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Hydrogenation and physical properties of the ternary germanide CeCoGe: an anisotropic expansion of the unit cell
Intermetallics 12 (2004) 437–442 www.elsevier.com/locate/intermet
Hydrogenation and physical properties of the ternary germanide CeCoGe: an anisotropic expansion of the unit cell B. Chevalier*, E. Gaudin, F. Weill, J.-L. Bobet Institut de Chimie de la Matie`re Condense´e de Bordeaux (ICMCB), CNRS [UPR 9048], Universite´ Bordeaux 1, Avenue du Docteur A. Schweitzer, 33608 Pessac Cedex, France Received 13 October 2003; received in revised form 20 December 2003; accepted 23 December 2003
Abstract The ternary germanide CeCoGe and its hydride CeCoGeH crystallize in the tetragonal CeFeSi-type structure but the hydrogenation induces an anisotropic expansion of the unit cell parameters: a decreases (3.1%) whereas c increases (+12.7%). These structural changes suggest that the hydrogen atoms occupy the [Ce4] tetrahedral sites empty in CeCoGe. Magnetization and electrical resistivity measurements evidence an interesting transition from antiferromagnetic ordering [CeCoGe; TN=5.0(2) K] to spin fluctuation behaviour [CeCoGeH; Tsf=13.3(5) K]. # 2004 Elsevier Ltd. All rights reserved. Keywords: A. Rare-earth intermetallics; B. Crystal chemistry of intermetallics; B. Magnetic properties; F. Diffraction (electron, neutron and X-ray)
1. Introduction The ternary germanide CeCoGe crystallizes in the tetragonal CeFeSi-type structure (space group P4/nmm, No 129) [1]. This structure can be described by a stacking along the c-axis of two layers formed by [Ce4Co4] antiprisms and separated by one layer of [Ce4] pseudotetrahedral units (Fig. 1). The [Ce4Co4] antiprisms surrounding the Ge-atom are also observed in the crystal structure of the other ternary germanide CeCo2Ge2 [2]. The layer of [Ce4] pseudo-tetrahedral units is potentially interesting for hydrogen insertion. Similar sites form the crystal structure of the dihydride CeH2 [3]. Extending our research on new hydrides based on cerium as CeNiGaH1.1 [4], CeNiInH1.8 [5] and CeNiSnH1.8 [6], we have studied the hydrogen sorption properties of CeCoGe. Our recent investigation by neutron powder diffraction reveals that this ternary germanide orders antiferromagnetically below TN=5.0(2) K [7]; (standard deviations in the data of the least-significant digits are given in brackets throughout the paper). In this paper, the synthesis of the hydride CeCoGeH1.1(1) and its characterization using X-ray diffraction (powder and * Corresponding author. Fax: +33-5-4000-2761. E-mail address: [email protected] (B. Chevalier). 0966-9795/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2003.12.007
single crystal), magnetization and electrical resistivity measurements are reported. It is shown that the insertion of hydrogen into CeCoGe keeps its crystal symmetry but causes a pronounced anisotropic expansion of the unit cell: the a-parameter decreases whereas the c-parameter strongly increases. As far as the physical properties are concerned, the insertion of hydrogen leads to a transition from antiferromagnet to spin fluctuation behaviour. For comparison, some results concerning the hydrogenation of the isomorphous ternary germanide LaCoGe are also presented.
2. Experimental details A polycrystalline CeCoGe sample was synthesized by arc-melting a stoichiometric mixture of pure elements (purity above 99.9%) in a high-purity argon atmosphere. Then, the sample was turned and remelted several times to ensure homogeneity. Annealing was done for one month at 1073 K by enclosing the sample in an evacuated quartz tube. X-ray powder diffraction confirms that this ternary germanide crystallizes in the tetragonal CeFeSi-type structure with the unit cell parameters a=0.4170(2) nm and c=0.6865(2) nm as reported previously [1]. The same procedure was used
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for the preparation and the characterization of the LaCoGe sample. Hydrogen absorption experiments were performed using the apparatus described previously [8]. An annealed ingot was heated under vacuum at 393 K for 12 h and then exposed to 5 MPa of hydrogen gas at the same temperature. The amount of H absorbed was determined volumetrically by monitoring pressure changes in a calibrated volume. The kinetic hydrogen absorption of CeCoGe was measured using an automated Sievert-type apparatus (HERA, Hydrogen Storage System, Que´bec, Canada) with a sample of about 500 mg [9]. X-ray powder diffraction with the use of a Philips 1050-diffractometer (Cu Ka radiation) was applied for the characterization of the structural type and for the phase identification of the samples. The unit cell parameters were determined by a least-squares refinement method using silicon (5N) as an internal standard. The refinement of the crystal structure of the hydride was performed using a tiny single crystal isolated from the pulverized sample. It was very difficult to find an acceptable single crystal in the different hydride samples. Eventually, after many tests a single crystal with strongly elongated diffraction spots was found. Data collection was carried out on an Enraf-Nonius Kappa CCD diffractometer using Mo Ka radiation. The intensity integration of such elongated spots was possible with the new soltware EVAL-14 [10]. Data processing was performed with the Jana 2000 software [11]. A Gaussian-type absorption correction was applied and the shape was estimated with the video microscope of the Kappa CCD.
