research, MURST, for ®nancial support, this work being part of the National Research Program ``Alloys and Intermetallic Compounds: Thermodynamics, Physical Properties, Reactivity''. This work has also been sponsored by the Italian National Research Council, CNR, under the
277
program entitled ``Progetto Finalizzato Materiali Speciali per Tecnologie Avanzate II''. This work was also supported by the ``Training and Mobility of Researchers РAccess to Large Scale Facilities'' programme of the European Community. Access to the neutron scattering facilities of the LLB-CEA, Saclay, France, is also gratefully acknowledged. References [1] Manfrinetti P, Canepa F, Palenzona A, Fornasini ML, Giannini E. J Alloys Compds 1997;247:109. [2] Parthe E, Chabot B. In: Gschneidner Jr. KA, Eyring L, editors. Handbook on the physics and chemistry of rare-earths, vol. 6. Amsterdam: North-Holland Publishers, 1984. p. 281. [3] Kalychak Ya M, Akselrud LG, Yarmolyuk Ya P, Bodak OI, Gladyshevskii EI. Sov Phys Crystallogr 1975;30:627. [4] Villars P, Calvert LD. Pearsons handbook of crystallographic data for intermetallic phases. 2nd ed. Materials Park (OH): ASM International, 1991. [5] Sears VF. Neutron scattering lengths and cross-sections, neutron news, vol. 3. Gordon and Breach Science Publishers, 1992. p. 26. [6] Rietveld HM. J Appl Cryst 1969;2:65. [7] Rodriguez-Carvajal J. Physica B 1990;192:55. [8] Shirane G. Acta Cryst 1959;12:282.
Magnetic and crystal structure of Th±Fe±Sn intermetallics: ThFe0.22Sn2 and Th4Fe13Sn5 O. Mozea, P. Manfrinettib,*, F. Canepab, A. Palenzonab, M.L. Fornasinic, J.R. Rodriguez-Carvajald a
Istituto Nazionale di Fisica della Materia (INFM), and Dipartimento di Fisica, UniversitaÁ di Modena e Reggio Emilia, Via G. Campi 213/A, 41100, Modena, Italy b Istituto Nazionale di Fisica della Materia (INFM), and Dipartimento di Chimica e Chimica Industriale, UniversitaÁ di Genova, Via Dodecaneso 31, 16146, Genova, Italy c Dipartimento di Chimica e Chimica Industriale, UniversitaÁ di Genova, Via Dodecaneso 31, 16146, Genova, Italy d Laboratoire LeÂon Brillouin, CEA-CNRS, Centre d'Etudes de Saclay, 91191 Gif Sur Yvette Cedex, France Received 14 April 1999; accepted 18 October 1999
Abstract The crystal and magnetic structures of two new ternary phases in the Th±Fe±Sn system: ThFe0.22Sn2 and Th4Fe13Sn5 have been investigated by high-resolution neutron powder diraction. The ThFe0.22Sn2 phase crystallizes in an orthorhombic crystal structure, space group Cmcm, whilst Th4Fe13Sn5 crystallizes in the tetragonal space group P4/mbm. In agreement with bulk magnetization measurements, the compound ThFe0.22Sn2 does not order magnetically in the temperature range investigated, while Th4Fe13Sn5 orders below 375 K, with the easy magnetization direction of the Fe moments aligned along the tetragonal c-axis. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Intermetallics, miscellaneous; B. Magnetic properties; D. Site occupancy; F. Diraction; G. Magnetic applications
1. Introduction The crystal structures of two newly identi®ed phases in the ternary Th±Fe±Sn system, ThFe0.22Sn2 and Th4Fe13Sn5, have been recently determined by single crystal X-ray diraction [1]. The phase ThFe0.22Sn2 was found to crystallize in an orthorhombic crystal structure (space group Cmcm) which is a defective structure of the CeNiSi2 type [2]. The phase Th4Fe13Sn5, which crystallizes in the tetragonal space group P4/mbm, was found to display a structure of a new type, formed by two different segments stacked alternatively along the tetragonal c-axis. The ®rst segment, with composition Th2Fe26, is similar to that of the Ce(Mn,Ni)11 structure (space group P4/mbm) [3], whilst the other segment, with composition Th6Sn10, corresponds to a half-cell of the type B3Cr5 (space group I4/mcm) [4]. Electrical resistivity, dierential scanning calorimetry and magnetization measurements reported for Th4Fe13Sn5 indicate * Corresponding author.
