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Crystal structure and magnetic properties of RCu5Sn compounds (R–Y, Gd–Yb)
PERGAMON
Solid State Communications 119 (2001) 423±427
www.elsevier.com/locate/ssc
Crystal structure and magnetic properties of RCu5Sn compounds (R±Y, Gd±Yb) Ya. Mudryk a, O. Isnard b,*, L. Romaka a,b, D. Fruchart b a Department of Inorganic Chemistry, I. Franko L'viv National University, Kyryl and Mephodiy str. 8, 79005 Lviv, Ukraine Laboratoire de Cristallographie, associe aÁ l'Universite CNRS-UJF, J. Fourier et aÁ l'INPG, BP 166, 38042 Grenoble Cedex 9, France
b
Received 14 April 2001; received in revised form 5 June 2001; accepted 5 June 2001 by P. Burlet
Abstract The new isostructural RCu5Sn compounds, where R Y, Gd±Yb, were prepared and their crystal structure was studied. These compounds crystallize in the CeCu6 structure type (space group Pnma). Rietveld re®nement of the X-ray powder diffraction has been carried out to analyze the crystal structure for R Er. The magnetic properties of RCu5Sn compounds were investigated from 2 to 300 K under a magnetic ®eld of 7 T. The ordering temperature and the effective magnetic moments were determined. The TmCu5Sn and YbCu5Sn compounds do not exhibit any magnetic order at least down to 2 K. X-ray absorption spectroscopy performed at the L edges reveals the intermediate valence state of Yb in YbCu5Sn. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 61.102i; 61.10.Ht; 75.302m; 75.30.Mb Keywords: C. Crystal structure; C. X-ray analysis; E. X-ray absorption spectroscopy
1. Introduction The existence of numerous isostructural series of intermetallic compounds in the R±M±X ternary systems (R Ð rare-earth element, M Ð transition metal, X Ð p-element of the III±V groups), have attracted large attention for a long period time. Especially many compounds are known with X Sn [1]. In the R±Cu±Sn ternary systems the following series were obtained: RCuSn with R Ce to Lu [2], R3Cu3Sn4 with R La to Tm [3], and RCu5Sn with R La to Sm [4,5]. The physical properties of these compounds were intensively investigated [2±7]. The RCu5Sn isostructural series was investigated for light rare earth elements [4,5], and the GdCu5Sn compound was also obtained [5]. These compounds were described in Ref. 4 with the CeCu6 structure type (ST) [8], (space group (SG) Pnma), and the existence of stability ranges was established (the phase composition is shown as RCu5^xSn1^x). Fornasini et al. [5], have proposed the CeCu5Au structure type [9], a superstructure of CeCu6, because their results led to the full * Corresponding author. Tel.: 133-4-76881146; fax: 133-476881038. E-mail address: [email protected] (O. Isnard).
atomic position ordering in these compounds. A detailed investigation of the CeCu5^xSn1^x compound has been done recently [10] and depending upon the concentration a cross over between antiferro-ordering and Kondo like behavior has been established. The CeCu5Sn compound orders antiferromagnetically at 10 K. PrCu5Sn is a Curie± Weiss. In the present paper we report on the existence of the RCu5Sn compounds with heavy rare-earth elements since the ErCu5Sn was obtained previously [11]. Besides we determine their crystal structure and measure their magnetic properties. These physical properties are presented and analyzed in the light of earlier results obtained. 2. Experimental Samples were prepared by direct arc melting of the constituent elements (rare earth purity better than 99.8, copper 99.9, tin 99.99 wt%) under high purity argon atmosphere on a water-cooled copper crucible. The weight losses were generally less than 1 wt%. Then the alloys were annealed at 870 K in evacuated quartz tubes for 1000 h and quenched in cold water.
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(01)00273-3
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Y. Mudryk et al. / Solid State Communications 119 (2001) 423±427
Table 1 Atomic and thermal parameters of the ErCu4.5Sn1.5 compound, obtained by Rietveld re®nement of X-ray powder diffraction pattern Atom
Site
x/a
y/b
z/c
B (nm 2)
Occupancy
Er Sn Cu1 Cu2 Cu3 Cu±Sn4 a
4c 4c 4c 4c 4c 8d
0.2509(8) 0.1410(7) 0.3178(13) 0.058(2) 0.4058(14) 0.4252(8)
0.2500 0.2500 0.2500 0.2500 0.2500 2 0.0026(14)
0.4369(5) 0.1405(6) 0.7562(10) 0.9011(12) 0.9827(11) 0.1891(9)
0.39 0.43 0.47 0.50 0.55 1.04
1 1 1 1 1 1
a
0.789(14) Cu 1 0.211(14) Sn.