Fig. 1. Projection on the (a, c)-plane of the crystal structure of CeCoGe. (Ce, Ge and Co atoms are respectively represented by large, medium and small circles). d gives the interatomic distance between two Ce-atoms in the antiprism blocks.
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. Electrical resistivity was measured above 4.2 K on a polycrystalline sample using standard dc four-probe measurements. For this investigation, the hydride was compacted at room temperature (compactness 80%) in order to form a pellet (diameter=6 mm and thickness=3 mm) and then heated for two days at 523 K under a pressure (5 MPa) of hydrogen.
3. Results and discussion Fig. 2 presents the first hydrogen absorption kinetic of CeCoGe performed at 393 K under a pressure of P(H2)=2 MPa. The amount of hydrogen absorbed increases very slowly with the time of hydrogenation showing saturation at times longer than 15 h; finally the composition CeCoGeH1.07(5) is attained. The same hydride is obtained when CeCoGe ingots are exposed at 523 K for 36 h under 5 MPa of hydrogen gas; the hydrogenation induces a decrepitation of the starting ingots. This hydride is stable in ambient conditions and no decomposition is observed after its heating under vacuum at 523 K. 3.1. Structural properties The structure of CeCoGeH1.07(5) has been determined from single-crystal X-ray data. The extinction conditions observed agree with the P4/nmm space group (No
Fig. 2. First hydrogen absorption at 393 K for CeCoGe (P(H2)= 2 MPa).
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129) already used for the refinement of the initial ternary germanide CeCoGe [1]. For data collection details, see Table 1. The starting atomic positions were those reported previously for CeCoGe (Ce in 2c-site (1/4 1/4 0.67), Co in 2a-site (3/4 1/4 0) and Ge in 2c-site (1/4 1/4 0.17)) [1]. The structure refinement was carried out with the Jana 2000 program package [11]. With anisotropic displacement parameters, the final residual factors converged to the value R(F)=0.0549 and wR(F)=0.0610 for 9 refined parameters and 88 observed reflections. The final atomic positions and anisotropic displacement parameters are given respectively in Tables 2 and 3.
Table 1 Crystallographic data and structure refinement for CeCoGeH (*deduced from X-ray powder diffraction) Crystal data Chemical Formula Molar mass (g mol1) Temperature Crystal system Space group Unit cell dimension (nm)
CeCoGeH 272.65 293 K tetragonal P4/nmm (No 129) a=0.4040(2) * c=0.7735(4) * 0.1262(4) * 2 7.18 MoKa; 0.071073 35.68 234 Metallic luster 0.20 0.15 0.15
Volume (nm3) Z (formula units per unit cell) Calculated density (g cm3) Radiation and wavelength (nm) Absorption coefficient (mm1) F(000) Crystal color Crystal size (mm) Data collection Diffractometer range for data collection Hkl range Measured reflections Absorption correction Independent reflections Refinement Refinement on No of independent reflections No of observed reflections [I>3s(I)] No of parameters R(F) WR(F) S Difference Fourier residues (e A˚3)
Enraf-Nonius Kappa CCD area-detector 2.6 30.0 54h 45, 54k 45, 104l 410 1298 Gaussian method 132, (Rint=0.1493) F (structure factor) 132 88 9 0.0549 0.0610 1.31 +2.90 and 3.70
Table 2 Positional and equivalent isotropic displacement parameters Ueq (A˚2) for CeCoGeH Atom
Site
x
y
z
Ueq
Ce Co Ge
2c 2a 2c
1/4 3/4 1/4
1/4 1/4 1/4
0.6709(2) 0 0.1643(4)
0.0118(5) 0.0122(10) 0.0126(8)
The crystal structure of CeCoGe and its hydride adopt the same space group but the hydrogenation of this ternary germanide causes a pronounced anisotropic expansion of the unit cell; the a-parameter decreases from 0.4170(2) nm to 0.4040(2) nm (3.1%) whereas the c-parameter increases strongly from 0.6865(2) nm to 0.7735(4) nm (+12.7%). In other words, the insertion of hydrogen into CeCoGe involves an expansion of the unit cell volume from 0.1194(2) nm3 to 0.1262(4) nm3 (+5.7%). It is also worth pointing out that the atomic positions of Ce, Co and Ge in CeCoGe and its hydride (Table 2) are practically similar. The strong increase of the c-parameter in the sequence CeCoGe ! CeCoGeH1.07(5) suggests