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Structural and magnetic properties of the new hydride CeAuAlH1.4(1)
Journal of Alloys and Compounds 334 (2002) 20–26
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Structural and magnetic properties of the new hydride CeAuAlH 1.4( 1 ) B. Chevalier*, J-L. Bobet, M.L. Kahn, F. Weill, J. Etourneau ` Condensee ´ de Bordeaux ( ICMCB), CNRS [ UPR 9048], Universite´ Bordeaux I, Avenue du Docteur A. Schweitzer, Institut de Chimie de la Matiere 33608 Pessac Cedex, France Received 19 June 2001; accepted 3 July 2001
Abstract CeAuAl which orders antiferromagnetically below T N 53.8 K is considered as a Kondo lattice. This intermetallic absorbs hydrogen at room temperature and at a pressure of 4.5 MPa. The new hydride CeAuAlH 1.4( 1 ) is stable in air and crystallises in a derivative structure of the hexagonal Ni 2 In-type with a50.4427(1) nm and c50.8467(1) nm as unit cell parameters. Moreover, it exhibits an antiferromagnetic order below T N 58.0(2) K and another magnetic transition at T 1 53.0(2) K. The strong increase of the T N -temperature after the insertion of hydrogen in CeAuAl can be explained on the basis of the Doniach diagram; the increase of the unit cell volume induced by this insertion leads to a strong reduction of the influence of the Kondo effect. 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Hydrogen absorbing materials; X-ray diffraction; Magnetic measurements
1. Introduction The unusual properties at low temperatures of many compounds based on cerium such as the Kondo effect, intermediate valence behaviour, heavy fermions behaviour, magnetic Kondo systems, . . . are governed by the strength of the interaction Jcf between the spins of localized 4f(Ce) electrons and conduction electrons. The competition between Kondo interaction which causes quenching of the magnetic moment of the 4f(Ce) ion due to the formation of a singlet state and Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction which favours the occurrence of a long-range magnetic order, is dependent on Jcf . Considering the Jcf values, Doniach has proposed a magnetic phase diagram [1]: (i) for small Jcf , the compound exhibits a magnetic RKKY metal behaviour; (ii) for larger Jcf , the Kondo effect is strong and the compound shows an intermediate valence behaviour; (iii) between these two domains appears the magnetic Kondo systems where the magnetic moment is reduced by the Kondo effect. The diagram indicates that, with increasing Jcf , the magnetic ordering temperature increases initially, then goes through a maximum and finally decreases rapidly by driving the compound to a non-magnetic Kondo state. *Corresponding author. Tel.: 133-5-5684-6336; fax: 133-5-56842761. E-mail address: [email protected] (B. Chevalier).
In this view, it is interesting to modify the Jcf interaction existing in one compound containing cerium atoms. Mainly two routes are known: (i) the application of hydrostatic pressure which increases Jcf ; (ii) the insertion of hydrogen in the lattice which induces on the contrary a decrease of Jcf . Recently, electrical resistivity measurements performed under pressure on CeNi 2 Ge 2 (unconventional non-magnetic metal) [2,3], CeRh 2 Si 2 (antiferromagnet below T N 5 36 K) [4,5] and CePd 2 Si 2 (Kondo antiferromagnet below T N 510.6 K) [2,3,6,7] indicate the occurrence of superconducting transition around 220, 350–400 and 430–520 mK, respectively. In all cases, the formation of a superconducting phase is closely linked to the disappearance of magnetism induced by an increase of the Kondo effect under pressure. On the other hand, the insertion of hydrogen leads to an expansion of the unit cell volume inducing a decrease of Jcf and driving the compound towards a magnetic state. For instance, the hydrogenation of CeNiAl [8] and CeIrAl [9] leads to a valence transition for cerium from intermediate valence state to trivalent state. Also, the hydride CePtAlH 1.1 exhibits a Curie temperature T C 511.6 K twice greater than that determined for the initial Kondo magnet CePtAl (T C 55.6 K) [10]. Extending our search on new hydrides based on cerium, we have studied the hydrogen absorption properties of CeAuAl. This intermetallic crystallises in the orthorhombic TiNiSi-type structure [11] and orders antiferromagnetically below T N 53.8 K [12]. Moreover, its investigation by
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01748-0
B. Chevalier et al. / Journal of Alloys and Compounds 334 (2002) 20 – 26
magnetoresistance measurements suggests that its Kondo ´ temperatemperature T K (3.6 K is comparable to the Neel ture T N indicating a strong competition between Kondo and RKKY interactions [13]. In this paper, we report on the synthesis of the hydride CeAuAlH x and on its characterization using X-ray powder diffraction and magnetization measurements. We show that the insertion of hydrogen in CeAuAl induces both a structural transition ´ temperature. Also, the and a strong increase of the Neel hydride CeAuAlH x presents an interesting magnetic phase diagram.