Phase control analysis was carried out using X-ray powder ®lm data obtained using the Debye±Scherrer technique (RKD-57.3 camera, Cr K radiation). Lattice parameters were calculated using powder patterns obtained on a DRON-2.0 powder diffractometer (Fe Ka radiation). For the crystal structure re®nements of the new compounds we used the ErCu5Sn powder pattern obtained on the HZG-4A automatic diffractometer (Cu Ka-radiation). Diffraction data were collected in the 2u range 15±1408 with 2u step of 0.058. The calculations of theoretical patterns and lattice parameters were performed using the CSD program package [12]. The determination of the crystal structure parameters was performed using the Rietveld re®nement with the DBWS 9807 program. The agreement factors used to evaluate the goodness of the Rietveld analysis are de®ned according to reference [13]. The magnetic measurements were performed using the so-called `axial extraction method'. From 2 to 300 K, the experiments were carried out in a liquid-helium cryostat with super conducting coils, providing a continuous magnetic ®eld up to 7 T. A complete description of this apparatus can be found in Ref. [14]. The X-ray absorption spectroscopy experiments were performed at the DCI synchrotron radiation storage ring at LURE (ORSAY), on the EXAFS 13 experimental station which uses a bending magnet. The monochromator consisted of two parallel Si crystals cut along the (311) plane. The rejection of harmonics was performed by adjustTable 2 Lattice parameters and cell volume for the RCu5Sn compounds Compound
YCu5Sn GdCu5Sn TbCu5Sn DyCu5Sn HoCu5Sn ErCu5Sn TmCu5Sn YbCu5Sn
Ê) Lattice parameters (A
Ê 3) V (A
a
b
c
8.203(3) 8.235(2) 8.207(3) 8.197(3) 8.183(3) 8.1719(4) 8.147(1) 8.284(2)
9.975(1) 4.989(1) 4.972(1) 4.964(1) 4.959(1) 4.9505(2) 4.932(1) 5.019(1)
10.520(4) 10.572(3) 10.555(3) 10.538(4) 10.542(3) 10.5564(6) 10.540(1) 10.526(3)
429.320(12) 434.344(6) 430.699(9) 428.790(12) 427.789(9) 426.357(1) 423.508(1) 437.644(6)
ing the parallelism between the crystals. The detection used two ionization chambers ®lled with air. The energy scale was calibrated by reference to the Cu K absorption edge. A calibrated amount of powder was mixed with cellulosis in order to optimize the edge jump. The determination of the valence state of Yb has been performed using a deconvolution technique from the LIII spectrum recorded at room temperature. The deconvolution technique is based on artan function which describes the transition from the 2p to the continuum states and a lorentzian function that takes into account both the 5d density of unoccupied states and the ®nite lifetime of the 2p core hole. The equation which is used is taken from Ref. [15].
3. Results The crystal structure of the RCu5Sn compounds was solved by powder method for ErCu5Sn compound. A detailed investigation of the crystal structure was carried out on an alloy of the Er14Cu72Sn14 composition. The Bragg re¯ections of the powder pattern were indexed on the basis of orthorhombic lattice with cell parameters a At ®rst, the 8:1719 4; b 4:9505 2; c 10:5564 6A: CeCu5Au structure type (SG Pnma, superstructure of the CeCu6 type) [9] was chosen as a starting model for structure re®nement. It was proposed that Er atom occupies the Ce position, Sn occupies the position of Au. The calculations were performed using a pseudo-Voigt pro®le function and a polynomial function was used for the background. A ®rst ®t has been performed using the CeCu5Sn structure type and constraining the stoichiometry to be ErCu5Sn. However, the B thermal parameter found for the 8d (Cu) position took a negative value. The presence of very small amounts of a second phase was observed (near 5 mass% content). This second phase was found to be ErCuSn. A better ®t was obtained with a (Cu,Sn) statistical distribution 0.789(14) Cu 1 0.211(14) Sn for the 8d atomic position. It is worth noting that with this model all the re®ned isotropic thermal parameters are found to be reasonable. Finally, we obtained better results characterized by agreement factors of Rp 3:58%; Rwp 5:65% as presented in Table 1. In this case, the stoichiometric composition of our sample weakly
Y. Mudryk et al. / Solid State Communications 119 (2001) 423±427
Fig. 1. Unit cell volume vs. rare earth elements in the RCu5Sn phases.