that the H-atoms are inserted in the [Ce4] pseudo-tetrahedral sites. According to this hypothesis, hydrogen occupies the 2bsite (1/4 3/4 1/2) giving interatomic distances dCeH=0.2410(4) nm between Ce and H atoms comparable to that observed in CeH2 dihydride (dCeH=0.2416 nm) [12]. The full occupancy of the 2bsite leads to the formulation CeCoGeH, close to that obtained experimentally. The possibility to insert hydrogen into the [Ce4Co4] antiprism seems to be unfavourable; our attempt to prepare hydride under the same condition [523 K, P(H2)=5 MPa] from the ternary germanide CeCo2Ge2 containing only the same antiprisms was unsuccessful. Neutron powder diffraction experiments need to be performed on CeCoGeD deuteride in order to verify this hypothesis. Table 4 compares the interatomic distances between Ce and its ligands in CeCoGe and its hydride. The distances dCeCo=0.3250 nm and dCeGe=0.3128 nm existing in the hydride CeCoGeH are comparable to that observed in the ternary germanide CeCo2Ge2; respectively 0.3257 nm and 0.3144 nm [2]. The hydrogenation of CeCoGe induces great changes in the values
Table 3 Anisotropic displacement parameters Uij (A˚2) for CeCoGeH Atom
U11
U22
U33
U12
U13
U23
Ce Co Ge
0.0097(7) 0.0113(14) 0.0106(12)
0.0097 0.0113 0.0106
0.0160(11) 0.014(2) 0.017(2)
0 0 0
0 0 0
0 0 0
Table 4 Selected interatomic distances (nm) around Ce atom in CeCoGe and its hydride CeCoGe
CeCoGeH
Ce -
Ce -
4 4 1 4 4
Ce 0.4170 Ce 0.3761 Ge 0.3433 Ge 0.3147 Co 0.3079
4 4 1 4 4
Ce 0.4040 Ce 0.3892 Ge 0.3919 Ge 0.3128 Co 0.3250
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Fig. 3 presents the temperature dependence of the reciprocal magnetic susceptibility w1 m of CeCoGe and its hydride. Above 70 K, the curve w1 m =f(T) relative to CeCoGe follows a Curie-Weiss law with the effective magnetic moment meff.=2.60(5) mB/Ce-ion and the paramagnetic Curie temperature p=39(1) K. The meff.
value is close to that calculated for a free Ce3+ ion (2.54 mB). Similar behaviour is observed above 30 K for the hydride CeCoGeH. In this case, the magnetic parameters are meff.=2.78(5) mB/Ce-ion and p= 29(1) K. The presence of a peak at TN=5.0(2) K in the temperature dependence of the magnetic susceptibility of CeCoGe (see inset of Fig. 4), agrees with the occurrence of an antiferromagnetic ordering of the Ce-moments [7]. Indeed, contrary to that reported previously by Welter et al. [1], our investigation of CeCoGe by neutron powder diffraction indicates that the Ce-ordered magnetic moment [0.43(6) mB/Ce at 1.6 K] forms a collinear antiferromagnetic structure associated to the k=(0 0 0) wavevector [7]. The reduced Ce-ordered magnetic moment observed at 1.6 K for CeCoGe leads to the presence of weak additional peaks of magnetic origin no detected previously in Ref. [1]. Below 20 K, the curve wm=f(T) relative to CeCoGeH shows a different behaviour (Fig. 4): (i) between 10 K and 4 K, wm tends to saturate; (ii) below 4 K, wm increases sharply with decreasing temperature. This behaviour is commonly observed in compounds based on cerium that show spin fluctuation arising from the Kondo effect and contain small amounts of impurity phase such as Ce2O3 leading to an increase of wm at low temperatures [14]. According to this scheme, the magnetic susceptibility wm of the hydride CeCoGeH can be expressed at low temperatures (T < 10 K) by wm=wm(0)+nC/T where wm(0) is the magnetic susceptibility at T=0 K, n is the proportion of stable Ce3+ moments composing the trace of magnetic impurities and C=0.807 emu K/mol is the Curie constant for free
Fig. 3. Temperature dependence of the reciprocal magnetic susceptibility w1 m , measured with an applied field m0H=4 T, of germanide CeCoGe and its hydride. Dashed lines show the fit to calculated w1 m with Curie-Weiss law. For clarity, the curve relative to CeCoGe is shifted vertically.
Fig. 4. Temperature dependence of the magnetic susceptibility wm, measured with an applied field m0H=0.1 T, of germanide CeCoGe (inset) and its hydride CeCoGeH. (Solid circles give the measured data and open circles show the corrected data (wmnC/T) (see text).
