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Magnetic properties of the R2CuGe6 (R = Gd, Tb, Dy, Er) ternary compounds
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Solid State Sciences 10 (2008) 1891e1894 www.elsevier.com/locate/ssscie
Magnetic properties of the R2CuGe6 (R ¼ Gd, Tb, Dy, Er) ternary compounds D. Kaczorowski a,*, M. Konyk b, A. Szytu1a c, L. Romaka b, O. Bodak b a
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław, Poland b Ivan Franko National University of Lviv, Kyryla i Mefodiya Street 6, 79005 Lviv, Ukraine c Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Krako´w, Poland Received 10 January 2008; received in revised form 9 March 2008; accepted 8 April 2008 Available online 22 April 2008
Abstract The magnetic behavior of the R2CuGe6 ternaries (R ¼ Gd, Tb, Dy and Er) was investigated in the temperature interval 1.72e300 K and in magnetic fields up to 5 T. All the compounds order antiferomagnetically with Ne´el temperatures of 20.6, 33.1, 22.0 and 5.6 K for R ¼ Gd, Tb, Dy and Er, respectively. Above the respective TN the reciprocal magnetic susceptibility obeys the CurieeWeiss law with the paramagnetic Curie temperature being negative for R ¼ Gd, Tb and Dy and positive for R ¼ Er. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Rare earth intermetallics; Germanides; Magnetic properties; Magnetically ordered compounds
1. Introduction The ternary compounds R2CuGe6 (R ¼ Y, Ce, Nd, Gd, Tb, Dy, Ho, Er, Yb) crystallize with an orthorhombic structure of their own type (Ce2CuGe6 type, space group Amm2), which is a linear combination of a few fragments of the AlB2, BaAl4, aPo and ZrSi2 types [1]. The study of the magnetic properties of the phases with R ¼ Ce, Pr, Nd and Sm in the temperature interval 4.2e100 K has revealed that all these compounds order antiferromagnetically below the Ne´el temperatures in the range 9.0e16.0 K [2]. The magnetic and electrical behaviors of the compounds with R ¼ Y, Gd, Tb, Dy, Ho, Er and Yb have been studied in the region 78e300 K only [3]. All these materials have been found to exhibit metallic electrical conductivity. The lanthanide-based ternaries have been characterized as CurieeWeiss paramagnets, whereas Pauli paramagnetism has been established for Y2CuGe6. The present work aimed at extending the region of magnetic investigations of the R2CuGe6 phases down to pumped helium temperatures.
* Corresponding author. E-mail address: [email protected] (D. Kaczorowski). 1293-2558/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.04.007
We report here the magnetic behavior of the compounds with R ¼ Gd, Tb, Dy and Er.
2. Experimental details Polycrystalline samples of the R2CuGe6 compounds with R ¼ Gd, Tb, Dy and Er were prepared by arc melting stoichiometric amounts of the constituent elements (nominal purities: rare earths e 99.9 wt.%, Cu e 99.99 wt.%, Ge e 99.999 wt.%) on a water-cooled copper hearth under a protective Ti-gettered argon atmosphere. Subsequently the buttons were annealed at 870 K for 720 h in evacuated silica tubes, followed by quenching in cold water. The quality of the samples was checked by X-ray powder diffraction using a DRON-3.0 powder diffractometer with CuKa radiation. The unit cell parameters were refined from the X-ray data employing the program CSD [4]. Analysis of the X-ray patterns revealed all the materials to be single phases with the expected Ce2CuGe6 type unit cells. The refined lattice parameters are collected in Table 1. Magnetic measurements were performed in the temperature range 1.72e300 K and in applied magnetic fields up to 5 T, using a Quantum Design MPMS-5 SQUID magnetometer.