deviates from ErCu5Sn to ErCu4.5Sn1.5 and reveals the existence of a small range of stability in the investigated compounds. The interatomic Er±Cu, Sn±Cu and Cu±Cu distances of the ErCu4.5Sn1.5 compound are slightly reduced in comparison with the sum of the appropriate atomic radii. For example, d Er2Cu3 distance is equal to 0.283 nm whereas the sum rEr 1 rCu is equal to 0.304 nm. This may be explained by the smaller atomic size of Cu in this compound than in the pure metallic copper. Isotypic compounds RCu5Sn were obtained with R Y, Gd, Tb, Dy, Ho, Tm and Yb. The lattice parameters for these investigated compounds were calculated and are summarized in Table 2. The dependence of the unit cell volume vs. rare earth elements is typical for lanthanide elements: the unit cell volume decreases from Gd to Tm as shown in Fig. 1. Whereas a nearly linear decrease of the volume is observed from Gd to Tm, a volume anomaly appears for YbCu5Sn. The unit cell volume of YbCu5Sn is higher than those of the other compounds. It bears witness to a divalent or intermediate valence state of Yb. It is to be reminded that for the earlier reported CeCu62xSnx phases a peculiar behavior of Ce was evidenced, supported by a cross over from Kondo lattice to an antiferromagnetic ordering. The RCu5Sn samples (R Gd to Yb) were investigated by iso®eld measurements in order to determine the magnetic
425
Fig. 3. Thermal variation of the susceptibility recorded for GdCu5Sn (left scale), TbCu5Sn and HoCu5Sn (right scale) compounds.
ordering temperatures. The used magnetic ®eld was kept at 0.1 T for all the iso-®eld magnetic measurements. All the RCu5Sn compounds exhibit a Curie±Weiss behavior at high temperature except for YbCu5Sn. The thermal variation of the inverse of the susceptibility is given in Fig. 2 for TbCu5Sn and GdCu5Sn and DyCu5Sn. The thermal variation of the susceptibility recorded for GdCu5Sn, TbCu5Sn and HoCu5Sn compounds is presented in Fig. 3. It is clearly evidenced that the NeÂel temperature TN of TbCu5Sn ranges about 20:5 1 0:5 K. The NeÂel temperatures of the other RCu5Sn compounds are gathered in Table 3. The Curie temperature u p and the corresponding effective magnetic moments m eff are also reported in Table 3. The experimental values of the effective magnetic moments derived from the thermal behaviors of the reciprocal susceptibility curves are found to be very close to the theoretical values of the corresponding rare-earth ion in the trivalent state. The obtained NeÂel temperatures are markedly low, all ranging below 25 K whatever the rare-earth. According to the magnetization curves measured in ®eld up to 7 T, none of the studied compounds exhibit any ferromagnetic or ferrimagnetic component. It is worth noting that TmCu5Sn (see Fig. 4) and YbCu5Sn do not exhibit any magnetic order at least down to 2 K. For TmCu5Sn the thermal evolution of the susceptibility is of Curie±Weiss type. For the YbCu5Sn Table 3 Magnetic characteristics of the RCu5Sn phases, the error bars of the Neel temperature TNeÂel and the Curie paramagnetic temperatures u p were estimated to ^0.5 K
Fig. 2. Thermal variation of the inverse of the magnetic susceptibility for GdCu5Sn, DyCu5Sn and TbCu5Sn compounds.
Compounds
TNeÂel (K)
u p (K)
m eff (m B)
M31 R (m B)
CeCu5Sn GdCu5Sn TbCu5Sn DyCu5Sn HoCu5Sn ErCu5Sn TmCu5Sn YbCu5Sn
10.0 14.0 20.5 13.0 5.0 1.8 ± ±
2 4.6 2 24.6 2 15.0 0 2 10.0 2 3.0 0 ±
2.46 8.60 10.10 10.90 10.90 9.76 7.66 ±
2.54 7.94 9.72 10.65 10.61 9.58 7.56 ±
426
Y. Mudryk et al. / Solid State Communications 119 (2001) 423±427
Fig. 4. Thermal variation of the susceptibility of TmCu5Sn (left scale) and DyCu5Sn (right scale).