of the dCeCe distances; these located in the (a, b)-plane decrease whereas the four other increase. The pseudotetrahedral [Ce4] unit is less distorted in the hydride CeCoGeH. Another interesting feature concerns the increase, after hydrogenation, of the interatomic distance d (Fig. 1) between two Ce atoms in the antiprism blocks. This distance changes from 0.5406 nm to 0.5838 nm (+8.0%); this last value is similar to that existing in the ternary germanide CeCo2Ge2 (d=0.5839 nm) [2]. In other words, many structural characteristics of the [Ce4Co4] antiprisms found in the hydride CeCoGeH are in agreement with those observed in CeCo2Ge2. This ternary germanide is considered as an intermediate valence compound [13]. A similar hydrogenation procedure [T=523 K and P(H2)=5 MPa] performed on an isomorphous ternary germanide based on lanthanum LaCoGe leads to the synthesis of the hydride LaCoGeH1.02(5). The evolution of the unit cell parameters observed during this procedure is very close to that evidenced for CeCoGe: the a-parameter decreases from 0.4194(2) nm to 0.4063(2) nm (3.1%) whereas the c-parameter increases strongly from 0.7028(3) nm to 0.7912(3) nm (+12.6%). 3.2. Magnetic and electrical properties
B. Chevalier et al. / Intermetallics 12 (2004) 437–442
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Fig. 5. Temperature dependence of the reduced electrical resistivity for hydrides CeCoGeH and LaCoGeH.
Ce3+ ions. The best agreement between experimental and calculated wm values leads to wm(0)=30.4(6).103 emu/mol and n=18.8(6).103 Ce3+ ions/mol. Fig. 4 presents the corrected (wmnC/T)=f(T) magnetic susceptibility versus temperature resulting from this calculation. This curve goes through a large maximum near 7(1) K; this temperature is almost consistent with the model offered by Lawrence et al. [14] which forecasts that the temperature dependence of the corrected susceptibility exhibits a maximum at T=Tsf/2 where Tsf=C/2wm(0)=13.3(5) K being the spin fluctuation temperature resulting from the hybridisation between 4f(Ce) and conduction electrons. This analysis suggests that the hydride CeCoGeH is a Kondo non-magnetic ordered system showing 13.3(5) K as spin fluctuation temperature. Fig. 5 compares the temperature dependence of the reduced resistivity of the two hydrides LaCoGeH and CeCoGeH. The curve relative to LaCoGeH characterizes a normal metal; its resistivity decreases with the temperature between 270 K and 15 K then takes a constant value at lowest temperatures. A different behaviour is observed for the resistivity of CeCoGeH. This curve reveals several anomalies: (i) a broad maximum appears around 90–100 K; (ii) a pronounced minimum occurs near 28(1) K and finally (iii) a decrease is evidenced below 7.0(5) K. This double-peak structure suggests a Kondo-lattice behaviour for CeCoGeH [15]. The presence of the maximum at 90–100 K represents the Kondo effect in the excited doublet whereas the minimum at 28(1) K and the maximum at 7.0(5) K characterize the onset of coherence effects of the Kondo lattice. The temperature 7.0(5) K where the curve =f(T) exhibits a maximum, is consistent with that 7(1) K where the magnetic susceptibility of CeCoGeH goes through a maximum (Fig. 4).
4. Conclusion Insertion of hydrogen into the crystal lattice of CeCoGe leads to an anisotropic expansion of the unit cell parameters but the symmetry is preserved. The suggested filling of the [Ce4] tetrahedral sites by hydrogen makes the crystallographic characteristics of the antiprism [Ce4Co4] layers of CeCoGeH comparable to those existing in the intermediate valence compound CeCo2Ge2. Magnetization and electrical resistivity measurements reveal that CeCoGeH shows spin fluctuation behaviour around Tsf=13.3(5) K. In other words, the hydrogenation of CeCoGe leads to the transition antiferromagnet ! spin fluctuation behaviour. This sequence is unusual since generally, the hydrogenation of intermetallics based on cerium induces a reduction of the hybridisation between 4f(Ce) electrons and conduction states. Other experiments such as neutron powder diffraction and specific heat measurements are planned in order to explain this interesting sequence. Nevertheless, we remark that the interatomic distances dCeCo and dCeGe existing in the hydride CeCoGeH are close to those determined in CeCo2Ge2, suggesting that the strength of the hybridisation between 4f(Ce) states and those of its ligands is comparable. This hybridisation plays an important role on the broadening of the 4f(Ce) states leading to a spin fluctuation behaviour or intermediate valence state.
Acknowledgements The authors thank J. Villot and R. Decourt for their technical help concerning the sample preparation and the electrical resistivity measurements.
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References [1] Welter R, Venturini G, Malaman B, Ressouche E. J Alloys Comp 1993;201:191. [2] McCall WM, Narasimhan KSVL, Butera RA. J Appl Crystallogr 1973;6:301. [3] Wiesinger G, Hilscher G. In: Buschow KHJ, editor. Handbook of Magnetic Materials, Vol. 6. Elsevier Science Publishers B.V; 1991. [4] Chevalier B, Bobet J-L, Gaudin E, Pasturel M, Etourneau J. J Solid State Chem 2002;168:28. [5] Chevalier B, Kahn ML, Bobet J-L, Pasturel M, Etourneau J. J Phys: Condens Matter 2002;14:L365. [6] Chevalier B, Bobet J-L, Pasturel M, Bauer E, Weill F, Decourt R, Etourneau J. Chem Materials 2003;15:2181.