2. Experimental techniques The sample CeAuAl was synthesised by arc-melting a stoichiometric mixture of pure elements in an atmosphere of high purity argon. Then, the sample was turned and remelted several times to ensure homogeneity. The weight loss during the arc-melting process was smaller than 0.5 wt.%. Finally, the ingot was enclosed in evacuated quartz tubes and annealed for 3 weeks at 1023 K. Its characterization by X-ray powder diffraction (Philips PW 1050 diffractometer, Cu Ka radiation) confirms that CeAuAl adopts the orthorhombic TiNiSi-type with unit cell parameters in agreement with those reported previously [11]. Hydrogen absorption–desorption experiments were performed using the apparatus described previously [14]. An ingot of sample was heated under vacuum at 473 K for 12 h and then exposed to hydrogen gas at various temperatures and pressures. The amount of H-atoms absorbed was determined volumetrically by monitoring pressure changes in a calibrated volume. Magnetization measurements were performed using a Superconducting Quantum Interference Device (SQUID) magnetometer in the temperature range 1.8–300 K and applied fields up to 5 T.
3. Results and discussion
3.1. Hydrogen absorption–desorption properties CeAuAl bulk compound was exposed to hydrogen gas at room temperature. The absorption starts only at pressures above 4.5 MPa even when an annealing treatment up to 473 K under vacuum is made before absorption. A complete decrepitation of the bulk material is observed and the X-ray powder pattern clearly shows that the hydrogenation leads to a structural change (Fig. 1). The hydride formed (i.e. CeAuAlH 1.4( 1) ) is stable in air suggesting that the equilibrium pressure is below 0.1 MPa. As a matter of fact, desorption is only observed after heating at 573 K under primary vacuum (i.e. about 10 Pa). The desorption is complete as the sample absorbs again 1.4(1) H per formula during the second absorption. However, the absorption
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pressure required is only 0.3 MPa. After a second desorption (i.e. 10 Pa and 573 K), a pressure below 0.1 MPa is sufficient to form CeAuAlH 1.4( 1) . The determination of the absorption and desorption equilibrium pressure at high temperature will be done in the near future in order to calculate the thermodynamic properties and then to estimate the equilibrium pressure at room temperature.
3.2. Structural properties The insertion of hydrogen in CeAuAl modifies its structural properties. The X-ray powder pattern of the CeAuAlH 1.4(1 ) hydride can be indexed on the basis of a hexagonal unit cell with the Ni 2 In-type structure (Fig. 1). The refined unit cell parameters are a50.4427(1) nm and c50.8467(1) nm. Table 1 summarizes the X-ray powder diffraction data of this hydride. Similar structural transition from orthorhombic TiNiSi-type structure to hexagonal Ni 2 In-type structure has been observed recently for the hydrogenation of equiatomic ternary compound CeIrAl [9]. This last type of structure adopts the P63 /mmc (no. 194) space group where Ce, Au and Al atoms occupy, respectively, the sites 2a (0 0 0), 2c (1 / 3 2 / 3 1 / 4) and 2d (1 / 3 2 / 3 3 / 4). Our study does not allow localizing the hydrogen atoms in the unit cell. Finally, we note that other ternary compounds such as LaCuSi [15] or UCu 0.96 Si 1.06 [16] crystallize in the same type structure where the transition metal Cu or Au and Si- or Al-atoms form a well-ordered network. The crystal structures of CeAuAl and CeAuAlH 1.4( 1) show some similarities (Fig. 2): the cerium three-dimensional network can be described by an intergrowth of trigonal [Ce 6 ] prisms surrounding alternatively the Au- and Al-atoms. But for CeAuAl, the [Ce 6 ] prisms are more distorted. Also, the unit cell volume per mol (Vm ) increases drastically after hydrogenation from 0.06709 nm 3 [11] to 0.07185 nm 3 (this work). In other words, the volume expansion is equal to 7.1% but is smaller than that observed ((18%) upon the hydrogenation of the intermediate valence compound CeIrAl [9]. However, this last intermetallic absorbs hydrogen up to a concentration of 2 H mol 21 . It is interesting to determine the influence of the insertion of hydrogen on the interatomic distances between Ce-atom and its ligands Au or Al in CeAuAl and CeAuAlH 1.4(1 ) . For these two compounds, each Ce-atom has six Au- and six Al-atoms as next nearest neighbours: (i) in CeAuAl the smallest interatomic distances d Ce – Au and d Ce – Al are respectively equal to 0.3140–0.3201 nm and 0.3032–0.3066 nm [11]; (ii) on the contrary in CeAuAlH 1.4(1 ) these similar distances are greater. They are both equal to 0.3319 nm. A comparison of these distances suggests that the strength of the hybridization between the 4f(Ce) orbital and the electronic states of the ligands Au and Al is somewhat reduced in CeAuAlH 1.4(1 ) when compared to CeAuAl which presents the Kondo effect
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B. Chevalier et al. / Journal of Alloys and Compounds 334 (2002) 20 – 26
Fig. 1. X-ray powder diffraction patterns of CeAuAl and CeAuAlH 1.4( 1 ) hydride. The Miller indices are relative to the hexagonal unit cell of the hydride having a50.4427(1) nm and c50.8467(1) nm as parameters.