compound the reciprocal susceptibility does not obey such a linear variation with temperature, and no magnetic moment is found either localized or delocalized on Yb. The case of the YbCu5Sn compound is peculiar. This can be due to a non magnetic state of Yb i.e. either a divalent Yb state characterized by an electronic 4f 14 con®guration or an intermediate valence state of Yb resulting from a strong hybridization of 4f electrons with 5d states. To distinguish between both hypotheses, X-ray absorption spectroscopy is required to probe the electronic con®guration of Yb. The LII and LIII edge spectra of Yb have been recorded for YbCu5Sn. As can be seen from Fig. 5, the Yb LIII X-ray absorption edge spectrum exhibits two white lines which are interpreted as 2p 54f 13 and 2p 54f 14 ®nal states and the average valence is simply obtained from the relative intensities of the two edges. Each white line is decomposed into a Lorentzian function that describes the transition towards the localized 5d states and an artan function that describes the transitions to the continuum states. Fig. 5 represent the observed LIII X-ray absorption edge spectrum together with the ®t to the experimental data. The re®nement of the LIII spectrum yielded a valence of 2.47(1) for Yb in YbCu5Sn. This demonstrates that in this compound Yb is in an intermediate valence state. This value is close to that has been reported
for other stannides compounds [16]. The energy shift between the two white lines in the LIII spectrum is re®ned to be 7 eV. The LIII X-ray absorption edge spectrum of Yb in YbCu5Sn is particularly similar to that reported for (SnYb) Yb4Rh6Sn18 [16] but differs signi®cantly from that of Yb2O3 or pure metallic Yb. Only one crystallographic site exists in the structure of YbCu5Sn. Consequently, the intermediate valence state is related to valence ¯uctuation state of Yb. At this point, it is worth reminding that the anomaly of YbCu5Sn among the RCu5Sn series is also evidenced by the evolution of the unit cell volume across the rare-earth series. This is consistent with the observed intermediate valence state of Yb in YbCu5Sn phase discovered by X-ray absorption spectroscopy. The Cu K absorption edge has also been recorded and is presented on Fig. 6. Its shape is found to be very similar to the XANES (X-ray absorption near edge structure) reported for pure metallic copper. 4. Conclusions 1. A new series of RCu5Sn compounds, where R Y, Gd± Yb, has been synthesized and their crystal structure studied. X-ray powder diffraction shows that the crystal structure of these compounds belongs to the CeCu6 type (SG Pnma). A detailed investigation of the crystal structure has been carried out by Rietveld re®nement on ErCu4.5Sn1.5. 2. The magnetic properties of RCu5Sn compounds were measured (2±300 K) and analyzed. The effective magnetic moment of the R elements are close to the values of the trivalent state. All the RCu5Sn compounds exhibit a Curie±Weiss behavior at high temperature except YbCu5Sn. 3. The ®t of the Yb LIII X-ray absorption edge spectrum yielded an intermediate valence state of 2.47(1) in
Fig. 5. XANES signal recorded at the Yb LIII absorption edge in YbCu5Sn. The diamonds and the solid line represent the experimental point and the ®t respectively. The corresponding model decompositions into 4f 13 and 4f 14 contributions are also shown in the lower part of the ®gure by the two sets of artan and lorentzian functions.
Y. Mudryk et al. / Solid State Communications 119 (2001) 423±427
427
Fig. 6. XANES spectra of the Cu K absorption edge in YbCu5Sn.
YbCu5Sn. This value is consistent with the anomalous unit cell parameters observed for YbCu5Sn. Acknowledgements This research was partly supported by a grant from the French MinisteÁre de l'Education Nationale (L.R.). The authors acknowledge with thanks the Laboratoire de l'Utilisation du Rayonnement ElectromagneÂtique at Orsay where the X-ray absorption spectroscopy has been carried out. The authors wish to thank the staff of LURE for experimental accommodations. References [1] R.V. Skolozdra, Handbook on the Physics and Chemistry of Rare-Earths, Vol. 24, Elsevier, Oxford, 1997, pp. 399±517. [2] L.P. Komarovskaya, R.V. Skolozdra, I.V. Filatova, Dokl. Akad. Nauk Ukr. SSR, Ser. A 1 (1983) 82. [3] R.V. Skolozdra, L.P. Komarovskaya, L.G. Akselrud, Ukr. Fiz. Zh. 29 (1984) 1395. [4] R.V. Skolozdra, L.P. Romaka, L.G. Akselrud, J. Pierre, J. Alloys Compd 262-263 (1997) 346±349.
