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Specific heat of mixed-valence intermetallic system Yb1−xCexInCu4
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 316 (2007) e428–e430 www.elsevier.com/locate/jmmm
Specific heat of mixed-valence intermetallic system Yb1xCexInCu4 T. Voloshoka,, E. Kadyrovaa, N. Mushnikovb, A. Vasilieva a
Low Temperature Physics and Superconductivity Department, Physics Faculty, Moscow State University, Moscow 119992, Russia b Institute of Metal Physics, Russian Academy of Sciences, Kovalevskaya Street 18, Ekaterinburg 620041, Russia Available online 12 March 2007
Abstract The YbInCu4 exhibits a first-order phase transition associated with a transformation of the valence state of the Yb ion. The effect of alloying on the valence transition in Yb1xCexInCu4 (0pxp0.2) has been investigated by specific heat C measurements in the range 5–300 K. The C vs. T dependence in YbInCu4 reveals sharp peak at TV ¼ 39 K. This peak shifts to higher temperatures with x and its shape gradually transforms to that of continuous phase transition. The specific heat measurements have also been made for nonmagnetic isomorphous compound YInCu. The comparison of YInCu4 data with those of Yb1xCexInCu4 allows an estimate of the value of entropy DS released at the phase transition. The Sommerfeld coefficient values in the Yb1xCexInCu4 family of compounds were found increasing with Ce concentration. r 2007 Elsevier B.V. All rights reserved. PACS: 71.27.+a; 75.30.Mb; 75.40.Cx Keywords: YbInCu4; Valence fluctuation; Specific heat
1. Introduction The intermetallic compound YbInCu4 exhibits a firstorder phase transition associated with a transformation of the valence state of the Yb ion from Yb3+ at elevated temperature to an intermediate valence state below TV ¼ 42 K [1]. This transition is accompanied by sharp changes in both thermodynamic and kinetic properties including specific heat, magnetic susceptibility and electrical resistivity [2]. From the thermal expansion measurements, the Yb valence state change is estimated to be about 0.1 [3–5]. At T4TV, the magnetic susceptibility follows the Curie–Weiss law with an effective magnetic moment close to the Yb3+ free-ion value. The high-temperature local moment state of Yb3+ ion transforms into an intermediate valence state at ToTV, where YbInCu4 exhibits almost temperature-independent Pauli paramagnetic behavior. Hence, a crossover from the stable local moments at high temperatures to a Fermi-liquid state occurs at TV Corresponding author. Tel.: +7 495 939 4811; fax: +7 495 932 9217.
E-mail address: [email protected] (T. Voloshok). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.169
accompanied by a strong hybridization of localized 4f electrons of Yb with conduction electrons [6]. The valence transition in YbInCu4 is strongly influenced by chemical substitution in both Yb and In sites [2,7,8]. The replacement of Yb ions by larger ionic radii rare-earth elements results in the increase of the phase-transition temperature, while the substitution of smaller radii rareearth elements reduces this temperature. At isovalent substitutions, these effects are due to chemical pressure. In case of Yb ions substituted by Ce ions, the phasetransition temperature increases, but the size effect is not the only cause. Here, it should be taken into account that as in majority of intermetallic compounds, the Ce ions possess an intermediate valence, thus changing the net concentration of carriers in Yb1xCexInCu4. In the present work, the systematic study of specific heat in the Yb1xCexInCu4 (0pxp0.2) series of compound in the range 5–300 K was performed. From the data obtained, the values of entropy DS released at phase transition were calculated. This quantity was found decreasing with Ce content. On the contrary, the linear term g in specific heat at low temperatures was found increasing with Ce content.