[13]. Moreover, the smallest distances d Ce – Ce 50.3744 nm and 0.3900 nm in CeAuAl are replaced by greater ones, 0.4233 nm and 0.4427 nm, in the hydride. These observations permit to forecast that the Kondo effect plays a minor role on the magnetic properties of CeAuAlH 1.4(1 ) .
3.3. Magnetic properties The temperature dependence of the magnetization of CeAuAl measured under low applied magnetic field m0 H 5 0.02 T, given in the inset of Fig. 3, exhibits a
typical maximum corresponding to an antiferromagnetic ´ temperature is in order at T N 53.6(2) K. This Neel agreement with that previously reported T N 53.8 K [12,13]. Two anomalies can be distinguished in the curves of the temperature dependence of the magnetization (Mag.) of the hydride CeAuAlH 1.4( 1) (Fig. 3): (i) a pronounced maximum appears at T N 58.0(2) K suggesting the occurrence of an antiferromagnetic ordering; (ii) a strong increase of the magnetization around T 1 53.0(2) K (temperature defined by a minimum in the derivative curve dMag. / dT )
B. Chevalier et al. / Journal of Alloys and Compounds 334 (2002) 20 – 26 Table 1 X-ray powder diffraction data for CeAuAlH 1.4( 1 ) hkl
2uobs (8)
d obs (nm)
Iobs
002 100 101 102 103 110 004 112 201 202 203 105 114 211 212 106 213 300 302
20.97 23.18 25.48 31.46 39.62 40.73 42.68 46.25 48.66 52.35 58.13 59.64 60.47 65.28 68.37 71.13 73.39 74.14 78.05
0.4233 0.3834 0.3493 0.2841 0.2273 0.2213 0.2117 0.1961 0.1870 0.1746 0.1586 0.1549 0.1530 0.1428 0.1371 0.1324 0.1289 0.1278 0.1223
7 2 50 100 14 74 21 5 13 28 6 4 32 12 24 9 5 12 2
Fig. 2. Crystal structure of CeAuAl and CeAuAlH 1.4( 1 ) projected respectively onto the (100) and (001) planes.
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attributed to a spin reorientation in the antiferromagnetic range. In order to obtain more information on the nature of the two magnetic transitions appearing at T N and T 1 , we have performed AC-magnetic susceptibility measurements on CeAuAlH 1.4( 1) hydride. Fig. 4 shows the real x 9 component of AC-susceptibility in an external field m0 Hdc 5 0.01 T, an AC-field m0 Hac 5 0.2 mT and a frequency n 512.5 Hz. The x 9 5 f(T ) curve exhibits two peaks: one at T N 58.0(2) K and a greater one around T 1 52.8(2) K. These two ordering temperatures are in agreement with those determined previously by DC-magnetization measurement (Fig. 3). On the contrary, only one anomaly, a strong increase, occurs at T 1 in the curve showing the thermal dependence of the imaginary x 0 part of the AC-susceptibility (inset of Fig. 4). The magnetic transition appearing at T N which is characterised by the absence of a peak in the x 0 5 f(T ) curve, can be associated with an antiferromagnetic ordering state. On the contrary, the other transition at T 1 giving a strong increase in the x 0 5 f(T ) curve, which reflects important energy losses in the magnetically ordered state, is presumably connected with domain effects appearing, for instance in ferromagnetic, ferrimagnetic or canted systems. In order to characterise the magnetic transition of the CeAuAlH 1.4(1 ) hydride occurring below T 1 53.0(2) K, we have measured its magnetization versus field. Fig. 5 shows two significant isothermal magnetization curves Mag. 5 f( m0 H ) respectively at 6 K (between T N and T 1 ) and 2 K (below T 1 ). Two behaviours are clearly observed from these curves. At 6 K, with rising magnetic field, Mag. increases linearly at low fields as expected for a normal antiferromagnet then more rapidly around m0 H1 5 0.8 T suggesting a transition induced by the magnetic field. The magnetization (0.53 m B / mol measured at 4.5 T, clearly smaller than that calculated for a free Ce 31 ion (2.14 m B ) may be due to: (i) the existence of another magnetic transition at higher fields; (ii) the effects of the crystalline electric field which can reduce the Ce-magnetic moment as existing for CeAuAl [13]. At 2 K, the curve Mag. 5 f( m0 H ) exhibits at low fields a downward curvature certainly due to a non-compensated long period magnetic structure (for instance occurrence of a slight canting of the Ce-magnetic moment) and a jump near m0 H2 5 1.5 T appearing for a field-induced transition. All these results show that the hydride CeAuAlH 1.4( 1) ´ temperapresents an antiferromagnetic transition at a Neel ture of T N 58.0(2) K more than twice greater than that determined for CeAuAl. This confirms that the insertion of hydrogen in CeAuAl increases the interatomic distances d Ce – (Ce, Au or Al ) and as a result diminishes the influence of the Kondo effect. The increase of T N -temperature following the sequence CeAuAl→CeAuAlH 1.4(1 ) is in perfect agreement with the Doniach diagram. Now, it is interesting to determine the ( m0 H–T )-magnetic phase diagram of the hydride CeAuAlH 1.4( 1) . Two types of measurements have been used: (i) field depen-
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B. Chevalier et al. / Journal of Alloys and Compounds 334 (2002) 20 – 26
Fig. 3. Temperature dependence of the magnetization of the hydride CeAuAlH 1.4( 1 ) measured at two applied magnetic fields m0 H 5 0.01 T and 0.02 T. In the inset, temperature dependence for CeAuAl.
