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From antiferromagnetic ordering to spin fluctuation behavior induced by hydrogenation of ternary compounds CeCoSi and CeCoGe
ARTICLE IN PRESS
Physica B 378–380 (2006) 795–796 www.elsevier.com/locate/physb
From antiferromagnetic ordering to spin fluctuation behavior induced by hydrogenation of ternary compounds CeCoSi and CeCoGe B. Chevaliera,, S.F. Matara, J. Sanchez Marcosb, J. Rodriguez Fernandezb a
ICMCB, CNRS [UPR 9048], Universite´ Bordeaux 1, Avenue Dr. A. Schweitzer, 33608 Pessac, France b CITIMAC, Universidad de Cantabria, 39005 Santander, Spain
Abstract The ternary compounds CeCoSi and CeCoGe exhibit an antiferromagnetic ordering at 8.8 and 5.0 K, respectively. We present magnetization and specific heat measurements on these compounds and their hydrides, CeCoSiH and CeCoGeH. The hydrogen absorption reveals an original transition from antiferromagnetic to spin fluctuation behavior. CeCoSiH and CeCoGeH present a spin fluctuation temperature T sf around 130 and 15 K, respectively. r 2006 Elsevier B.V. All rights reserved. PACS: 65.40.Ba; 71.20.LP; 75.50.Ee; 75.50.Cc Keywords: Hydrogenation; Heat capacity; Spin fluctuations
Hydrogenation of the cerium-based equiatomic compounds CeNiX where X is a p-electron element, has received much attention in recent years because the hydrogen insertion leads to an expansion of the unit cell volume inducing a decrease of the strength of the interaction Jcf between 4f(Ce) electrons and conduction electrons [1]. This favors the occurrence of interesting magnetic transitions as intermediate valence state-ferromagnetic ordering in the CeNiInHy system or the Kondo semiconductor–antiferromagnet–ferromagnet sequence evidenced in the CeNiSnHy system. On the contrary, the hydrogenation of the compounds crystallizing in the tetragonal CeFeSi-type structure as CeCoSi, CeCoGe or CeMnGe shows that hydrogen atoms are inserted inside the [Ce4 ]-tetrahedral sites and that the chemical bonding between Ce- and H-atoms prevails over the cell expansion effect [2]. This latter influence leads to an unusual transition induced by hydrogenation from antiferromagnetism to spin fluctuation behavior as observed in the CeCoSiHy system. For a better understanding of these hydrides CeCoSiH and CeCoGeH, we report here on their Corresponding author. Tel.: +33 540006336; fax: +33 540002761.
E-mail address: [email protected] (B. Chevalier). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.291
investigation by magnetization and specific heat measurements. CeCoSiH and CeCoGeH were obtained by exposure of CeCoSi and CeCoGe for 48 h to 5 MPa of hydrogen gas at 523 K. These hydrides crystallize as the parent compounds in the tetragonal CeFeSi-type structure where H-atoms are inserted in the [Ce4 ] tetrahedral interstices [3]. The hydrogenation of CeCoSi and CeCoGe involves an expansion of the unit cell volume 7.8% and 5.7%, respectively. Fig. 1 presents the temperature dependence of the magnetic susceptibility wm of CeCoSiH and CeCoGeH. An appreciable increase in the wm value of CeCoGeH is observed relative to that of CeCoSiH. The curve wm ¼ f ðTÞ concerning CeCoSiH exhibits a broad maximum around 70 K characteristic of valence fluctuating systems and an upturn below 25 K attributed to the presence of traces of a magnetic impurity phase. Below 25 K, wm of CeCoSiH follows the expression wm ¼ wm ð0Þ þ nC=T where C ¼ 0:807 emu K=mol is the Curie constant for free Ce3þ ions, n ¼ 13:6 103 Ce3þ ion/mol the proportion of Ce3þ in the impurity phase and wm ð0Þ ¼ 3:85 103 emu/mol the susceptibility at 0 K. The corrected susceptibility (wm nC=TÞ ¼ f ðTÞ (full line in Fig. 1) goes through a
ARTICLE IN PRESS B. Chevalier et al. / Physica B 378–380 (2006) 795–796
796 0.025
10 CeCoGe CeCoGeH
6
Cp (J/mol K)
χm (emu/mol)
8
CeCoGeH
0.020
0.015
4
0.010
CeCoSiH
2 0.005
0
40
80 120 Temperature (K)
160
200
Fig. 1. Temperature dependence of the magnetic susceptibility of CeCoSiH and CeCoGeH.
0
0
5
10 Temperature (K)
15
20
Fig. 3. Low-temperature data of specific heat for CeCoGe and CeCoGeH.