[5] M.L. Fornasini, R. Marazza, D. Mazzone, P. Riani, G. Zanicchi, Z. Kristallogr. 213 (1998) 108. [6] S. Baran, V. Ivanov, J. Leciejewicz, N. Stusser, A. Szytula, A. Zygmunt, Yong Fan Ding, J. Alloys Compd 257 (1997) 5±13. [7] S. Singh, S.K. Dhar, P. Manfrinetti, A. Palenzona, J. Alloys Compd 298 (2000) 8±72. [8] D.T. Cromer, A.C. Larson, R.B. Roof Jr., J. Acta Crystallogr. 13 (1960) 913. [9] M. Ruck, G. Portisch, H.G. Schlager, M. Sieck, H.V. Lohneysen, Acta Crystallogr.: Sec. B 49 (1993) 936±941. [10] O. Isnard, J. Pierre, D. Fruchart, L.P. Romaka, R.V. Skolozdra, Solid State Commun. 113 (1999) 335±340. [11] I.V. Senkovska, Ya.S. Mudryk, L.P. Romaka, O.I. Bodak, J. Alloys Compd 312 (2000) 124±129. [12] L.G. Akselrud, Yu.N. Gryn, P.Yu. Zavalij, V.K. Pecharsky, V.S. Fundamensky, XII European Crystallography Meeting, Collected Abstracts, Moscow, 1989, p. 15. [13] L.B. McCuskerR, R.B. Von Dreele, D.E. Cox, D. LouÈer, P. Scardi, J. Appl. Crystallogr. 32 (1999) 36. [14] A. Barlet, J.C. Genna, P. Lethuillier, Cryogenics 31 (1991) 801. [15] J. RoÈhler, J. Magn. Magn. Mater. 47 (1985) 175. [16] P. Bordet, J.L. Hodeau, P. Wolfers, G. Krill, F. Weiss, M. Marezio, J. Magn. Magn. Mater. 63-64 (1987) 524.
Solid State Communications 119 (2001) 423±427
www.elsevier.com/locate/ssc
Crystal structure and magnetic properties of RCu5Sn compounds (R±Y, Gd±Yb) Ya. Mudryk a, O. Isnard b,*, L. Romaka a,b, D. Fruchart b a Department of Inorganic Chemistry, I. Franko L'viv National University, Kyryl and Mephodiy str. 8, 79005 Lviv, Ukraine Laboratoire de Cristallographie, associe aÁ l'Universite CNRS-UJF, J. Fourier et aÁ l'INPG, BP 166, 38042 Grenoble Cedex 9, France
b
Received 14 April 2001; received in revised form 5 June 2001; accepted 5 June 2001 by P. Burlet
Abstract The new isostructural RCu5Sn compounds, where R Y, Gd±Yb, were prepared and their crystal structure was studied. These compounds crystallize in the CeCu6 structure type (space group Pnma). Rietveld re®nement of the X-ray powder diffraction has been carried out to analyze the crystal structure for R Er. The magnetic properties of RCu5Sn compounds were investigated from 2 to 300 K under a magnetic ®eld of 7 T. The ordering temperature and the effective magnetic moments were determined. The TmCu5Sn and YbCu5Sn compounds do not exhibit any magnetic order at least down to 2 K. X-ray absorption spectroscopy performed at the L edges reveals the intermediate valence state of Yb in YbCu5Sn. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 61.102i; 61.10.Ht; 75.302m; 75.30.Mb Keywords: C. Crystal structure; C. X-ray analysis; E. X-ray absorption spectroscopy
1. Introduction The existence of numerous isostructural series of intermetallic compounds in the R±M±X ternary systems (R Ð rare-earth element, M Ð transition metal, X Ð p-element of the III±V groups), have attracted large attention for a long period time. Especially many compounds are known with X Sn [1]. In the R±Cu±Sn ternary systems the following series were obtained: RCuSn with R Ce to Lu [2], R3Cu3Sn4 with R La to Tm [3], and RCu5Sn with R La to Sm [4,5]. The physical properties of these compounds were intensively investigated [2±7]. The RCu5Sn isostructural series was investigated for light rare earth elements [4,5], and the GdCu5Sn compound was also obtained [5]. These compounds were described in Ref. 4 with the CeCu6 structure type (ST) [8], (space group (SG) Pnma), and the existence of stability ranges was established (the phase composition is shown as RCu5^xSn1^x). Fornasini et al. [5], have proposed the CeCu5Au structure type [9], a superstructure of CeCu6, because their results led to the full * Corresponding author. Tel.: 133-4-76881146; fax: 133-476881038. E-mail address: [email protected] (O. Isnard).
atomic position ordering in these compounds. A detailed investigation of the CeCu5^xSn1^x compound has been done recently [10] and depending upon the concentration a cross over between antiferro-ordering and Kondo like behavior has been established. The CeCu5Sn compound orders antiferromagnetically at 10 K. PrCu5Sn is a Curie± Weiss. In the present paper we report on the existence of the RCu5Sn compounds with heavy rare-earth elements since the ErCu5Sn was obtained previously [11]. Besides we determine their crystal structure and measure their magnetic properties. These physical properties are presented and analyzed in the light of earlier results obtained. 2. Experimental Samples were prepared by direct arc melting of the constituent elements (rare earth purity better than 99.8, copper 99.9, tin 99.99 wt%) under high purity argon atmosphere on a water-cooled copper crucible. The weight losses were generally less than 1 wt%. Then the alloys were annealed at 870 K in evacuated quartz tubes for 1000 h and quenched in cold water.