Fig. 4. Temperature dependence of the real part of the AC-magnetic susceptibility for CeAuAlH 1.4( 1 ) . In the inset, temperature dependence of the imaginary part.
B. Chevalier et al. / Journal of Alloys and Compounds 334 (2002) 20 – 26
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Fig. 5. Field dependence of the magnetization for CeAuAlH 1.4( 1 ) hydride measured at 6 and 2 K.
dence of the magnetization at constant temperature giving the critical field where a field-induced phase transition appears (Fig. 5); (ii) temperature dependence of the
magnetization at constant field showing the temperature where a magnetic phase substitutes another (Fig. 6). For instance, the curve Mag. 5 f(T ) measured for m0 H 5 1.1 T
Fig. 6. Temperature dependence of the magnetization of the hydride CeAuAlH 1.4( 1 ) measured at various magnetic fields. In the inset, temperature dependence of the derivative curve for m0 H 5 1.1 T.
B. Chevalier et al. / Journal of Alloys and Compounds 334 (2002) 20 – 26
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and a strong increase of its antiferromagnetic character. ´ temperature increases from 3.6(2) to 8.0(2) K in The Neel agreement with a reduction (or disappearance) of the influence of the Kondo effect. In order to check this assumption, an electronic band calculation based on spin polarised density functional theory is now in progress for CeAuAl and its hydride CeAuAlH 1.4(1 ) .
References
Fig. 7. ( m0 H–T )-magnetic phase diagram of CeAuAlH 1.4( 1 ) .
with decreasing temperature (field cooled curve), exhibits a large maximum followed by an increase at low temperature. Its derivative curve, presented in the inset of Fig. 6, allows to define three transition temperatures corresponding respectively to the presence of a minimum at 8.0(2) K, a maximum at 4.5(2) K and a shoulder near 3.0(2) K. On the contrary, similar measurement performed for m0 H 5 2 T indicates the existence of only one transition at 8.0(2) K. Using these various measurements, we have constructed the ( m0 H–T )-magnetic phase diagram of the hydride CeAuAlH 1.4(1 ) summarized in Fig. 7. Three magnetic ordered phases can be distinguished: (i) an antiferromagnetic phase (AF) stable between T N 58.0(2) K and T 1 53.0(2) K; (ii) for T , T 1 and m0 H , 1.5–1.6 T an antiferromagnetic phase (AF1) having a small ferromagnetic component; (iii) finally a phase (P1) induced by the magnetic field.