D. Kaczorowski et al. / Solid State Sciences 10 (2008) 1891e1894
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Table 1 Unit cell parameters of the R2CuGe6 compounds Compound
Lattice parameters a (nm)
b (nm)
Gd2CuGe6 Tb2CuGe6 Dy2CuGe6 Er2CuGe6
0.4003(3) 0.4000(2) 0.3993(1) 0.3977(2)
0.4120(3) 0.4113(2) 0.4100(1) 0.4080(2)
Table 2 Magnetic characteristics of the R2CuGe6 compounds Compound TN (K) Tt (K)
c (nm)
Volume, V (103 nm3)
2.1096(7) 2.1010(2) 2.0939(3) 2.0837(5)
347.92 345.65 342.79 338.10
Gd2CuGe6 Tb2CuGe6 Dy2CuGe6 Er2CuGe6
3. Results The temperature dependencies of the reciprocal molar magnetic susceptibility of the R2CuGe6 compounds with R ¼ Gd, Tb, Dy and Er, measured in a magnetic field of 0.1 T, are shown in Fig. 1. All these materials order antiferromagnetically at low temperatures and exhibit CurieeWeiss behavior in the paramagnetic state, with the parameters given in Table 2, which are in reasonable agreement with the data reported in Ref. [3]. The paramagnetic Curie temperatures are negative for R ¼ Gd, Tb and Dy and positive for R ¼ Er. The effective magnetic moments are slightly smaller than the values calculated within the RusselleSaunders approach for the respective R3þ ions (cf. Table 2). The magnetization measured at T ¼ 1.72 K as a function of applied magnetic field strength (see the insets to Fig. 1) is characteristic of systems with antiferromagnetic ground state. For all the compounds except Gd2CuGe6 a metamagnetic phase transition occurs in a certain critical field below 1.5 T.
20.6 33.1 22.0 5.6 (?)
qp (K) meff (mB)
m5T (mB) ms (mB) Bc (T) theor. Exp. Theor. exp.
<1.72 (?) 37.7 7.71 7.94 0.9 8.5 18.0 9.48 9.72 1.4 7.5 10.2 9.90 10.65 2.5 3.8 þ2 9.30 9.58 5.8
7 9 10 9
e 1.3 1.0 0.4
However, no saturation of s(B) is observed up to 5 T. The magnetic moment measured in this terminal field is only a small fraction of the free R3þ ion values (cf. Table 2). The low-temperature magnetic behavior of the compounds studied is presented in Fig. 2. Clearly, for Tb2CuGe6 and Dy2CuGe6, besides the anomalies in s(T ) related to the onset of antiferromagnetism at TN ¼ 33.1 and 22.0 K, respectively, some additional features occurring at a temperature Tt of 8.5 and 7.5 K, respectively, are seen, which might be interpreted as being due to some changes in the magnetic structures (the singularity observed for Tb2CuGe6 at 2.2 K is probably caused by some small amount of terbium oxide impurity). Likely, also in the case of Er2CuGe6 the magnetic behavior is more complex than simple antiferromagnetism, and possibly the actual Ne´el temperature is 5.6 K. Then, the pronounced magnetization maximum at 3.8 K might signal an ordereorder phase transition (see Fig. 2). In this context it is worthwhile noting that for the Gd-based compound a little upturn in s(T ) is observed at low temperatures, which may provoke some speculation
Fig. 1. Temperature dependencies of the inverse molar magnetic susceptibility of (a) Gd2CuGe6; (b) Tb2CuGe6; (c) Dy2CuGe6 and (d) Er2CuGe6, measured in a field of 0.1 T. The solid lines represent the CurieeWeiss fits with the parameters given in Table 2. The insets show the magnetic field variations of the magnetization in the respective compounds, measured at 1.72 K with increasing (full symbols) and decreasing (open symbols) magnetic field.
D. Kaczorowski et al. / Solid State Sciences 10 (2008) 1891e1894
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Fig. 2. Temperature dependencies of the magnetization in (a) Gd2CuGe6; (b) Tb2CuGe6; (c) Dy2CuGe6 and (d) Er2CuGe6, measured in a field of 0.1 T upon cooling the samples in applied field (open circles). For Tb2CuGe6 and Er2CuGe6 the measurements were additionally performed in a field of 0.01 T upon cooling the samples either in zero (full triangles) or applied (open triangles) field. For the sake of clarity these magnetization data were multiplied by 10 (Tb2CuGe6) or 8 (Er2CuGe6).