ARTICLE IN PRESS T. Voloshok et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e428–e430
2. Experimental Polycrystalline samples of Yb1xCexInCu4 (x ¼ 0, 0.04, 0.08, 0.12, 0.16, 0.20) were prepared by induction melting of high-purity constituents in alumina crucibles. Due to the volatile nature of Yb, a small excess of this constituent was used in the initial melt. The ingots were crushed and
a
C, J/(molK)
500 400 300
YbInCu4
200 100
C, J/(molK)
25000
b
2000 1500
120 80
1000
40
Yb0.96Ce0.04InCu4
500 40
0
c
500 C, J/(molK)
60
400 300
Yb0.92Ce0.08InCu4
200 100 0
d
C, J/(molK)
250 200 150 100
Yb0.88Ce0.12InCu4
50
C, J/(molK)
re-melted using semi-levitation technique to achieve better mixing of the components and smaller grain size. The ingots were wrapped in Ta foils and sealed in a quartz tube filled with purified Ar. The annealing of the samples obtained was performed for 1 month at 870 1C [9]. The specific heat measurements in a range 5–300 K were performed in a quasi-adiabatic microcalorimeter ‘Termis’. The magnetic susceptibility in the Yb1xCexInCu4 series of compound indicates that the valence phase transition shifts by the alloying to the high-temperature region. The transition is very sharp at low Ce content [9]. At higher x, the anomaly in magnetic susceptibility broadens significantly showing a behavior similar to that of the dense Kondo lattice compound YbAgCu4 [10]. The temperature dependences of specific heat in Yb1xCex InCu4 are shown in Fig. 1a–f. In the parent compound YbInCu4, a very sharp, almost symmetrical singularity marks a first-order phase transition at TV ¼ 39 K. Note that the high-temperature shoulder of the peak is especially abrupt indicating a rather high quality of the sample studied. At increasing Ce content, the phase transition shifts to higher temperatures, the anomaly broadens and its shape changes qualitatively. At high Ce content, it is more of a l-type shape with a well-pronounced high-temperature shoulder. In the series of the compounds studied, the phasetransition features in the Yb0.96Ce0.04InCu4 sample differ from the other ones, as shown in Fig. 1b. It is because originally this compound had shown a double-peak structure at phase transition assumingly due to a spinodal decomposition. The newly prepared sample of Yb0.96Ce0.04 InCu4 had shown an extra-high single peak at phase transition. The specific heat measurements have also been done for the nonmagnetic isomorphous compound YInCu4, which shows monotonous dependence in the temperature range studied. 3. Discussion
0 150
e429
e
At low temperatures, the C vs. T dependence in every member of Yb1xCexInCu4 series can be represented by the sum of linear and cubic terms:
100 50 Yb0.0.84Ce0.16InCu4
C ¼ gT þ bT 3 .
(1) 2
0
f
C, J/(molK)
150 100 50
Yb0.80Ce0.20InCu4 0 0
50
100
150
200
250
T, K Fig. 1. Temperature dependences of specific heat in the Yb1xCexInCu4 series of compounds.
The C/T vs. T curves in the Yb1xCexInCu4 series of compounds are shown in Fig. 2. At low temperatures, these curves appear as parallel straight lines shifted with respect to each other. The linear term in specific heat is due to electrons’ contribution while the cubic term relates to phonons’ contribution. The Sommerfeld coefficient g serves as a measure of the electronic density of states at the Fermi energy D(EF), g ¼ 13p2 DðE F Þk2B
(2)
being directly proportional to the effective mass m* of the electrons. The Sommerfeld coefficient in YbInCu4 is about
ARTICLE IN PRESS T. Voloshok et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e428–e430
e430
0.20 C / T, J / molK2
scaling procedure was employed to compare the lattice contributions [11]. The magnetic phase transition entropy was calculated as Z T C mag dT, (3) DS mag ¼ T 0
x=0 x = 0.04 x = 0.08 x = 0.12 x = 0.16 x = 0.20
0.24
0.16 0.12 0.08 0.04 0
20
40
60
80
100
120
140
160
180
T2, K2 Fig. 2. The C/T vs. T2 dependences in the Yb1xCexInCu4 series of compounds.