[7] Chevalier B, Malaman B. J. Alloys Comp. (in preparation). [8] Bobet J-L, Pechev S, Chevalier B, Darriet B. J Alloys Comp 1998;267:136. [9] Schulz R, Boily S, Huot J. Canadian patent, Ser.-Nr.: 2207149; 1999. [10] Duisenberg AJM, Kroon-Batenburg LMJ, Schreurs AMM. J Appl Cryst 2003;36:220. [11] Petricek V, Dusek M. The crystallographic computing system Jana 2000. Praha, Czech Republic: Institute of Physics; 2000. [12] Korst WL, Warf JC. Inorganic Chem 1966;5:1719. [13] Fujii H, Ueda E, Uwatoko Y, Shigeoka T. J Magn Magn Mat 1988;76-77:179. [14] Lawrence JM, Riseborough PS, Parks RD. Rep Prog Phys 1981; 44:1. [15] Cornut B, Coqblin B. Phys Rev B 1972;5:4541.
Hydrogenation and physical properties of the ternary germanide CeCoGe: an anisotropic expansion of the unit cell B. Chevalier*, E. Gaudin, F. Weill, J.-L. Bobet Institut de Chimie de la Matie`re Condense´e de Bordeaux (ICMCB), CNRS [UPR 9048], Universite´ Bordeaux 1, Avenue du Docteur A. Schweitzer, 33608 Pessac Cedex, France Received 13 October 2003; received in revised form 20 December 2003; accepted 23 December 2003
Abstract The ternary germanide CeCoGe and its hydride CeCoGeH crystallize in the tetragonal CeFeSi-type structure but the hydrogenation induces an anisotropic expansion of the unit cell parameters: a decreases (3.1%) whereas c increases (+12.7%). These structural changes suggest that the hydrogen atoms occupy the [Ce4] tetrahedral sites empty in CeCoGe. Magnetization and electrical resistivity measurements evidence an interesting transition from antiferromagnetic ordering [CeCoGe; TN=5.0(2) K] to spin fluctuation behaviour [CeCoGeH; Tsf=13.3(5) K]. # 2004 Elsevier Ltd. All rights reserved. Keywords: A. Rare-earth intermetallics; B. Crystal chemistry of intermetallics; B. Magnetic properties; F. Diffraction (electron, neutron and X-ray)
1. Introduction The ternary germanide CeCoGe crystallizes in the tetragonal CeFeSi-type structure (space group P4/nmm, No 129) [1]. This structure can be described by a stacking along the c-axis of two layers formed by [Ce4Co4] antiprisms and separated by one layer of [Ce4] pseudotetrahedral units (Fig. 1). The [Ce4Co4] antiprisms surrounding the Ge-atom are also observed in the crystal structure of the other ternary germanide CeCo2Ge2 [2]. The layer of [Ce4] pseudo-tetrahedral units is potentially interesting for hydrogen insertion. Similar sites form the crystal structure of the dihydride CeH2 [3]. Extending our research on new hydrides based on cerium as CeNiGaH1.1 [4], CeNiInH1.8 [5] and CeNiSnH1.8 [6], we have studied the hydrogen sorption properties of CeCoGe. Our recent investigation by neutron powder diffraction reveals that this ternary germanide orders antiferromagnetically below TN=5.0(2) K [7]; (standard deviations in the data of the least-significant digits are given in brackets throughout the paper). In this paper, the synthesis of the hydride CeCoGeH1.1(1) and its characterization using X-ray diffraction (powder and * Corresponding author. Fax: +33-5-4000-2761. E-mail address: [email protected] (B. Chevalier). 0966-9795/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2003.12.007
single crystal), magnetization and electrical resistivity measurements are reported. It is shown that the insertion of hydrogen into CeCoGe keeps its crystal symmetry but causes a pronounced anisotropic expansion of the unit cell: the a-parameter decreases whereas the c-parameter strongly increases. As far as the physical properties are concerned, the insertion of hydrogen leads to a transition from antiferromagnet to spin fluctuation behaviour. For comparison, some results concerning the hydrogenation of the isomorphous ternary germanide LaCoGe are also presented.