about spin reorientation occurring also in this germanide, as in all the other counterparts. In order to verify the above presumptions and determine the magnetic structures in the R2CuGe6 ternaries neutron diffraction experiments are planned. 4. Discussion The results presented in this work indicate that all the investigated R2CuGe6 compounds order antiferromagnetically at low temperatures. The experimental values of the effective magnetic moment correspond to those expected for the magnetic moments localized on the rare earth atoms. Large interatomic ReR distances as well as metallic character of the electrical conductivity imply that the magnetic interactions can be predominantly described as an indirect coupling via conduction electrons, i.e. within the RKKY model. In this approach the ordering temperatures are proportional to the de Gennes factor G ¼ ( gJ 1)2J(J þ 1) and may be expressed as [5] 2 2 TN ¼ IðgJ 1Þ JðJ þ 1Þ; 3
ð1Þ
where I stands for the exchange interaction parameter, J is the total angular momentum of the Hund’s rule ground state of the rare earth ion, while gJ denotes the Lande´ factor. In Fig. 3(a) the experimental values of the ordering temperatures
of the R2CuGe6 phases are shown as a function of the G factor. Whereas the overall trend of increasing TN with rising G is consistent with the RKKY interaction, the observed dependence is strongly non-linear, presumably because of strong crystalline electric field (CEF) effect. As is apparent from Fig. 3(a), quite similar behavior can be anticipated for the isostructural R2NiGe6 compounds [6]. Within the mean field approximation the paramagnetic Curie temperature measures the indirect interactions between the local magnetic moments. Therefore qp may also be expected to scale with the strength of the RKKY interactions and hence with the magnitude of the de Gennes factor. The relevant formula is given by [5] kqp ¼
3p z2 m Jsf2 ðgJ 1Þ2 JðJ þ 1Þ; 4 Z2 kF2
ð2Þ
where Jsf is the effective interaction between the conduction electrons and the local moments, kF is the Fermi wave-vector, m* stands for the effective mass and z is the average conduction electron concentration. Fig. 3(b) presents the respective data for the R2NiGe6 and R2CuGe6 series. Interestingly, for the Ni-containing ternaries the relation (2) is nearly linear, yet for the R2CuGe6 family some deviation from a straightline behavior is observed, which may result from the CEF effect.
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Fig. 4. The critical magnetic fields of the metamagnetic phase transitions in the R2NiGe6 and R2CuGe6 series plotted against the ratio defined in Eq. (3). The data for R2NiGe6 were taken from [6].
value of the paramagnetic Curie temperature, and is given by the formula [6] 3kB TN þ qp : ð3Þ Bc f 2gJ ðJ þ 1ÞmB The above dependence derived for the R2CuGe6 and R2NiGe6 compounds is presented in Fig. 4. Apparently, for neither of the series a firm proportionality is observed, however, the values of the critical field Bc continuously decrease with increasing the number of the 4f electrons. References Fig. 3. (a) The Ne´el temperatures and (b) the paramagnetic Curie temperatures for the R2NiGe6 and R2CuGe6 compounds plotted as a function of the de Gennes parameter. The data for R2NiGe6 were taken from [6].
It is known that in intermetallics based on the rare earth metals with the orbital quantum number L s 0 the crystalline electric field effects lead to large magnetocrystalline anisotropy. In a simple model of an antiferromagnet in which all atoms experience the same molecular field it can be shown that the critical field of the metamagnetic process is proportional to the sum of the Ne´el temperature and the absolute
[1] M.B. Konyk, P.S. Salamakha, O.I. Bodak, V.K. Pecharsky, Kristallografiya 33 (1988) 838. [2] O. Sologub, K. Hiebl, P. Rogl, O.I. Bodak, J. Alloys Compd. 227 (1995) 37. [3] M.B. Konyk, L.P. Romaka, Yu.K. Gorelenko, O.I. Bodak, J. Alloys Compd. 311 (2000) 120. [4] L.G. Akselrud, Yu.N. Grin, P.Yu. Zavalii, V.K. Pecharsky, V.S. Fundamenskii, CSD-Universal Program Package for Single Crystal or Powder Structure Data Treatment, Collected Abstracts, 12th European Crystallography Meeting, vol. 3, Nauka, Moscow, 1989, p. 155. [5] D.R. Noakes, G.K. Shenoy, Phys. Lett. A91 (1982) 35. [6] M. Konyk, L. Romaka, D. Gignoux, D. Fruchart, O. Bodak, Yu. Gorelenko, J. Alloys Compd. 398 (2005) 8.