where Cmag represents the difference in specific heat between scaled Yb1xCexInCu4 and YInCu4 data. This quantity as shown in Table 1 was found decreasing with increasing Ce content. This observation once again is in direct correspondence with reduction in magnetic susceptibility [9]. In principle, the volume change at phase transition is accompanied by an entropy jump too. However, its values in the Yb1xCexInCu4 series of compounds were found negligible as compared to magnetic specific heat jumps. 4. Conclusion
Table 1 Thermal parameters of Yb1xCexInCu4 Compound
TV (K)
g (mJ/mol K2)
DS (J/mol K)
YbInCu4 Yb0.96Ce0.04InCu4 Yb0.92Ce0.08InCu4 Yb0.88Ce0.12InCu4 Yb0.84Ce0.16InCu4 Yb0.80Ce0.20InCu4
39 45 50 55 59 63
39 57 61 77 81 90
12.6 9.3 7.1 5.6 5.8 4.45
We have presented results of the alloying effect on the first-order valence phase transition in the Yb1xCexInCu4 series of compounds as obtained in specific heat measurements. The electronic specific heat was found increasing with Ce content indicating increase in the density of states at the Fermi level. The magnetic entropy jump at phase transition was found decreasing with the Ce content indicating a tendency to stronger hybridization of 4f-electrons with conduction electrons. References
2
40 mJ/mol K indicating strong renormalization of kinetic parameters in this compound. With increasing Ce content, the g value also increases as shown in Table 1. This evidently reflects the fact that Ce3+/4+ ions raise the density of states at the Fermi level. The absolute values of Sommerfeld coefficients in the Yb1xCexInCu4 series of compounds allow to classify this family as a ‘light’ heavyfermion system. The increase in electronic specific heat with x is in direct correspondence with the increase in Pauli magnetic susceptibility at ToTV as found in magneticsusceptibility measurements [9]. The coefficient b responsible for the slope of C/T vs. T2 lines is practically independent of Ce content. It means that the Debye temperature is not very sensitive to composition and equals to (23575) K. To subtract the specific heat associated with phase transition in the Yb1xCexInCu4 series of compounds, the specific heat data for YInCu4 were used. Due to a large difference in the masses of Y and Yb ions, the special
[1] I. Felner, I. Nowik, Phys. Rev. B 33 (1986) 617. [2] J.L. Sarrao, Physica B 259–261 (1999) 128. [3] A.L. Kornelius, J.M. Lawrence, J.L. Sarrao, Z. Fisk, M.F. Hundley, G.H. Kwei, J.D. Thompson, C.H. Booth, F. Bridges, Phys. Rev. B 56 (1997) 7993. [4] I. Felner, I. Nowik, D. Vaknin, U. Potzel, J. Moser, G.M. Kalvius, G. Wortmann, G. Schmiester, G. Hilscher, G. Gratz, C. Schmitzer, N. Pillmayr, K.G. Prasad, H. de Waard, H. Pinto, Phys. Rev. B 35 (1987) 6956. [5] J.M. de Teresa, Z. Arnold, A. del Moral, M.R. Ibarra, J. Kamarad, D.T. Adroja, B. Rainford, Solid State Commun. 99 (1996) 911. [6] H. Nakamura, K. Nakagima, Y. Kitaoka, K. Asayama, K. Yoshimura, T. Nitta, J. Phys. Soc. Jpn. 59 (1990) 28. [7] H. Nakamura, M. Shiga, Physica B 206–207 (1995) 364. [8] C.D. Immer, J.L. Sarrao, Z. Fisk, A. Lacerda, C. Mielke, J.D. Thompson, Phys. Rev. B 56 (1997) 71. [9] N.V. Mushnikov, T. Goto, F. Ishikawa, W. Zhang, K. Yoshimura, V.S. Gaviko, J. Alloys Compds. 345 (2002) 20. [10] C. Rossel, K.N. Yang, M.B. Maple, Z. Fisk, E. Zirngiebl, J.D. Thompson, Phys. Rev. B 35 (1987) 1914. [11] D.B. Losee, J.N. McElearney, G.E. Shankle, R.L. Carlin, P.J. Cresswell, W.T. Robinson, Phys. Rev. B 8 (1973) 2185.