2. Experimental details A polycrystalline CeCoGe sample was synthesized by arc-melting a stoichiometric mixture of pure elements (purity above 99.9%) in a high-purity argon atmosphere. Then, the sample was turned and remelted several times to ensure homogeneity. Annealing was done for one month at 1073 K by enclosing the sample in an evacuated quartz tube. X-ray powder diffraction confirms that this ternary germanide crystallizes in the tetragonal CeFeSi-type structure with the unit cell parameters a=0.4170(2) nm and c=0.6865(2) nm as reported previously [1]. The same procedure was used
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for the preparation and the characterization of the LaCoGe sample. Hydrogen absorption experiments were performed using the apparatus described previously [8]. An annealed ingot was heated under vacuum at 393 K for 12 h and then exposed to 5 MPa of hydrogen gas at the same temperature. The amount of H absorbed was determined volumetrically by monitoring pressure changes in a calibrated volume. The kinetic hydrogen absorption of CeCoGe was measured using an automated Sievert-type apparatus (HERA, Hydrogen Storage System, Que´bec, Canada) with a sample of about 500 mg [9]. X-ray powder diffraction with the use of a Philips 1050-diffractometer (Cu Ka radiation) was applied for the characterization of the structural type and for the phase identification of the samples. The unit cell parameters were determined by a least-squares refinement method using silicon (5N) as an internal standard. The refinement of the crystal structure of the hydride was performed using a tiny single crystal isolated from the pulverized sample. It was very difficult to find an acceptable single crystal in the different hydride samples. Eventually, after many tests a single crystal with strongly elongated diffraction spots was found. Data collection was carried out on an Enraf-Nonius Kappa CCD diffractometer using Mo Ka radiation. The intensity integration of such elongated spots was possible with the new soltware EVAL-14 [10]. Data processing was performed with the Jana 2000 software [11]. A Gaussian-type absorption correction was applied and the shape was estimated with the video microscope of the Kappa CCD.
Fig. 1. Projection on the (a, c)-plane of the crystal structure of CeCoGe. (Ce, Ge and Co atoms are respectively represented by large, medium and small circles). d gives the interatomic distance between two Ce-atoms in the antiprism blocks.
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. Electrical resistivity was measured above 4.2 K on a polycrystalline sample using standard dc four-probe measurements. For this investigation, the hydride was compacted at room temperature (compactness 80%) in order to form a pellet (diameter=6 mm and thickness=3 mm) and then heated for two days at 523 K under a pressure (5 MPa) of hydrogen.
3. Results and discussion Fig. 2 presents the first hydrogen absorption kinetic of CeCoGe performed at 393 K under a pressure of P(H2)=2 MPa. The amount of hydrogen absorbed increases very slowly with the time of hydrogenation showing saturation at times longer than 15 h; finally the composition CeCoGeH1.07(5) is attained. The same hydride is obtained when CeCoGe ingots are exposed at 523 K for 36 h under 5 MPa of hydrogen gas; the hydrogenation induces a decrepitation of the starting ingots. This hydride is stable in ambient conditions and no decomposition is observed after its heating under vacuum at 523 K. 3.1. Structural properties The structure of CeCoGeH1.07(5) has been determined from single-crystal X-ray data. The extinction conditions observed agree with the P4/nmm space group (No
Fig. 2. First hydrogen absorption at 393 K for CeCoGe (P(H2)= 2 MPa).
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129) already used for the refinement of the initial ternary germanide CeCoGe [1]. For data collection details, see Table 1. The starting atomic positions were those reported previously for CeCoGe (Ce in 2c-site (1/4 1/4 0.67), Co in 2a-site (3/4 1/4 0) and Ge in 2c-site (1/4 1/4 0.17)) [1]. The structure refinement was carried out with the Jana 2000 program package [11]. With anisotropic displacement parameters, the final residual factors converged to the value R(F)=0.0549 and wR(F)=0.0610 for 9 refined parameters and 88 observed reflections. The final atomic positions and anisotropic displacement parameters are given respectively in Tables 2 and 3.
Table 1 Crystallographic data and structure refinement for CeCoGeH (*deduced from X-ray powder diffraction) Crystal data Chemical Formula Molar mass (g mol1) Temperature Crystal system Space group Unit cell dimension (nm)
CeCoGeH 272.65 293 K tetragonal P4/nmm (No 129) a=0.4040(2) * c=0.7735(4) * 0.1262(4) * 2 7.18 MoKa; 0.071073 35.68 234 Metallic luster 0.20 0.15 0.15
Volume (nm3) Z (formula units per unit cell) Calculated density (g cm3) Radiation and wavelength (nm) Absorption coefficient (mm1) F(000) Crystal color Crystal size (mm) Data collection Diffractometer range for data collection Hkl range Measured reflections Absorption correction Independent reflections Refinement Refinement on No of independent reflections No of observed reflections [I>3s(I)] No of parameters R(F) WR(F) S Difference Fourier residues (e A˚3)