4. Conclusion We have shown that the insertion of hydrogen in the Kondo lattice CeAuAl induces both a structural transition
[1] S. Doniach, Physica B 91 (1977) 231. [2] S.J.S. Lister, F.M. Grosche, F.V. Carter, R.K.W. Haselwimmer, S.S. Saxema, N.D. Mathur, S.R. Julian, G.G. Lonzarich, Z. Phys. B 103 (1997) 263. [3] F.M. Grosche, S.J.S. Lister, F.V. Carter, S.S. Saxema, R.K.W. Haselwimmer, N.D. Mathur, S.R. Julian, G.G. Lonzarich, Physica B 239 (1997) 62. [4] R. Mowshovich, T. Graf, D. Mandrus, J.D. Thompson, J.L. Smith, Z. Fisk, Phys. Rev. B 53 (1996) 8241. [5] R. Movshovich, T. Graf, D. Mandrus, M.F. Hundley, J.D. Thompson, R.A. Fisher, N.E. Philips, J.L. Smith, Physica B 223–224 (1996) 126. [6] F.M. Grosche, S.R. Julian, N.D. Mathur, G.G. Lonzarich, Physica B 223–224 (1996) 50. [7] S. Raymond, D. Jaccard, H. Wilhelm, R. Cerny, Solid State Commun. 112 (1999) 617. [8] J-L. Bobet, B. Chevalier, B. Darriet, M. Nakhl, F. Weill, J. Etourneau, J. Alloys Comp. 317–318 (2001) 67. [9] S.K. Malik, P. Raj, A. Sathyamoorthy, K. Shashikala, N. Harish Kumar, L. Menon, Phys. Rev. B 63 (2001) 172418. [10] J-L. Bobet, B. Chevalier, F. Weill, J. Etourneau, J. Alloys Comp., submitted. [11] F. Hulliger, J. Alloys Comp. 200 (1993) 75. [12] L. Menon, S.K. Malik, Phys. Rev. B 51 (1995) 5858. [13] D.T. Adroja, B.D. Rainford, L. Menon, S.K. Malik, J. Phys.: Condens. Matter 9 (1997) 4743. [14] J-L. Bobet, S. Pechev, B. Chevalier, B. Darriet, J. Alloys Comp. 267 (1998) 136. [15] A. Mugnoli, A. Albinati, A.W. Hewat, J. Less-Common Metals 97 (1984) L1. [16] S. Pechev, B. Chevalier, B. Darriet, J. Etourneau, J. Alloys Comp. 282 (1999) 44.
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Structural and magnetic properties of the new hydride CeAuAlH 1.4( 1 ) B. Chevalier*, J-L. Bobet, M.L. Kahn, F. Weill, J. Etourneau ` Condensee ´ de Bordeaux ( ICMCB), CNRS [ UPR 9048], Universite´ Bordeaux I, Avenue du Docteur A. Schweitzer, Institut de Chimie de la Matiere 33608 Pessac Cedex, France Received 19 June 2001; accepted 3 July 2001
Abstract CeAuAl which orders antiferromagnetically below T N 53.8 K is considered as a Kondo lattice. This intermetallic absorbs hydrogen at room temperature and at a pressure of 4.5 MPa. The new hydride CeAuAlH 1.4( 1 ) is stable in air and crystallises in a derivative structure of the hexagonal Ni 2 In-type with a50.4427(1) nm and c50.8467(1) nm as unit cell parameters. Moreover, it exhibits an antiferromagnetic order below T N 58.0(2) K and another magnetic transition at T 1 53.0(2) K. The strong increase of the T N -temperature after the insertion of hydrogen in CeAuAl can be explained on the basis of the Doniach diagram; the increase of the unit cell volume induced by this insertion leads to a strong reduction of the influence of the Kondo effect. 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Hydrogen absorbing materials; X-ray diffraction; Magnetic measurements
1. Introduction The unusual properties at low temperatures of many compounds based on cerium such as the Kondo effect, intermediate valence behaviour, heavy fermions behaviour, magnetic Kondo systems, . . . are governed by the strength of the interaction Jcf between the spins of localized 4f(Ce) electrons and conduction electrons. The competition between Kondo interaction which causes quenching of the magnetic moment of the 4f(Ce) ion due to the formation of a singlet state and Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction which favours the occurrence of a long-range magnetic order, is dependent on Jcf . Considering the Jcf values, Doniach has proposed a magnetic phase diagram [1]: (i) for small Jcf , the compound exhibits a magnetic RKKY metal behaviour; (ii) for larger Jcf , the Kondo effect is strong and the compound shows an intermediate valence behaviour; (iii) between these two domains appears the magnetic Kondo systems where the magnetic moment is reduced by the Kondo effect. The diagram indicates that, with increasing Jcf , the magnetic ordering temperature increases initially, then goes through a maximum and finally decreases rapidly by driving the compound to a non-magnetic Kondo state. *Corresponding author. Tel.: 133-5-5684-6336; fax: 133-5-56842761. E-mail address: [email protected] (B. Chevalier).