Solid State Sciences 10 (2008) 1891e1894 www.elsevier.com/locate/ssscie
Magnetic properties of the R2CuGe6 (R ¼ Gd, Tb, Dy, Er) ternary compounds D. Kaczorowski a,*, M. Konyk b, A. Szytu1a c, L. Romaka b, O. Bodak b a
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław, Poland b Ivan Franko National University of Lviv, Kyryla i Mefodiya Street 6, 79005 Lviv, Ukraine c Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Krako´w, Poland Received 10 January 2008; received in revised form 9 March 2008; accepted 8 April 2008 Available online 22 April 2008
Abstract The magnetic behavior of the R2CuGe6 ternaries (R ¼ Gd, Tb, Dy and Er) was investigated in the temperature interval 1.72e300 K and in magnetic fields up to 5 T. All the compounds order antiferomagnetically with Ne´el temperatures of 20.6, 33.1, 22.0 and 5.6 K for R ¼ Gd, Tb, Dy and Er, respectively. Above the respective TN the reciprocal magnetic susceptibility obeys the CurieeWeiss law with the paramagnetic Curie temperature being negative for R ¼ Gd, Tb and Dy and positive for R ¼ Er. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Rare earth intermetallics; Germanides; Magnetic properties; Magnetically ordered compounds
1. Introduction The ternary compounds R2CuGe6 (R ¼ Y, Ce, Nd, Gd, Tb, Dy, Ho, Er, Yb) crystallize with an orthorhombic structure of their own type (Ce2CuGe6 type, space group Amm2), which is a linear combination of a few fragments of the AlB2, BaAl4, aPo and ZrSi2 types [1]. The study of the magnetic properties of the phases with R ¼ Ce, Pr, Nd and Sm in the temperature interval 4.2e100 K has revealed that all these compounds order antiferromagnetically below the Ne´el temperatures in the range 9.0e16.0 K [2]. The magnetic and electrical behaviors of the compounds with R ¼ Y, Gd, Tb, Dy, Ho, Er and Yb have been studied in the region 78e300 K only [3]. All these materials have been found to exhibit metallic electrical conductivity. The lanthanide-based ternaries have been characterized as CurieeWeiss paramagnets, whereas Pauli paramagnetism has been established for Y2CuGe6. The present work aimed at extending the region of magnetic investigations of the R2CuGe6 phases down to pumped helium temperatures.
* Corresponding author. E-mail address: [email protected] (D. Kaczorowski). 1293-2558/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.04.007
We report here the magnetic behavior of the compounds with R ¼ Gd, Tb, Dy and Er.
2. Experimental details Polycrystalline samples of the R2CuGe6 compounds with R ¼ Gd, Tb, Dy and Er were prepared by arc melting stoichiometric amounts of the constituent elements (nominal purities: rare earths e 99.9 wt.%, Cu e 99.99 wt.%, Ge e 99.999 wt.%) on a water-cooled copper hearth under a protective Ti-gettered argon atmosphere. Subsequently the buttons were annealed at 870 K for 720 h in evacuated silica tubes, followed by quenching in cold water. The quality of the samples was checked by X-ray powder diffraction using a DRON-3.0 powder diffractometer with CuKa radiation. The unit cell parameters were refined from the X-ray data employing the program CSD [4]. Analysis of the X-ray patterns revealed all the materials to be single phases with the expected Ce2CuGe6 type unit cells. The refined lattice parameters are collected in Table 1. Magnetic measurements were performed in the temperature range 1.72e300 K and in applied magnetic fields up to 5 T, using a Quantum Design MPMS-5 SQUID magnetometer.