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(01)00273-3
424
Y. Mudryk et al. / Solid State Communications 119 (2001) 423±427
Table 1 Atomic and thermal parameters of the ErCu4.5Sn1.5 compound, obtained by Rietveld re®nement of X-ray powder diffraction pattern Atom
Site
x/a
y/b
z/c
B (nm 2)
Occupancy
Er Sn Cu1 Cu2 Cu3 Cu±Sn4 a
4c 4c 4c 4c 4c 8d
0.2509(8) 0.1410(7) 0.3178(13) 0.058(2) 0.4058(14) 0.4252(8)
0.2500 0.2500 0.2500 0.2500 0.2500 2 0.0026(14)
0.4369(5) 0.1405(6) 0.7562(10) 0.9011(12) 0.9827(11) 0.1891(9)
0.39 0.43 0.47 0.50 0.55 1.04
1 1 1 1 1 1
a
0.789(14) Cu 1 0.211(14) Sn.
Phase control analysis was carried out using X-ray powder ®lm data obtained using the Debye±Scherrer technique (RKD-57.3 camera, Cr K radiation). Lattice parameters were calculated using powder patterns obtained on a DRON-2.0 powder diffractometer (Fe Ka radiation). For the crystal structure re®nements of the new compounds we used the ErCu5Sn powder pattern obtained on the HZG-4A automatic diffractometer (Cu Ka-radiation). Diffraction data were collected in the 2u range 15±1408 with 2u step of 0.058. The calculations of theoretical patterns and lattice parameters were performed using the CSD program package [12]. The determination of the crystal structure parameters was performed using the Rietveld re®nement with the DBWS 9807 program. The agreement factors used to evaluate the goodness of the Rietveld analysis are de®ned according to reference [13]. The magnetic measurements were performed using the so-called `axial extraction method'. From 2 to 300 K, the experiments were carried out in a liquid-helium cryostat with super conducting coils, providing a continuous magnetic ®eld up to 7 T. A complete description of this apparatus can be found in Ref. [14]. The X-ray absorption spectroscopy experiments were performed at the DCI synchrotron radiation storage ring at LURE (ORSAY), on the EXAFS 13 experimental station which uses a bending magnet. The monochromator consisted of two parallel Si crystals cut along the (311) plane. The rejection of harmonics was performed by adjustTable 2 Lattice parameters and cell volume for the RCu5Sn compounds Compound
YCu5Sn GdCu5Sn TbCu5Sn DyCu5Sn HoCu5Sn ErCu5Sn TmCu5Sn YbCu5Sn
Ê) Lattice parameters (A
Ê 3) V (A
a
b
c
8.203(3) 8.235(2) 8.207(3) 8.197(3) 8.183(3) 8.1719(4) 8.147(1) 8.284(2)
9.975(1) 4.989(1) 4.972(1) 4.964(1) 4.959(1) 4.9505(2) 4.932(1) 5.019(1)
10.520(4) 10.572(3) 10.555(3) 10.538(4) 10.542(3) 10.5564(6) 10.540(1) 10.526(3)
429.320(12) 434.344(6) 430.699(9) 428.790(12) 427.789(9) 426.357(1) 423.508(1) 437.644(6)
ing the parallelism between the crystals. The detection used two ionization chambers ®lled with air. The energy scale was calibrated by reference to the Cu K absorption edge. A calibrated amount of powder was mixed with cellulosis in order to optimize the edge jump. The determination of the valence state of Yb has been performed using a deconvolution technique from the LIII spectrum recorded at room temperature. The deconvolution technique is based on artan function which describes the transition from the 2p to the continuum states and a lorentzian function that takes into account both the 5d density of unoccupied states and the ®nite lifetime of the 2p core hole. The equation which is used is taken from Ref. [15].
3. Results The crystal structure of the RCu5Sn compounds was solved by powder method for ErCu5Sn compound. A detailed investigation of the crystal structure was carried out on an alloy of the Er14Cu72Sn14 composition. The Bragg re¯ections of the powder pattern were indexed on the basis of orthorhombic lattice with cell parameters a At ®rst, the 8:1719 4; b 4:9505 2; c 10:5564 6A: CeCu5Au structure type (SG Pnma, superstructure of the CeCu6 type) [9] was chosen as a starting model for structure re®nement. It was proposed that Er atom occupies the Ce position, Sn occupies the position of Au. The calculations were performed using a pseudo-Voigt pro®le function and a polynomial function was used for the background. A ®rst ®t has been performed using the CeCu5Sn structure type and constraining the stoichiometry to be ErCu5Sn. However, the B thermal parameter found for the 8d (Cu) position took a negative value. The presence of very small amounts of a second phase was observed (near 5 mass% content). This second phase was found to be ErCuSn. A better ®t was obtained with a (Cu,Sn) statistical distribution 0.789(14) Cu 1 0.211(14) Sn for the 8d atomic position. It is worth noting that with this model all the re®ned isotropic thermal parameters are found to be reasonable. Finally, we obtained better results characterized by agreement factors of Rp 3:58%; Rwp 5:65% as presented in Table 1. In this case, the stoichiometric composition of our sample weakly
Y. Mudryk et al. / Solid State Communications 119 (2001) 423±427
Fig. 1. Unit cell volume vs. rare earth elements in the RCu5Sn phases.