that a ferromagnetic-paramagnetic phase transition occurs at approximately 375 K, whereas no such magnetic transition is reported for the ThFe0.22Sn2 phase, even down to low temperatures. This paper reports a high-resolution neutron powder diraction investigation of these two compounds and in particular, the magnetic structure of Th4Fe13Sn5. From these data, the lowtemperature easy magnetization direction and magnitude of the Fe magnetic moments in Th4Fe13Sn5 have been inferred from model calculations of the observed neutron intensities. 2. Experimental details and data analysis 2.1. Sample preparation Samples were prepared from commercially available metals: Th of 99.8 wt% purity from Metal Crystals and Oxides Ltd, UK, Fe (99.998 wt% purity) and Sn (99.999 wt% purity) from Koch-Light Laboratories, UK. Turnings of Th (prepared under Ar) were mixed with Fe
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00109-0
274
O. Moze et al. / Intermetallics 8 (2000) 273±277
2.2. Neutron diraction
diractometer G4.1, located at the Orphee reactor, Laboratoire LeÂon Brillouin, CEA, Saclay, France. An incident neutron wavelength of 234.186 pm was selected from the (111) re¯ection of a Ge monochromator. The following neutron scattering lengths were used: bTh=1.031 10ÿ12 cm, bFe=0.954 10ÿ12 cm and bSn=0.6225 10ÿ12 cm [5]. Measurements were carried out at 300, 150 and 1.5 K for ThFe0.22Sn2, and at 300 and 1.5 K for Th4Fe13Sn5. In each case, approximately 10 g of sample were used.
Neutron powder diraction measurements were carried out on the high resolution multi-detector powder
3. Results and discussion
and Sn ®lings and then cold pressed in a steel die. These pellets were then melted under pure Ar in a highfrequency induction furnace on a water-cooled copper boat. Remelting was repeated three times. The resulting ingots were then anealed at 1223 K for 7 days. X-ray powder diractograms were collected by a Guinier-Stoe camera with a Cu Ka tube. The samples were also examined by metallographic and microprobe analysis.
Table 1 Re®ned atomic co-ordinates, lattice and thermal parameters for ThFe0.22Sn2 at 1.5 Ka Atom
Position
x
y
z
B (AÊ2)
Th Fe Sn1 Sn2
4c 4c 4c 4c
0 0 0 0
0.3965(2) 0.1926(2) 0.0603(4) 0.7504(4)
1/4 1/4 1/4 1/4
0.10 0.10 0.10 0.10
Occupancy 0.19(3)
a=446.485(60) pm, b =1701.448(20) pm, c=440.227(9) pm a
Rwp=6.7%; Rexp=2.4%; 2=7.8%.
Data analysis of the neutron powder diraction results was carried out using the FULLPROF Rietveld code for re®nement of crystal and magnetic structures [6,7]. 3.1. ThFe0.22Sn2 The structural parameters obtained from a previous single crystal X-ray diraction determination of the crystal structure were used as trial starting parameters for re®nement of the room temperature data for
Fig. 1. Observed and calculated neutron diraction pattern for ThFe0.22Sn2 measured at 1.5 K. The tick marks, from top to bottom, refer to calculated peak positions for (a) the main phase, ThFe0.22Sn2; (b), an alpha-Fe impurity and (c) cubic ThO2.
O. Moze et al. / Intermetallics 8 (2000) 273±277
ThFe0.22Sn2. These place Th at site 4c (0, 0.3969, 1/4), Fe at 4c (0, 0.197, 1/4) with an occupancy of 0.22 and Sn atoms at sites 4c (0, 0.0600, 1/4) and 4c (0, 0.7498, 1/ 4). With these starting parameters, the re®nement converged immediately. Inspection of the observed and re®ned powder diractograms showed the presence of additional impurity phases. These were identi®ed as alpha-Fe and cubic ThO2. A multi-phase re®nement was then performed with inclusion of these two materials as secondary and tertiary phases. The low-temperature re®nement results, together with R factors, are reported in Table 1; the observed and calculated diraction pattern for ThFe0.22Sn2 is showed in Fig. 1. The atomic positions and lattice parameters are in excellent accord with results obtained from X-ray single crystal diraction. There is a small discrepancy between the X-ray and neutron diraction determinations of the occupancy of Fe in the 4c defective site, but given the rather large
275
neutron scattering length of Fe, the value determined by neutron diraction can be considered as highly reliable. A schematic of the crystal structure of ThFe0.22Sn2 is displayed in Fig. 2. 3.2. Th4Fe13Sn5 Starting parameters for re®nement of the crystal structure were those obtained from a previous X-ray single crystal diraction study [1]. As for ThFe0.22Sn2, inspection of the re®ned room temperature diraction pattern revealed the presence of at least two impurity phases, alpha-Fe and ThO2. These were included in the overall re®nement of the crystal structure. The results from re®nement, with respective R factors values, are reported in Table 2: they are in excellent accord with the single crystal diraction structure determination. The crystal structure is displayed in Fig. 3. The low-temperature diraction pattern for this compound revealed the presence of some very intense re¯ections at low scattering angles (Fig. 4). In particular, a strong re¯ection occurs at 2y = 11 ; this indexes as an (001) peak. The (001) peak has a large nuclear structure factor and model calculations showed that all of this observed intensity can be accounted for just by the nuclear scattering. The magnitude and orientation of magnetic moments for magnetic structures with a con®gurational symmetry lower than cubic can be determined even for polycrystalline materials [8]. For a tetragonal crystal, the scattered neutron intensity for a powder specimen I(hkl), for a speci®ed re¯ection with Miller indices (hkl), can be most simply written as: " X jnuc F2hkl nuclear I hkl A hkl
X jmag hq2hkl iF2hkl magnetic
#
hkl
Table 2 Re®ned atomic co-ordinates, Th4Fe13Sn5 at 1.5 Ka
lattice,
thermal
parameters
Atom
Position
x
y
z
B (AÊ2)
Th1 Th2 Fe1 Fe2 Fe3 Fe4 Sn1 Sn2
4h 4c 8k 8k 8i 2d 8k 2h
0.6579(1) 0 0.1751(8) 0.6219(7) 0.0687(1) 0 0.1462(9) 0
0.1579(1) 0 0.6751(8) 0.1219(7) 0.2057(9) 1/2 0.6462(9) 0
1/2 0.2243(1) 0.1074(6) 0.1848(6) 0 0 0.3206(8) 1/2
0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10
a=825.51(6) pm, c=1193.77(9) pm Fig. 2. A schematic of the crystal structure of ThFe0.22Sn2.