Journal of Magnetism and Magnetic Materials 316 (2007) e428–e430 www.elsevier.com/locate/jmmm
Specific heat of mixed-valence intermetallic system Yb1xCexInCu4 T. Voloshoka,, E. Kadyrovaa, N. Mushnikovb, A. Vasilieva a
Low Temperature Physics and Superconductivity Department, Physics Faculty, Moscow State University, Moscow 119992, Russia b Institute of Metal Physics, Russian Academy of Sciences, Kovalevskaya Street 18, Ekaterinburg 620041, Russia Available online 12 March 2007
Abstract The YbInCu4 exhibits a first-order phase transition associated with a transformation of the valence state of the Yb ion. The effect of alloying on the valence transition in Yb1xCexInCu4 (0pxp0.2) has been investigated by specific heat C measurements in the range 5–300 K. The C vs. T dependence in YbInCu4 reveals sharp peak at TV ¼ 39 K. This peak shifts to higher temperatures with x and its shape gradually transforms to that of continuous phase transition. The specific heat measurements have also been made for nonmagnetic isomorphous compound YInCu. The comparison of YInCu4 data with those of Yb1xCexInCu4 allows an estimate of the value of entropy DS released at the phase transition. The Sommerfeld coefficient values in the Yb1xCexInCu4 family of compounds were found increasing with Ce concentration. r 2007 Elsevier B.V. All rights reserved. PACS: 71.27.+a; 75.30.Mb; 75.40.Cx Keywords: YbInCu4; Valence fluctuation; Specific heat
1. Introduction The intermetallic compound YbInCu4 exhibits a firstorder phase transition associated with a transformation of the valence state of the Yb ion from Yb3+ at elevated temperature to an intermediate valence state below TV ¼ 42 K [1]. This transition is accompanied by sharp changes in both thermodynamic and kinetic properties including specific heat, magnetic susceptibility and electrical resistivity [2]. From the thermal expansion measurements, the Yb valence state change is estimated to be about 0.1 [3–5]. At T4TV, the magnetic susceptibility follows the Curie–Weiss law with an effective magnetic moment close to the Yb3+ free-ion value. The high-temperature local moment state of Yb3+ ion transforms into an intermediate valence state at ToTV, where YbInCu4 exhibits almost temperature-independent Pauli paramagnetic behavior. Hence, a crossover from the stable local moments at high temperatures to a Fermi-liquid state occurs at TV Corresponding author. Tel.: +7 495 939 4811; fax: +7 495 932 9217.
E-mail address: [email protected] (T. Voloshok). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.169
accompanied by a strong hybridization of localized 4f electrons of Yb with conduction electrons [6]. The valence transition in YbInCu4 is strongly influenced by chemical substitution in both Yb and In sites [2,7,8]. The replacement of Yb ions by larger ionic radii rare-earth elements results in the increase of the phase-transition temperature, while the substitution of smaller radii rareearth elements reduces this temperature. At isovalent substitutions, these effects are due to chemical pressure. In case of Yb ions substituted by Ce ions, the phasetransition temperature increases, but the size effect is not the only cause. Here, it should be taken into account that as in majority of intermetallic compounds, the Ce ions possess an intermediate valence, thus changing the net concentration of carriers in Yb1xCexInCu4. In the present work, the systematic study of specific heat in the Yb1xCexInCu4 (0pxp0.2) series of compound in the range 5–300 K was performed. From the data obtained, the values of entropy DS released at phase transition were calculated. This quantity was found decreasing with Ce content. On the contrary, the linear term g in specific heat at low temperatures was found increasing with Ce content.