deviates from ErCu5Sn to ErCu4.5Sn1.5 and reveals the existence of a small range of stability in the investigated compounds. The interatomic Er±Cu, Sn±Cu and Cu±Cu distances of the ErCu4.5Sn1.5 compound are slightly reduced in comparison with the sum of the appropriate atomic radii. For example, d Er2Cu3 distance is equal to 0.283 nm whereas the sum rEr 1 rCu is equal to 0.304 nm. This may be explained by the smaller atomic size of Cu in this compound than in the pure metallic copper. Isotypic compounds RCu5Sn were obtained with R Y, Gd, Tb, Dy, Ho, Tm and Yb. The lattice parameters for these investigated compounds were calculated and are summarized in Table 2. The dependence of the unit cell volume vs. rare earth elements is typical for lanthanide elements: the unit cell volume decreases from Gd to Tm as shown in Fig. 1. Whereas a nearly linear decrease of the volume is observed from Gd to Tm, a volume anomaly appears for YbCu5Sn. The unit cell volume of YbCu5Sn is higher than those of the other compounds. It bears witness to a divalent or intermediate valence state of Yb. It is to be reminded that for the earlier reported CeCu62xSnx phases a peculiar behavior of Ce was evidenced, supported by a cross over from Kondo lattice to an antiferromagnetic ordering. The RCu5Sn samples (R Gd to Yb) were investigated by iso®eld measurements in order to determine the magnetic
425
Fig. 3. Thermal variation of the susceptibility recorded for GdCu5Sn (left scale), TbCu5Sn and HoCu5Sn (right scale) compounds.
ordering temperatures. The used magnetic ®eld was kept at 0.1 T for all the iso-®eld magnetic measurements. All the RCu5Sn compounds exhibit a Curie±Weiss behavior at high temperature except for YbCu5Sn. The thermal variation of the inverse of the susceptibility is given in Fig. 2 for TbCu5Sn and GdCu5Sn and DyCu5Sn. The thermal variation of the susceptibility recorded for GdCu5Sn, TbCu5Sn and HoCu5Sn compounds is presented in Fig. 3. It is clearly evidenced that the NeÂel temperature TN of TbCu5Sn ranges about 20:5 1 0:5 K. The NeÂel temperatures of the other RCu5Sn compounds are gathered in Table 3. The Curie temperature u p and the corresponding effective magnetic moments m eff are also reported in Table 3. The experimental values of the effective magnetic moments derived from the thermal behaviors of the reciprocal susceptibility curves are found to be very close to the theoretical values of the corresponding rare-earth ion in the trivalent state. The obtained NeÂel temperatures are markedly low, all ranging below 25 K whatever the rare-earth. According to the magnetization curves measured in ®eld up to 7 T, none of the studied compounds exhibit any ferromagnetic or ferrimagnetic component. It is worth noting that TmCu5Sn (see Fig. 4) and YbCu5Sn do not exhibit any magnetic order at least down to 2 K. For TmCu5Sn the thermal evolution of the susceptibility is of Curie±Weiss type. For the YbCu5Sn Table 3 Magnetic characteristics of the RCu5Sn phases, the error bars of the Neel temperature TNeÂel and the Curie paramagnetic temperatures u p were estimated to ^0.5 K
Fig. 2. Thermal variation of the inverse of the magnetic susceptibility for GdCu5Sn, DyCu5Sn and TbCu5Sn compounds.