CeCoSi CeCoSiH
Cp (J/mol K)
12
8
4
0
0
4
8 12 Temperature (K)
16
20
Fig. 2. Low-temperature data of specific heat for CeCoSi and CeCoSiH.
large maximum near 65 K giving a spin fluctuation temperature T sf ¼ 130 K. On the contrary, the curve wm ¼ f ðTÞ obtained from CeCoGeH shows only a broad maximum around 7.5 K suggesting that T sf ¼ 15 K. The specific heat C p vs. T curve for CeCoSi, see Fig. 2, shows a sharp l-type peak centering at 9.0 K, confirming the antiferromagnetic transition observed in the DC susceptibility (8.8 K) [2]. On the contrary, no anomaly can be distinguished from the curve C p ¼ f ðTÞ observed for CeCoSiH in agreement with the suppression of magnetic ordering by hydrogenation of CeCoSi.
The specific heat of CeCoGe and CeCoGeH below 20 K is shown in Fig. 3. For CeCoGe, a double transition is seen at 5.5 and 4.8 K indicating that the antiferromagnetic ordering is of a more complex form than suggested by neutron powder diffraction; this investigation reveals only an antiferromagnetic structure below T N ¼ 5:0 K [4]. The curve C p ¼ f ðTÞ obtained from CeCoGeH exhibits (i) no anomaly around 7.5 K where wm goes through a broad maximum and (ii) a very small bump close to 4.6 K. This behavior suggests that hydrogenation of CeCoGe destroys its long-range magnetic ordering but leads to the existence of some magnetic correlations. In conclusion, the hydrogenation of CeCoSi and CeCoGe induces a large anisotropic expansion of the unit cell but also the disappearance of its antiferromagnetic ordering. The transition from antiferromagnetism to an intermediate valence state is clearly evidenced during the hydrogenation of CeCoSi. On the contrary, a similar transition could not be observed in the CeCoGeHy system because the specific heat measurements performed on CeCoGeH reveals the presence of magnetic correlations. In order to explain this difference, the calculation of the electronic structure of CeCoGe and CeCoGeH is necessary.
References [1] [2] [3] [4]
B. B. B. B.
Chevalier, Chevalier, Chevalier, Chevalier,
et et et et
al., al., al., al.,
J. Alloys Compounds 383 (2004) 4. Phys. Rev. B 70 (2004) 174408. Solid State Comm. 134 (2005) 529. Solid State Comm. 130 (2004) 711.
Physica B 378–380 (2006) 795–796 www.elsevier.com/locate/physb
From antiferromagnetic ordering to spin fluctuation behavior induced by hydrogenation of ternary compounds CeCoSi and CeCoGe B. Chevaliera,, S.F. Matara, J. Sanchez Marcosb, J. Rodriguez Fernandezb a
ICMCB, CNRS [UPR 9048], Universite´ Bordeaux 1, Avenue Dr. A. Schweitzer, 33608 Pessac, France b CITIMAC, Universidad de Cantabria, 39005 Santander, Spain
Abstract The ternary compounds CeCoSi and CeCoGe exhibit an antiferromagnetic ordering at 8.8 and 5.0 K, respectively. We present magnetization and specific heat measurements on these compounds and their hydrides, CeCoSiH and CeCoGeH. The hydrogen absorption reveals an original transition from antiferromagnetic to spin fluctuation behavior. CeCoSiH and CeCoGeH present a spin fluctuation temperature T sf around 130 and 15 K, respectively. r 2006 Elsevier B.V. All rights reserved. PACS: 65.40.Ba; 71.20.LP; 75.50.Ee; 75.50.Cc Keywords: Hydrogenation; Heat capacity; Spin fluctuations
Hydrogenation of the cerium-based equiatomic compounds CeNiX where X is a p-electron element, has received much attention in recent years because the hydrogen insertion leads to an expansion of the unit cell volume inducing a decrease of the strength of the interaction Jcf between 4f(Ce) electrons and conduction electrons [1]. This favors the occurrence of interesting magnetic transitions as intermediate valence state-ferromagnetic ordering in the CeNiInHy system or the Kondo semiconductor–antiferromagnet–ferromagnet sequence evidenced in the CeNiSnHy system. On the contrary, the hydrogenation of the compounds crystallizing in the tetragonal CeFeSi-type structure as CeCoSi, CeCoGe or CeMnGe shows that hydrogen atoms are inserted inside the [Ce4 ]-tetrahedral sites and that the chemical bonding between Ce- and H-atoms prevails over the cell expansion effect [2]. This latter influence leads to an unusual transition induced by hydrogenation from antiferromagnetism to spin fluctuation behavior as observed in the CeCoSiHy system. For a better understanding of these hydrides CeCoSiH and CeCoGeH, we report here on their Corresponding author. Tel.: +33 540006336; fax: +33 540002761.