ARTICLE IN PRESS T. Voloshok et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e428–e430
2. Experimental Polycrystalline samples of Yb1xCexInCu4 (x ¼ 0, 0.04, 0.08, 0.12, 0.16, 0.20) were prepared by induction melting of high-purity constituents in alumina crucibles. Due to the volatile nature of Yb, a small excess of this constituent was used in the initial melt. The ingots were crushed and
a
C, J/(molK)
500 400 300
YbInCu4
200 100
C, J/(molK)
25000
b
2000 1500
120 80
1000
40
Yb0.96Ce0.04InCu4
500 40
0
c
500 C, J/(molK)
60
400 300
Yb0.92Ce0.08InCu4
200 100 0
d
C, J/(molK)
250 200 150 100
Yb0.88Ce0.12InCu4
50
C, J/(molK)
re-melted using semi-levitation technique to achieve better mixing of the components and smaller grain size. The ingots were wrapped in Ta foils and sealed in a quartz tube filled with purified Ar. The annealing of the samples obtained was performed for 1 month at 870 1C [9]. The specific heat measurements in a range 5–300 K were performed in a quasi-adiabatic microcalorimeter ‘Termis’. The magnetic susceptibility in the Yb1xCexInCu4 series of compound indicates that the valence phase transition shifts by the alloying to the high-temperature region. The transition is very sharp at low Ce content [9]. At higher x, the anomaly in magnetic susceptibility broadens significantly showing a behavior similar to that of the dense Kondo lattice compound YbAgCu4 [10]. The temperature dependences of specific heat in Yb1xCex InCu4 are shown in Fig. 1a–f. In the parent compound YbInCu4, a very sharp, almost symmetrical singularity marks a first-order phase transition at TV ¼ 39 K. Note that the high-temperature shoulder of the peak is especially abrupt indicating a rather high quality of the sample studied. At increasing Ce content, the phase transition shifts to higher temperatures, the anomaly broadens and its shape changes qualitatively. At high Ce content, it is more of a l-type shape with a well-pronounced high-temperature shoulder. In the series of the compounds studied, the phasetransition features in the Yb0.96Ce0.04InCu4 sample differ from the other ones, as shown in Fig. 1b. It is because originally this compound had shown a double-peak structure at phase transition assumingly due to a spinodal decomposition. The newly prepared sample of Yb0.96Ce0.04 InCu4 had shown an extra-high single peak at phase transition. The specific heat measurements have also been done for the nonmagnetic isomorphous compound YInCu4, which shows monotonous dependence in the temperature range studied. 3. Discussion
0 150
e429
e
At low temperatures, the C vs. T dependence in every member of Yb1xCexInCu4 series can be represented by the sum of linear and cubic terms:
100 50 Yb0.0.84Ce0.16InCu4
C ¼ gT þ bT 3 .
(1) 2
0
f
C, J/(molK)
150 100 50
Yb0.80Ce0.20InCu4 0 0
50
100
150
200
250
T, K Fig. 1. Temperature dependences of specific heat in the Yb1xCexInCu4 series of compounds.
The C/T vs. T curves in the Yb1xCexInCu4 series of compounds are shown in Fig. 2. At low temperatures, these curves appear as parallel straight lines shifted with respect to each other. The linear term in specific heat is due to electrons’ contribution while the cubic term relates to phonons’ contribution. The Sommerfeld coefficient g serves as a measure of the electronic density of states at the Fermi energy D(EF), g ¼ 13p2 DðE F Þk2B
(2)
being directly proportional to the effective mass m* of the electrons. The Sommerfeld coefficient in YbInCu4 is about
ARTICLE IN PRESS T. Voloshok et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e428–e430
e430
0.20 C / T, J / molK2
scaling procedure was employed to compare the lattice contributions [11]. The magnetic phase transition entropy was calculated as Z T C mag dT, (3) DS mag ¼ T 0
x=0 x = 0.04 x = 0.08 x = 0.12 x = 0.16 x = 0.20
0.24
0.16 0.12 0.08 0.04 0
20
40
60
80
100
120
140
160
180
T2, K2 Fig. 2. The C/T vs. T2 dependences in the Yb1xCexInCu4 series of compounds.