Compounds
TNeÂel (K)
u p (K)
m eff (m B)
M31 R (m B)
CeCu5Sn GdCu5Sn TbCu5Sn DyCu5Sn HoCu5Sn ErCu5Sn TmCu5Sn YbCu5Sn
10.0 14.0 20.5 13.0 5.0 1.8 ± ±
2 4.6 2 24.6 2 15.0 0 2 10.0 2 3.0 0 ±
2.46 8.60 10.10 10.90 10.90 9.76 7.66 ±
2.54 7.94 9.72 10.65 10.61 9.58 7.56 ±
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Y. Mudryk et al. / Solid State Communications 119 (2001) 423±427
Fig. 4. Thermal variation of the susceptibility of TmCu5Sn (left scale) and DyCu5Sn (right scale).
compound the reciprocal susceptibility does not obey such a linear variation with temperature, and no magnetic moment is found either localized or delocalized on Yb. The case of the YbCu5Sn compound is peculiar. This can be due to a non magnetic state of Yb i.e. either a divalent Yb state characterized by an electronic 4f 14 con®guration or an intermediate valence state of Yb resulting from a strong hybridization of 4f electrons with 5d states. To distinguish between both hypotheses, X-ray absorption spectroscopy is required to probe the electronic con®guration of Yb. The LII and LIII edge spectra of Yb have been recorded for YbCu5Sn. As can be seen from Fig. 5, the Yb LIII X-ray absorption edge spectrum exhibits two white lines which are interpreted as 2p 54f 13 and 2p 54f 14 ®nal states and the average valence is simply obtained from the relative intensities of the two edges. Each white line is decomposed into a Lorentzian function that describes the transition towards the localized 5d states and an artan function that describes the transitions to the continuum states. Fig. 5 represent the observed LIII X-ray absorption edge spectrum together with the ®t to the experimental data. The re®nement of the LIII spectrum yielded a valence of 2.47(1) for Yb in YbCu5Sn. This demonstrates that in this compound Yb is in an intermediate valence state. This value is close to that has been reported
for other stannides compounds [16]. The energy shift between the two white lines in the LIII spectrum is re®ned to be 7 eV. The LIII X-ray absorption edge spectrum of Yb in YbCu5Sn is particularly similar to that reported for (SnYb) Yb4Rh6Sn18 [16] but differs signi®cantly from that of Yb2O3 or pure metallic Yb. Only one crystallographic site exists in the structure of YbCu5Sn. Consequently, the intermediate valence state is related to valence ¯uctuation state of Yb. At this point, it is worth reminding that the anomaly of YbCu5Sn among the RCu5Sn series is also evidenced by the evolution of the unit cell volume across the rare-earth series. This is consistent with the observed intermediate valence state of Yb in YbCu5Sn phase discovered by X-ray absorption spectroscopy. The Cu K absorption edge has also been recorded and is presented on Fig. 6. Its shape is found to be very similar to the XANES (X-ray absorption near edge structure) reported for pure metallic copper. 4. Conclusions 1. A new series of RCu5Sn compounds, where R Y, Gd± Yb, has been synthesized and their crystal structure studied. X-ray powder diffraction shows that the crystal structure of these compounds belongs to the CeCu6 type (SG Pnma). A detailed investigation of the crystal structure has been carried out by Rietveld re®nement on ErCu4.5Sn1.5. 2. The magnetic properties of RCu5Sn compounds were measured (2±300 K) and analyzed. The effective magnetic moment of the R elements are close to the values of the trivalent state. All the RCu5Sn compounds exhibit a Curie±Weiss behavior at high temperature except YbCu5Sn. 3. The ®t of the Yb LIII X-ray absorption edge spectrum yielded an intermediate valence state of 2.47(1) in
Fig. 5. XANES signal recorded at the Yb LIII absorption edge in YbCu5Sn. The diamonds and the solid line represent the experimental point and the ®t respectively. The corresponding model decompositions into 4f 13 and 4f 14 contributions are also shown in the lower part of the ®gure by the two sets of artan and lorentzian functions.
Y. Mudryk et al. / Solid State Communications 119 (2001) 423±427
427
Fig. 6. XANES spectra of the Cu K absorption edge in YbCu5Sn.
YbCu5Sn. This value is consistent with the anomalous unit cell parameters observed for YbCu5Sn. Acknowledgements This research was partly supported by a grant from the French MinisteÁre de l'Education Nationale (L.R.). The authors acknowledge with thanks the Laboratoire de l'Utilisation du Rayonnement ElectromagneÂtique at Orsay where the X-ray absorption spectroscopy has been carried out. The authors wish to thank the staff of LURE for experimental accommodations. References [1] R.V. Skolozdra, Handbook on the Physics and Chemistry of Rare-Earths, Vol. 24, Elsevier, Oxford, 1997, pp. 399±517. [2] L.P. Komarovskaya, R.V. Skolozdra, I.V. Filatova, Dokl. Akad. Nauk Ukr. SSR, Ser. A 1 (1983) 82. [3] R.V. Skolozdra, L.P. Komarovskaya, L.G. Akselrud, Ukr. Fiz. Zh. 29 (1984) 1395. [4] R.V. Skolozdra, L.P. Romaka, L.G. Akselrud, J. Pierre, J. Alloys Compd 262-263 (1997) 346±349.
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