Enraf-Nonius Kappa CCD area-detector 2.6 30.0 54h 45, 54k 45, 104l 410 1298 Gaussian method 132, (Rint=0.1493) F (structure factor) 132 88 9 0.0549 0.0610 1.31 +2.90 and 3.70
Table 2 Positional and equivalent isotropic displacement parameters Ueq (A˚2) for CeCoGeH Atom
Site
x
y
z
Ueq
Ce Co Ge
2c 2a 2c
1/4 3/4 1/4
1/4 1/4 1/4
0.6709(2) 0 0.1643(4)
0.0118(5) 0.0122(10) 0.0126(8)
The crystal structure of CeCoGe and its hydride adopt the same space group but the hydrogenation of this ternary germanide causes a pronounced anisotropic expansion of the unit cell; the a-parameter decreases from 0.4170(2) nm to 0.4040(2) nm (3.1%) whereas the c-parameter increases strongly from 0.6865(2) nm to 0.7735(4) nm (+12.7%). In other words, the insertion of hydrogen into CeCoGe involves an expansion of the unit cell volume from 0.1194(2) nm3 to 0.1262(4) nm3 (+5.7%). It is also worth pointing out that the atomic positions of Ce, Co and Ge in CeCoGe and its hydride (Table 2) are practically similar. The strong increase of the c-parameter in the sequence CeCoGe ! CeCoGeH1.07(5) suggests that the H-atoms are inserted in the [Ce4] pseudo-tetrahedral sites. According to this hypothesis, hydrogen occupies the 2bsite (1/4 3/4 1/2) giving interatomic distances dCeH=0.2410(4) nm between Ce and H atoms comparable to that observed in CeH2 dihydride (dCeH=0.2416 nm) [12]. The full occupancy of the 2bsite leads to the formulation CeCoGeH, close to that obtained experimentally. The possibility to insert hydrogen into the [Ce4Co4] antiprism seems to be unfavourable; our attempt to prepare hydride under the same condition [523 K, P(H2)=5 MPa] from the ternary germanide CeCo2Ge2 containing only the same antiprisms was unsuccessful. Neutron powder diffraction experiments need to be performed on CeCoGeD deuteride in order to verify this hypothesis. Table 4 compares the interatomic distances between Ce and its ligands in CeCoGe and its hydride. The distances dCeCo=0.3250 nm and dCeGe=0.3128 nm existing in the hydride CeCoGeH are comparable to that observed in the ternary germanide CeCo2Ge2; respectively 0.3257 nm and 0.3144 nm [2]. The hydrogenation of CeCoGe induces great changes in the values
Table 3 Anisotropic displacement parameters Uij (A˚2) for CeCoGeH Atom
U11
U22
U33
U12
U13
U23
Ce Co Ge
0.0097(7) 0.0113(14) 0.0106(12)
0.0097 0.0113 0.0106
0.0160(11) 0.014(2) 0.017(2)
0 0 0
0 0 0
0 0 0
Table 4 Selected interatomic distances (nm) around Ce atom in CeCoGe and its hydride CeCoGe
CeCoGeH
Ce -
Ce -
4 4 1 4 4
Ce 0.4170 Ce 0.3761 Ge 0.3433 Ge 0.3147 Co 0.3079
4 4 1 4 4
Ce 0.4040 Ce 0.3892 Ge 0.3919 Ge 0.3128 Co 0.3250
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Fig. 3 presents the temperature dependence of the reciprocal magnetic susceptibility w1 m of CeCoGe and its hydride. Above 70 K, the curve w1 m =f(T) relative to CeCoGe follows a Curie-Weiss law with the effective magnetic moment meff.=2.60(5) mB/Ce-ion and the paramagnetic Curie temperature p=39(1) K. The meff.
value is close to that calculated for a free Ce3+ ion (2.54 mB). Similar behaviour is observed above 30 K for the hydride CeCoGeH. In this case, the magnetic parameters are meff.=2.78(5) mB/Ce-ion and p= 29(1) K. The presence of a peak at TN=5.0(2) K in the temperature dependence of the magnetic susceptibility of CeCoGe (see inset of Fig. 4), agrees with the occurrence of an antiferromagnetic ordering of the Ce-moments [7]. Indeed, contrary to that reported previously by Welter et al. [1], our investigation of CeCoGe by neutron powder diffraction indicates that the Ce-ordered magnetic moment [0.43(6) mB/Ce at 1.6 K] forms a collinear antiferromagnetic structure associated to the k=(0 0 0) wavevector [7]. The reduced Ce-ordered magnetic moment observed at 1.6 K for CeCoGe leads to the presence of weak additional peaks of magnetic origin no detected previously in Ref. [1]. Below 20 K, the curve wm=f(T) relative to CeCoGeH shows a different behaviour (Fig. 4): (i) between 10 K and 4 K, wm tends to saturate; (ii) below 4 K, wm increases sharply with decreasing temperature. This behaviour is commonly observed in compounds based on cerium that show spin fluctuation arising from the Kondo effect and contain small amounts of impurity phase such as Ce2O3 leading to an increase of wm at low temperatures [14]. According to this scheme, the magnetic susceptibility wm of the hydride CeCoGeH can be expressed at low temperatures (T < 10 K) by wm=wm(0)+nC/T where wm(0) is the magnetic susceptibility at T=0 K, n is the proportion of stable Ce3+ moments composing the trace of magnetic impurities and C=0.807 emu K/mol is the Curie constant for free
Fig. 3. Temperature dependence of the reciprocal magnetic susceptibility w1 m , measured with an applied field m0H=4 T, of germanide CeCoGe and its hydride. Dashed lines show the fit to calculated w1 m with Curie-Weiss law. For clarity, the curve relative to CeCoGe is shifted vertically.