D. Kaczorowski et al. / Solid State Sciences 10 (2008) 1891e1894
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Table 1 Unit cell parameters of the R2CuGe6 compounds Compound
Lattice parameters a (nm)
b (nm)
Gd2CuGe6 Tb2CuGe6 Dy2CuGe6 Er2CuGe6
0.4003(3) 0.4000(2) 0.3993(1) 0.3977(2)
0.4120(3) 0.4113(2) 0.4100(1) 0.4080(2)
Table 2 Magnetic characteristics of the R2CuGe6 compounds Compound TN (K) Tt (K)
c (nm)
Volume, V (103 nm3)
2.1096(7) 2.1010(2) 2.0939(3) 2.0837(5)
347.92 345.65 342.79 338.10
Gd2CuGe6 Tb2CuGe6 Dy2CuGe6 Er2CuGe6
3. Results The temperature dependencies of the reciprocal molar magnetic susceptibility of the R2CuGe6 compounds with R ¼ Gd, Tb, Dy and Er, measured in a magnetic field of 0.1 T, are shown in Fig. 1. All these materials order antiferromagnetically at low temperatures and exhibit CurieeWeiss behavior in the paramagnetic state, with the parameters given in Table 2, which are in reasonable agreement with the data reported in Ref. [3]. The paramagnetic Curie temperatures are negative for R ¼ Gd, Tb and Dy and positive for R ¼ Er. The effective magnetic moments are slightly smaller than the values calculated within the RusselleSaunders approach for the respective R3þ ions (cf. Table 2). The magnetization measured at T ¼ 1.72 K as a function of applied magnetic field strength (see the insets to Fig. 1) is characteristic of systems with antiferromagnetic ground state. For all the compounds except Gd2CuGe6 a metamagnetic phase transition occurs in a certain critical field below 1.5 T.
20.6 33.1 22.0 5.6 (?)
qp (K) meff (mB)
m5T (mB) ms (mB) Bc (T) theor. Exp. Theor. exp.
<1.72 (?) 37.7 7.71 7.94 0.9 8.5 18.0 9.48 9.72 1.4 7.5 10.2 9.90 10.65 2.5 3.8 þ2 9.30 9.58 5.8
7 9 10 9
e 1.3 1.0 0.4
However, no saturation of s(B) is observed up to 5 T. The magnetic moment measured in this terminal field is only a small fraction of the free R3þ ion values (cf. Table 2). The low-temperature magnetic behavior of the compounds studied is presented in Fig. 2. Clearly, for Tb2CuGe6 and Dy2CuGe6, besides the anomalies in s(T ) related to the onset of antiferromagnetism at TN ¼ 33.1 and 22.0 K, respectively, some additional features occurring at a temperature Tt of 8.5 and 7.5 K, respectively, are seen, which might be interpreted as being due to some changes in the magnetic structures (the singularity observed for Tb2CuGe6 at 2.2 K is probably caused by some small amount of terbium oxide impurity). Likely, also in the case of Er2CuGe6 the magnetic behavior is more complex than simple antiferromagnetism, and possibly the actual Ne´el temperature is 5.6 K. Then, the pronounced magnetization maximum at 3.8 K might signal an ordereorder phase transition (see Fig. 2). In this context it is worthwhile noting that for the Gd-based compound a little upturn in s(T ) is observed at low temperatures, which may provoke some speculation
Fig. 1. Temperature dependencies of the inverse molar magnetic susceptibility of (a) Gd2CuGe6; (b) Tb2CuGe6; (c) Dy2CuGe6 and (d) Er2CuGe6, measured in a field of 0.1 T. The solid lines represent the CurieeWeiss fits with the parameters given in Table 2. The insets show the magnetic field variations of the magnetization in the respective compounds, measured at 1.72 K with increasing (full symbols) and decreasing (open symbols) magnetic field.
D. Kaczorowski et al. / Solid State Sciences 10 (2008) 1891e1894
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Fig. 2. Temperature dependencies of the magnetization in (a) Gd2CuGe6; (b) Tb2CuGe6; (c) Dy2CuGe6 and (d) Er2CuGe6, measured in a field of 0.1 T upon cooling the samples in applied field (open circles). For Tb2CuGe6 and Er2CuGe6 the measurements were additionally performed in a field of 0.01 T upon cooling the samples either in zero (full triangles) or applied (open triangles) field. For the sake of clarity these magnetization data were multiplied by 10 (Tb2CuGe6) or 8 (Er2CuGe6).