E-mail address: [email protected] (B. Chevalier). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.291
investigation by magnetization and specific heat measurements. CeCoSiH and CeCoGeH were obtained by exposure of CeCoSi and CeCoGe for 48 h to 5 MPa of hydrogen gas at 523 K. These hydrides crystallize as the parent compounds in the tetragonal CeFeSi-type structure where H-atoms are inserted in the [Ce4 ] tetrahedral interstices [3]. The hydrogenation of CeCoSi and CeCoGe involves an expansion of the unit cell volume 7.8% and 5.7%, respectively. Fig. 1 presents the temperature dependence of the magnetic susceptibility wm of CeCoSiH and CeCoGeH. An appreciable increase in the wm value of CeCoGeH is observed relative to that of CeCoSiH. The curve wm ¼ f ðTÞ concerning CeCoSiH exhibits a broad maximum around 70 K characteristic of valence fluctuating systems and an upturn below 25 K attributed to the presence of traces of a magnetic impurity phase. Below 25 K, wm of CeCoSiH follows the expression wm ¼ wm ð0Þ þ nC=T where C ¼ 0:807 emu K=mol is the Curie constant for free Ce3þ ions, n ¼ 13:6 103 Ce3þ ion/mol the proportion of Ce3þ in the impurity phase and wm ð0Þ ¼ 3:85 103 emu/mol the susceptibility at 0 K. The corrected susceptibility (wm nC=TÞ ¼ f ðTÞ (full line in Fig. 1) goes through a
ARTICLE IN PRESS B. Chevalier et al. / Physica B 378–380 (2006) 795–796
796 0.025
10 CeCoGe CeCoGeH
6
Cp (J/mol K)
χm (emu/mol)
8
CeCoGeH
0.020
0.015
4
0.010
CeCoSiH
2 0.005
0
40
80 120 Temperature (K)
160
200
Fig. 1. Temperature dependence of the magnetic susceptibility of CeCoSiH and CeCoGeH.
0
0
5
10 Temperature (K)
15
20
Fig. 3. Low-temperature data of specific heat for CeCoGe and CeCoGeH.
CeCoSi CeCoSiH
Cp (J/mol K)
12
8
4
0
0
4
8 12 Temperature (K)
16
20
Fig. 2. Low-temperature data of specific heat for CeCoSi and CeCoSiH.
large maximum near 65 K giving a spin fluctuation temperature T sf ¼ 130 K. On the contrary, the curve wm ¼ f ðTÞ obtained from CeCoGeH shows only a broad maximum around 7.5 K suggesting that T sf ¼ 15 K. The specific heat C p vs. T curve for CeCoSi, see Fig. 2, shows a sharp l-type peak centering at 9.0 K, confirming the antiferromagnetic transition observed in the DC susceptibility (8.8 K) [2]. On the contrary, no anomaly can be distinguished from the curve C p ¼ f ðTÞ observed for CeCoSiH in agreement with the suppression of magnetic ordering by hydrogenation of CeCoSi.
The specific heat of CeCoGe and CeCoGeH below 20 K is shown in Fig. 3. For CeCoGe, a double transition is seen at 5.5 and 4.8 K indicating that the antiferromagnetic ordering is of a more complex form than suggested by neutron powder diffraction; this investigation reveals only an antiferromagnetic structure below T N ¼ 5:0 K [4]. The curve C p ¼ f ðTÞ obtained from CeCoGeH exhibits (i) no anomaly around 7.5 K where wm goes through a broad maximum and (ii) a very small bump close to 4.6 K. This behavior suggests that hydrogenation of CeCoGe destroys its long-range magnetic ordering but leads to the existence of some magnetic correlations. In conclusion, the hydrogenation of CeCoSi and CeCoGe induces a large anisotropic expansion of the unit cell but also the disappearance of its antiferromagnetic ordering. The transition from antiferromagnetism to an intermediate valence state is clearly evidenced during the hydrogenation of CeCoSi. On the contrary, a similar transition could not be observed in the CeCoGeHy system because the specific heat measurements performed on CeCoGeH reveals the presence of magnetic correlations. In order to explain this difference, the calculation of the electronic structure of CeCoGe and CeCoGeH is necessary.
References [1] [2] [3] [4]
B. B. B. B.
Chevalier, Chevalier, Chevalier, Chevalier,
et et et et
al., al., al., al.,
J. Alloys Compounds 383 (2004) 4. Phys. Rev. B 70 (2004) 174408. Solid State Comm. 134 (2005) 529. Solid State Comm. 130 (2004) 711.