where Cmag represents the difference in specific heat between scaled Yb1xCexInCu4 and YInCu4 data. This quantity as shown in Table 1 was found decreasing with increasing Ce content. This observation once again is in direct correspondence with reduction in magnetic susceptibility [9]. In principle, the volume change at phase transition is accompanied by an entropy jump too. However, its values in the Yb1xCexInCu4 series of compounds were found negligible as compared to magnetic specific heat jumps. 4. Conclusion
Table 1 Thermal parameters of Yb1xCexInCu4 Compound
TV (K)
g (mJ/mol K2)
DS (J/mol K)
YbInCu4 Yb0.96Ce0.04InCu4 Yb0.92Ce0.08InCu4 Yb0.88Ce0.12InCu4 Yb0.84Ce0.16InCu4 Yb0.80Ce0.20InCu4
39 45 50 55 59 63
39 57 61 77 81 90
12.6 9.3 7.1 5.6 5.8 4.45
We have presented results of the alloying effect on the first-order valence phase transition in the Yb1xCexInCu4 series of compounds as obtained in specific heat measurements. The electronic specific heat was found increasing with Ce content indicating increase in the density of states at the Fermi level. The magnetic entropy jump at phase transition was found decreasing with the Ce content indicating a tendency to stronger hybridization of 4f-electrons with conduction electrons. References
2
40 mJ/mol K indicating strong renormalization of kinetic parameters in this compound. With increasing Ce content, the g value also increases as shown in Table 1. This evidently reflects the fact that Ce3+/4+ ions raise the density of states at the Fermi level. The absolute values of Sommerfeld coefficients in the Yb1xCexInCu4 series of compounds allow to classify this family as a ‘light’ heavyfermion system. The increase in electronic specific heat with x is in direct correspondence with the increase in Pauli magnetic susceptibility at ToTV as found in magneticsusceptibility measurements [9]. The coefficient b responsible for the slope of C/T vs. T2 lines is practically independent of Ce content. It means that the Debye temperature is not very sensitive to composition and equals to (23575) K. To subtract the specific heat associated with phase transition in the Yb1xCexInCu4 series of compounds, the specific heat data for YInCu4 were used. Due to a large difference in the masses of Y and Yb ions, the special
[1] I. Felner, I. Nowik, Phys. Rev. B 33 (1986) 617. [2] J.L. Sarrao, Physica B 259–261 (1999) 128. [3] A.L. Kornelius, J.M. Lawrence, J.L. Sarrao, Z. Fisk, M.F. Hundley, G.H. Kwei, J.D. Thompson, C.H. Booth, F. Bridges, Phys. Rev. B 56 (1997) 7993. [4] I. Felner, I. Nowik, D. Vaknin, U. Potzel, J. Moser, G.M. Kalvius, G. Wortmann, G. Schmiester, G. Hilscher, G. Gratz, C. Schmitzer, N. Pillmayr, K.G. Prasad, H. de Waard, H. Pinto, Phys. Rev. B 35 (1987) 6956. [5] J.M. de Teresa, Z. Arnold, A. del Moral, M.R. Ibarra, J. Kamarad, D.T. Adroja, B. Rainford, Solid State Commun. 99 (1996) 911. [6] H. Nakamura, K. Nakagima, Y. Kitaoka, K. Asayama, K. Yoshimura, T. Nitta, J. Phys. Soc. Jpn. 59 (1990) 28. [7] H. Nakamura, M. Shiga, Physica B 206–207 (1995) 364. [8] C.D. Immer, J.L. Sarrao, Z. Fisk, A. Lacerda, C. Mielke, J.D. Thompson, Phys. Rev. B 56 (1997) 71. [9] N.V. Mushnikov, T. Goto, F. Ishikawa, W. Zhang, K. Yoshimura, V.S. Gaviko, J. Alloys Compds. 345 (2002) 20. [10] C. Rossel, K.N. Yang, M.B. Maple, Z. Fisk, E. Zirngiebl, J.D. Thompson, Phys. Rev. B 35 (1987) 1914. [11] D.B. Losee, J.N. McElearney, G.E. Shankle, R.L. Carlin, P.J. Cresswell, W.T. Robinson, Phys. Rev. B 8 (1973) 2185.