In this view, it is interesting to modify the Jcf interaction existing in one compound containing cerium atoms. Mainly two routes are known: (i) the application of hydrostatic pressure which increases Jcf ; (ii) the insertion of hydrogen in the lattice which induces on the contrary a decrease of Jcf . Recently, electrical resistivity measurements performed under pressure on CeNi 2 Ge 2 (unconventional non-magnetic metal) [2,3], CeRh 2 Si 2 (antiferromagnet below T N 5 36 K) [4,5] and CePd 2 Si 2 (Kondo antiferromagnet below T N 510.6 K) [2,3,6,7] indicate the occurrence of superconducting transition around 220, 350–400 and 430–520 mK, respectively. In all cases, the formation of a superconducting phase is closely linked to the disappearance of magnetism induced by an increase of the Kondo effect under pressure. On the other hand, the insertion of hydrogen leads to an expansion of the unit cell volume inducing a decrease of Jcf and driving the compound towards a magnetic state. For instance, the hydrogenation of CeNiAl [8] and CeIrAl [9] leads to a valence transition for cerium from intermediate valence state to trivalent state. Also, the hydride CePtAlH 1.1 exhibits a Curie temperature T C 511.6 K twice greater than that determined for the initial Kondo magnet CePtAl (T C 55.6 K) [10]. Extending our search on new hydrides based on cerium, we have studied the hydrogen absorption properties of CeAuAl. This intermetallic crystallises in the orthorhombic TiNiSi-type structure [11] and orders antiferromagnetically below T N 53.8 K [12]. Moreover, its investigation by
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01748-0
B. Chevalier et al. / Journal of Alloys and Compounds 334 (2002) 20 – 26
magnetoresistance measurements suggests that its Kondo ´ temperatemperature T K (3.6 K is comparable to the Neel ture T N indicating a strong competition between Kondo and RKKY interactions [13]. In this paper, we report on the synthesis of the hydride CeAuAlH x and on its characterization using X-ray powder diffraction and magnetization measurements. We show that the insertion of hydrogen in CeAuAl induces both a structural transition ´ temperature. Also, the and a strong increase of the Neel hydride CeAuAlH x presents an interesting magnetic phase diagram.
2. Experimental techniques The sample CeAuAl was synthesised by arc-melting a stoichiometric mixture of pure elements in an atmosphere of high purity argon. Then, the sample was turned and remelted several times to ensure homogeneity. The weight loss during the arc-melting process was smaller than 0.5 wt.%. Finally, the ingot was enclosed in evacuated quartz tubes and annealed for 3 weeks at 1023 K. Its characterization by X-ray powder diffraction (Philips PW 1050 diffractometer, Cu Ka radiation) confirms that CeAuAl adopts the orthorhombic TiNiSi-type with unit cell parameters in agreement with those reported previously [11]. Hydrogen absorption–desorption experiments were performed using the apparatus described previously [14]. An ingot of sample was heated under vacuum at 473 K for 12 h and then exposed to hydrogen gas at various temperatures and pressures. The amount of H-atoms absorbed was determined volumetrically by monitoring pressure changes in a calibrated volume. Magnetization measurements were performed using a Superconducting Quantum Interference Device (SQUID) magnetometer in the temperature range 1.8–300 K and applied fields up to 5 T.
3. Results and discussion
3.1. Hydrogen absorption–desorption properties CeAuAl bulk compound was exposed to hydrogen gas at room temperature. The absorption starts only at pressures above 4.5 MPa even when an annealing treatment up to 473 K under vacuum is made before absorption. A complete decrepitation of the bulk material is observed and the X-ray powder pattern clearly shows that the hydrogenation leads to a structural change (Fig. 1). The hydride formed (i.e. CeAuAlH 1.4( 1) ) is stable in air suggesting that the equilibrium pressure is below 0.1 MPa. As a matter of fact, desorption is only observed after heating at 573 K under primary vacuum (i.e. about 10 Pa). The desorption is complete as the sample absorbs again 1.4(1) H per formula during the second absorption. However, the absorption
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pressure required is only 0.3 MPa. After a second desorption (i.e. 10 Pa and 573 K), a pressure below 0.1 MPa is sufficient to form CeAuAlH 1.4( 1) . The determination of the absorption and desorption equilibrium pressure at high temperature will be done in the near future in order to calculate the thermodynamic properties and then to estimate the equilibrium pressure at room temperature.