for
a
Rwp=7.8%; Rexp=3.2%; Rmag=8.6%; w2=5.9%.
276
O. Moze et al. / Intermetallics 8 (2000) 273±277
where A() is an angle-dependent absorption factor, jnuc and F2hkl are multiplicities and structure factors for nuclear re¯ections, with their corresponding counterparts for magnetic re¯ections. The term hq2hkl i in the part containing information about the magnetic structure can be expressed as: 1 hq2hkl i 1 ÿ h2 k2 a 2 sin2 12 c 2 cos2 d2 2 where is the angle between the spin direction and the c-axis, a* and c* are the reciprocal lattice parameters, and d is the d-spacing for the re¯ection, which for a tetragonal crystal is given by; 1 h2 k2 12 2 2 a2 c d
Fig. 3. A schematic of the crystal structure of Th4Fe13Sn5.
It is the hq2hkl i term which allows one to deduce the orientation of the magnetic moments with respect to the unique tetragonal axis, i.e. if is 0 or 90 , or even at an intermediate angle between these. Naturally for a powder specimen, the moment direction in the (a±b) plane cannot be determined. Assuming that the Fe magnetic moments on the four available sites are aligned collinearly, the easy
Fig. 4. Observed and calculated neutron diraction pattern of Th4Fe13Sn5, measured at 1.5 K. The tick marks, from top to bottom, refer to calculated peak positions for (a) the main phase, Th4Fe13Sn5; (b) the magnetic re¯ections for Th4Fe13Sn5, and (c), (d) and (e), respectively, for alphaFe, cubic ThO2 and ThFe0.22Sn2 impurity.
O. Moze et al. / Intermetallics 8 (2000) 273±277
magnetization direction most likely lies along the tetragonal axis of the crystal, since this gives the most consistent agreement with the observed low-temperature intensites. Attempts were made to re®ne the magnitude of the magnetic moments at the four Fe sites, but the presence of impurity phases led to extremely unstable re®nements. A model calculation showed that an Fe moment of approximately 1.5 Bohr magnetons gives a good agreement with the observed intensities of the low angle re¯ections. This is in good accord with the observed magnitude for the Fe moment in a wide range of Th intermetallic compounds. Acknowledgements The authors would like to thank the Italian Ministry of University and Scienti®c Research, MURST, for ®nancial support, this work being part of the National Research Program ``Alloys and Intermetallic Compounds: Thermodynamics, Physical Properties, Reactivity''. This work has also been sponsored by the Italian National Research Council, CNR, under the
277
program entitled ``Progetto Finalizzato Materiali Speciali per Tecnologie Avanzate II''. This work was also supported by the ``Training and Mobility of Researchers РAccess to Large Scale Facilities'' programme of the European Community. Access to the neutron scattering facilities of the LLB-CEA, Saclay, France, is also gratefully acknowledged. References [1] Manfrinetti P, Canepa F, Palenzona A, Fornasini ML, Giannini E. J Alloys Compds 1997;247:109. [2] Parthe E, Chabot B. In: Gschneidner Jr. KA, Eyring L, editors. Handbook on the physics and chemistry of rare-earths, vol. 6. Amsterdam: North-Holland Publishers, 1984. p. 281. [3] Kalychak Ya M, Akselrud LG, Yarmolyuk Ya P, Bodak OI, Gladyshevskii EI. Sov Phys Crystallogr 1975;30:627. [4] Villars P, Calvert LD. Pearsons handbook of crystallographic data for intermetallic phases. 2nd ed. Materials Park (OH): ASM International, 1991. [5] Sears VF. Neutron scattering lengths and cross-sections, neutron news, vol. 3. Gordon and Breach Science Publishers, 1992. p. 26. [6] Rietveld HM. J Appl Cryst 1969;2:65. [7] Rodriguez-Carvajal J. Physica B 1990;192:55. [8] Shirane G. Acta Cryst 1959;12:282.