about spin reorientation occurring also in this germanide, as in all the other counterparts. In order to verify the above presumptions and determine the magnetic structures in the R2CuGe6 ternaries neutron diffraction experiments are planned. 4. Discussion The results presented in this work indicate that all the investigated R2CuGe6 compounds order antiferromagnetically at low temperatures. The experimental values of the effective magnetic moment correspond to those expected for the magnetic moments localized on the rare earth atoms. Large interatomic ReR distances as well as metallic character of the electrical conductivity imply that the magnetic interactions can be predominantly described as an indirect coupling via conduction electrons, i.e. within the RKKY model. In this approach the ordering temperatures are proportional to the de Gennes factor G ¼ ( gJ 1)2J(J þ 1) and may be expressed as [5] 2 2 TN ¼ IðgJ 1Þ JðJ þ 1Þ; 3
ð1Þ
where I stands for the exchange interaction parameter, J is the total angular momentum of the Hund’s rule ground state of the rare earth ion, while gJ denotes the Lande´ factor. In Fig. 3(a) the experimental values of the ordering temperatures
of the R2CuGe6 phases are shown as a function of the G factor. Whereas the overall trend of increasing TN with rising G is consistent with the RKKY interaction, the observed dependence is strongly non-linear, presumably because of strong crystalline electric field (CEF) effect. As is apparent from Fig. 3(a), quite similar behavior can be anticipated for the isostructural R2NiGe6 compounds [6]. Within the mean field approximation the paramagnetic Curie temperature measures the indirect interactions between the local magnetic moments. Therefore qp may also be expected to scale with the strength of the RKKY interactions and hence with the magnitude of the de Gennes factor. The relevant formula is given by [5] kqp ¼
3p z2 m Jsf2 ðgJ 1Þ2 JðJ þ 1Þ; 4 Z2 kF2
ð2Þ
where Jsf is the effective interaction between the conduction electrons and the local moments, kF is the Fermi wave-vector, m* stands for the effective mass and z is the average conduction electron concentration. Fig. 3(b) presents the respective data for the R2NiGe6 and R2CuGe6 series. Interestingly, for the Ni-containing ternaries the relation (2) is nearly linear, yet for the R2CuGe6 family some deviation from a straightline behavior is observed, which may result from the CEF effect.
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D. Kaczorowski et al. / Solid State Sciences 10 (2008) 1891e1894
Fig. 4. The critical magnetic fields of the metamagnetic phase transitions in the R2NiGe6 and R2CuGe6 series plotted against the ratio defined in Eq. (3). The data for R2NiGe6 were taken from [6].
value of the paramagnetic Curie temperature, and is given by the formula [6] 3kB TN þ qp : ð3Þ Bc f 2gJ ðJ þ 1ÞmB The above dependence derived for the R2CuGe6 and R2NiGe6 compounds is presented in Fig. 4. Apparently, for neither of the series a firm proportionality is observed, however, the values of the critical field Bc continuously decrease with increasing the number of the 4f electrons. References Fig. 3. (a) The Ne´el temperatures and (b) the paramagnetic Curie temperatures for the R2NiGe6 and R2CuGe6 compounds plotted as a function of the de Gennes parameter. The data for R2NiGe6 were taken from [6].
It is known that in intermetallics based on the rare earth metals with the orbital quantum number L s 0 the crystalline electric field effects lead to large magnetocrystalline anisotropy. In a simple model of an antiferromagnet in which all atoms experience the same molecular field it can be shown that the critical field of the metamagnetic process is proportional to the sum of the Ne´el temperature and the absolute
[1] M.B. Konyk, P.S. Salamakha, O.I. Bodak, V.K. Pecharsky, Kristallografiya 33 (1988) 838. [2] O. Sologub, K. Hiebl, P. Rogl, O.I. Bodak, J. Alloys Compd. 227 (1995) 37. [3] M.B. Konyk, L.P. Romaka, Yu.K. Gorelenko, O.I. Bodak, J. Alloys Compd. 311 (2000) 120. [4] L.G. Akselrud, Yu.N. Grin, P.Yu. Zavalii, V.K. Pecharsky, V.S. Fundamenskii, CSD-Universal Program Package for Single Crystal or Powder Structure Data Treatment, Collected Abstracts, 12th European Crystallography Meeting, vol. 3, Nauka, Moscow, 1989, p. 155. [5] D.R. Noakes, G.K. Shenoy, Phys. Lett. A91 (1982) 35. [6] M. Konyk, L. Romaka, D. Gignoux, D. Fruchart, O. Bodak, Yu. Gorelenko, J. Alloys Compd. 398 (2005) 8.