Fig. 4. Temperature dependence of the magnetic susceptibility wm, measured with an applied field m0H=0.1 T, of germanide CeCoGe (inset) and its hydride CeCoGeH. (Solid circles give the measured data and open circles show the corrected data (wmnC/T) (see text).
of the dCeCe distances; these located in the (a, b)-plane decrease whereas the four other increase. The pseudotetrahedral [Ce4] unit is less distorted in the hydride CeCoGeH. Another interesting feature concerns the increase, after hydrogenation, of the interatomic distance d (Fig. 1) between two Ce atoms in the antiprism blocks. This distance changes from 0.5406 nm to 0.5838 nm (+8.0%); this last value is similar to that existing in the ternary germanide CeCo2Ge2 (d=0.5839 nm) [2]. In other words, many structural characteristics of the [Ce4Co4] antiprisms found in the hydride CeCoGeH are in agreement with those observed in CeCo2Ge2. This ternary germanide is considered as an intermediate valence compound [13]. A similar hydrogenation procedure [T=523 K and P(H2)=5 MPa] performed on an isomorphous ternary germanide based on lanthanum LaCoGe leads to the synthesis of the hydride LaCoGeH1.02(5). The evolution of the unit cell parameters observed during this procedure is very close to that evidenced for CeCoGe: the a-parameter decreases from 0.4194(2) nm to 0.4063(2) nm (3.1%) whereas the c-parameter increases strongly from 0.7028(3) nm to 0.7912(3) nm (+12.6%). 3.2. Magnetic and electrical properties
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Fig. 5. Temperature dependence of the reduced electrical resistivity for hydrides CeCoGeH and LaCoGeH.
Ce3+ ions. The best agreement between experimental and calculated wm values leads to wm(0)=30.4(6).103 emu/mol and n=18.8(6).103 Ce3+ ions/mol. Fig. 4 presents the corrected (wmnC/T)=f(T) magnetic susceptibility versus temperature resulting from this calculation. This curve goes through a large maximum near 7(1) K; this temperature is almost consistent with the model offered by Lawrence et al. [14] which forecasts that the temperature dependence of the corrected susceptibility exhibits a maximum at T=Tsf/2 where Tsf=C/2wm(0)=13.3(5) K being the spin fluctuation temperature resulting from the hybridisation between 4f(Ce) and conduction electrons. This analysis suggests that the hydride CeCoGeH is a Kondo non-magnetic ordered system showing 13.3(5) K as spin fluctuation temperature. Fig. 5 compares the temperature dependence of the reduced resistivity of the two hydrides LaCoGeH and CeCoGeH. The curve relative to LaCoGeH characterizes a normal metal; its resistivity decreases with the temperature between 270 K and 15 K then takes a constant value at lowest temperatures. A different behaviour is observed for the resistivity of CeCoGeH. This curve reveals several anomalies: (i) a broad maximum appears around 90–100 K; (ii) a pronounced minimum occurs near 28(1) K and finally (iii) a decrease is evidenced below 7.0(5) K. This double-peak structure suggests a Kondo-lattice behaviour for CeCoGeH [15]. The presence of the maximum at 90–100 K represents the Kondo effect in the excited doublet whereas the minimum at 28(1) K and the maximum at 7.0(5) K characterize the onset of coherence effects of the Kondo lattice. The temperature 7.0(5) K where the curve =f(T) exhibits a maximum, is consistent with that 7(1) K where the magnetic susceptibility of CeCoGeH goes through a maximum (Fig. 4).
4. Conclusion Insertion of hydrogen into the crystal lattice of CeCoGe leads to an anisotropic expansion of the unit cell parameters but the symmetry is preserved. The suggested filling of the [Ce4] tetrahedral sites by hydrogen makes the crystallographic characteristics of the antiprism [Ce4Co4] layers of CeCoGeH comparable to those existing in the intermediate valence compound CeCo2Ge2. Magnetization and electrical resistivity measurements reveal that CeCoGeH shows spin fluctuation behaviour around Tsf=13.3(5) K. In other words, the hydrogenation of CeCoGe leads to the transition antiferromagnet ! spin fluctuation behaviour. This sequence is unusual since generally, the hydrogenation of intermetallics based on cerium induces a reduction of the hybridisation between 4f(Ce) electrons and conduction states. Other experiments such as neutron powder diffraction and specific heat measurements are planned in order to explain this interesting sequence. Nevertheless, we remark that the interatomic distances dCeCo and dCeGe existing in the hydride CeCoGeH are close to those determined in CeCo2Ge2, suggesting that the strength of the hybridisation between 4f(Ce) states and those of its ligands is comparable. This hybridisation plays an important role on the broadening of the 4f(Ce) states leading to a spin fluctuation behaviour or intermediate valence state.
Acknowledgements The authors thank J. Villot and R. Decourt for their technical help concerning the sample preparation and the electrical resistivity measurements.
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