3.2. Structural properties The insertion of hydrogen in CeAuAl modifies its structural properties. The X-ray powder pattern of the CeAuAlH 1.4(1 ) hydride can be indexed on the basis of a hexagonal unit cell with the Ni 2 In-type structure (Fig. 1). The refined unit cell parameters are a50.4427(1) nm and c50.8467(1) nm. Table 1 summarizes the X-ray powder diffraction data of this hydride. Similar structural transition from orthorhombic TiNiSi-type structure to hexagonal Ni 2 In-type structure has been observed recently for the hydrogenation of equiatomic ternary compound CeIrAl [9]. This last type of structure adopts the P63 /mmc (no. 194) space group where Ce, Au and Al atoms occupy, respectively, the sites 2a (0 0 0), 2c (1 / 3 2 / 3 1 / 4) and 2d (1 / 3 2 / 3 3 / 4). Our study does not allow localizing the hydrogen atoms in the unit cell. Finally, we note that other ternary compounds such as LaCuSi [15] or UCu 0.96 Si 1.06 [16] crystallize in the same type structure where the transition metal Cu or Au and Si- or Al-atoms form a well-ordered network. The crystal structures of CeAuAl and CeAuAlH 1.4( 1) show some similarities (Fig. 2): the cerium three-dimensional network can be described by an intergrowth of trigonal [Ce 6 ] prisms surrounding alternatively the Au- and Al-atoms. But for CeAuAl, the [Ce 6 ] prisms are more distorted. Also, the unit cell volume per mol (Vm ) increases drastically after hydrogenation from 0.06709 nm 3 [11] to 0.07185 nm 3 (this work). In other words, the volume expansion is equal to 7.1% but is smaller than that observed ((18%) upon the hydrogenation of the intermediate valence compound CeIrAl [9]. However, this last intermetallic absorbs hydrogen up to a concentration of 2 H mol 21 . It is interesting to determine the influence of the insertion of hydrogen on the interatomic distances between Ce-atom and its ligands Au or Al in CeAuAl and CeAuAlH 1.4(1 ) . For these two compounds, each Ce-atom has six Au- and six Al-atoms as next nearest neighbours: (i) in CeAuAl the smallest interatomic distances d Ce – Au and d Ce – Al are respectively equal to 0.3140–0.3201 nm and 0.3032–0.3066 nm [11]; (ii) on the contrary in CeAuAlH 1.4(1 ) these similar distances are greater. They are both equal to 0.3319 nm. A comparison of these distances suggests that the strength of the hybridization between the 4f(Ce) orbital and the electronic states of the ligands Au and Al is somewhat reduced in CeAuAlH 1.4(1 ) when compared to CeAuAl which presents the Kondo effect
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B. Chevalier et al. / Journal of Alloys and Compounds 334 (2002) 20 – 26
Fig. 1. X-ray powder diffraction patterns of CeAuAl and CeAuAlH 1.4( 1 ) hydride. The Miller indices are relative to the hexagonal unit cell of the hydride having a50.4427(1) nm and c50.8467(1) nm as parameters.
[13]. Moreover, the smallest distances d Ce – Ce 50.3744 nm and 0.3900 nm in CeAuAl are replaced by greater ones, 0.4233 nm and 0.4427 nm, in the hydride. These observations permit to forecast that the Kondo effect plays a minor role on the magnetic properties of CeAuAlH 1.4(1 ) .
3.3. Magnetic properties The temperature dependence of the magnetization of CeAuAl measured under low applied magnetic field m0 H 5 0.02 T, given in the inset of Fig. 3, exhibits a
typical maximum corresponding to an antiferromagnetic ´ temperature is in order at T N 53.6(2) K. This Neel agreement with that previously reported T N 53.8 K [12,13]. Two anomalies can be distinguished in the curves of the temperature dependence of the magnetization (Mag.) of the hydride CeAuAlH 1.4( 1) (Fig. 3): (i) a pronounced maximum appears at T N 58.0(2) K suggesting the occurrence of an antiferromagnetic ordering; (ii) a strong increase of the magnetization around T 1 53.0(2) K (temperature defined by a minimum in the derivative curve dMag. / dT )
B. Chevalier et al. / Journal of Alloys and Compounds 334 (2002) 20 – 26 Table 1 X-ray powder diffraction data for CeAuAlH 1.4( 1 ) hkl
2uobs (8)
d obs (nm)
Iobs
002 100 101 102 103 110 004 112 201 202 203 105 114 211 212 106 213 300 302
20.97 23.18 25.48 31.46 39.62 40.73 42.68 46.25 48.66 52.35 58.13 59.64 60.47 65.28 68.37 71.13 73.39 74.14 78.05
0.4233 0.3834 0.3493 0.2841 0.2273 0.2213 0.2117 0.1961 0.1870 0.1746 0.1586 0.1549 0.1530 0.1428 0.1371 0.1324 0.1289 0.1278 0.1223
7 2 50 100 14 74 21 5 13 28 6 4 32 12 24 9 5 12 2
Fig. 2. Crystal structure of CeAuAl and CeAuAlH 1.4( 1 ) projected respectively onto the (100) and (001) planes.
23
attributed to a spin reorientation in the antiferromagnetic range. In order to obtain more information on the nature of the two magnetic transitions appearing at T N and T 1 , we have performed AC-magnetic susceptibility measurements on CeAuAlH 1.4( 1) hydride. Fig. 4 shows the real x 9 component of AC-susceptibility in an external field m0 Hdc 5 0.01 T, an AC-field m0 Hac 5 0.2 mT and a frequency n 512.5 Hz. The x 9 5 f(T ) curve exhibits two peaks: one at T N 58.0(2) K and a greater one around T 1 52.8(2) K. These two ordering temperatures are in agreement with those determined previously by DC-magnetization measurement (Fig. 3). On the contrary, only one anomaly, a strong increase, occurs at T 1 in the curve showing the thermal dependence of the imaginary x 0 part of the AC-susceptibility (inset of Fig. 4). The magnetic transition appearing at T N which is characterised by the absence of a peak in the x 0 5 f(T ) curve, can be associated with an antiferromagnetic ordering state. On the contrary, the other transition at T 1 giving a strong increase in the x 0 5 f(T ) curve, which reflects important energy losses in the magnetically ordered state, is presumably connected with domain effects appearing, for instance in ferromagnetic, ferrimagnetic or canted systems. In order to characterise the magnetic transition of the CeAuAlH 1.4(1 ) hydride occurring below T 1 53.0(2) K, we have measured its magnetization versus field. Fig. 5 shows two significant isothermal magnetization curves Mag. 5 f( m0 H ) respectively at 6 K (between T N and T 1 ) and 2 K (below T 1 ). Two behaviours are clearly observed from these curves. At 6 K, with rising magnetic field, Mag. increases linearly at low fields as expected for a normal antiferromagnet then more rapidly around m0 H1 5 0.8 T suggesting a transition induced by the magnetic field. The magnetization (0.53 m B / mol measured at 4.5 T, clearly smaller than that calculated for a free Ce 31 ion (2.14 m B ) may be due to: (i) the existence of another magnetic transition at higher fields; (ii) the effects of the crystalline electric field which can reduce the Ce-magnetic moment as existing for CeAuAl [13]. At 2 K, the curve Mag. 5 f( m0 H ) exhibits at low fields a downward curvature certainly due to a non-compensated long period magnetic structure (for instance occurrence of a slight canting of the Ce-magnetic moment) and a jump near m0 H2 5 1.5 T appearing for a field-induced transition. All these results show that the hydride CeAuAlH 1.4( 1) ´ temperapresents an antiferromagnetic transition at a Neel ture of T N 58.0(2) K more than twice greater than that determined for CeAuAl. This confirms that the insertion of hydrogen in CeAuAl increases the interatomic distances d Ce – (Ce, Au or Al ) and as a result diminishes the influence of the Kondo effect. The increase of T N -temperature following the sequence CeAuAl→CeAuAlH 1.4(1 ) is in perfect agreement with the Doniach diagram. Now, it is interesting to determine the ( m0 H–T )-magnetic phase diagram of the hydride CeAuAlH 1.4( 1) . Two types of measurements have been used: (i) field depen-
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B. Chevalier et al. / Journal of Alloys and Compounds 334 (2002) 20 – 26
Fig. 3. Temperature dependence of the magnetization of the hydride CeAuAlH 1.4( 1 ) measured at two applied magnetic fields m0 H 5 0.01 T and 0.02 T. In the inset, temperature dependence for CeAuAl.
Fig. 4. Temperature dependence of the real part of the AC-magnetic susceptibility for CeAuAlH 1.4( 1 ) . In the inset, temperature dependence of the imaginary part.
B. Chevalier et al. / Journal of Alloys and Compounds 334 (2002) 20 – 26
25
Fig. 5. Field dependence of the magnetization for CeAuAlH 1.4( 1 ) hydride measured at 6 and 2 K.
dence of the magnetization at constant temperature giving the critical field where a field-induced phase transition appears (Fig. 5); (ii) temperature dependence of the
magnetization at constant field showing the temperature where a magnetic phase substitutes another (Fig. 6). For instance, the curve Mag. 5 f(T ) measured for m0 H 5 1.1 T
Fig. 6. Temperature dependence of the magnetization of the hydride CeAuAlH 1.4( 1 ) measured at various magnetic fields. In the inset, temperature dependence of the derivative curve for m0 H 5 1.1 T.
B. Chevalier et al. / Journal of Alloys and Compounds 334 (2002) 20 – 26
26
and a strong increase of its antiferromagnetic character. ´ temperature increases from 3.6(2) to 8.0(2) K in The Neel agreement with a reduction (or disappearance) of the influence of the Kondo effect. In order to check this assumption, an electronic band calculation based on spin polarised density functional theory is now in progress for CeAuAl and its hydride CeAuAlH 1.4(1 ) .
References
Fig. 7. ( m0 H–T )-magnetic phase diagram of CeAuAlH 1.4( 1 ) .
with decreasing temperature (field cooled curve), exhibits a large maximum followed by an increase at low temperature. Its derivative curve, presented in the inset of Fig. 6, allows to define three transition temperatures corresponding respectively to the presence of a minimum at 8.0(2) K, a maximum at 4.5(2) K and a shoulder near 3.0(2) K. On the contrary, similar measurement performed for m0 H 5 2 T indicates the existence of only one transition at 8.0(2) K. Using these various measurements, we have constructed the ( m0 H–T )-magnetic phase diagram of the hydride CeAuAlH 1.4(1 ) summarized in Fig. 7. Three magnetic ordered phases can be distinguished: (i) an antiferromagnetic phase (AF) stable between T N 58.0(2) K and T 1 53.0(2) K; (ii) for T , T 1 and m0 H , 1.5–1.6 T an antiferromagnetic phase (AF1) having a small ferromagnetic component; (iii) finally a phase (P1) induced by the magnetic field.
4. Conclusion We have shown that the insertion of hydrogen in the Kondo lattice CeAuAl induces both a structural transition
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