- Email: [email protected]
Temperature- and pressure-induced valence transition in EuCo2Ge2
ARTICLE IN PRESS
Physica B 378–380 (2006) 724–725 www.elsevier.com/locate/physb
Temperature- and pressure-induced valence transition in EuCo2 Ge2 G. Dionicioa,, H. Wilhelma, Z. Hossainb, C. Geibela a
Max Planck Institute for Chemical Physics of Solids, No¨tnitzer Str. 40, 01187 Dresden, Germany b Department of Physics, Indian Institute of Technology, Kanpur-208016, India
Abstract The temperature dependence of the electrical resistivity rðTÞ of EuCo2 Ge2 was measured up to 7 GPa in the temperature range 4 KoTo300 K. At ambient pressure, the magnetic Eu2þ moments order antiferromagnetically below T N ¼ 23 K. Upon applying pressure, T N increases monotonously. For pressures higher than 3 GPa, the change of the overall shape of rðTÞ suggests a temperatureand pressure-induced continuous valence transition from a high-temperature Eu2þ state to a low-temperature Eu3þ state. r 2006 Elsevier B.V. All rights reserved. PACS: 74.62.Fj; 75; 75.30.Mb; 74.62.c Keywords: EuCo2 Ge2 ; Resistivity; Valence transition; High pressure
Among the intermetallic compounds that crystallize in the tetragonal ThCr2 Si2 structure, those which contain Eu certainly are of interest since their electronic configuration is sensitive to chemical substitution and pressure (p). The Eu ions can adopt either a magnetic (S ¼ 72Þ Eu2þ (4f 7 ) configuration or a non-magnetic (J ¼ 0Þ Eu3þ (4f 6 ) configuration. Thus, while EuCo2 Si2 , e.g., a stable nonmagnetic state [1], EuNi2 Ge2 orders antiferromagnetically below 45 K. In the latter compound, pressures higher than 1.5 GPa destabilizes the Eu2þ state and a isostructural firstorder transition towards a non-magnetic Eu3þ occurs. An average reduction of the volume of the unit cell of EuNi2 Ge2 can be achieved with a partial substitution of Ge with Si, which has an equivalent effect to pressure in this compound [2,3]. In quite a number of compounds, one observes a continuous evolution from a divalent state at high-temperature to a trivalent state at low-temperature with an extended valence fluctuating regime in between [1,4]. Studies at ambient pressure performed on single-crystalline samples of EuCo2 Ge2 reveal that the Eu ions are divalent. Magnetic susceptibility, resistivity and specific Corresponding author. Tel.: +49 351 4646 3129; fax: +49 351 4646 3119. E-mail address: [email protected] (G. Dionicio).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.258
measurements confirm that the Eu2þ ions orders antiferromagnetically (AFM) below T N ¼ 23 K [5]. Considering that a non-magnetic Eu3þ state can be achieved by a complete substitution of Ge with Si, the system is an ideal candidate to study the destabilization of the Eu valence state with pressure. We present the temperature dependence of electrical resistivity of EuCo2 Ge2 measured up to 7 GPa in the temperature range 4 KoTo300 K. The measurements were performed on single crystals grown using a self-flux method [5]. The experiment was carried out in a Bridgmantype of pressure cell with lead as the pressure gauge [6]. The temperature dependence of electrical resistivity rðTÞ of EuCo2 Ge2 at selected pressures is shown in Fig. 1. For pp2:6 GPa, rðTÞ is typical of a well-localized Eu-system having a linear temperature-dependence of rðTÞ in the paramagnetic state (PM) followed by a sudden drop at ToT N . The onset of long-range order causes a reduction of the scattering of the conduction electrons on the magnetic sites. For pressures higher than 3 GPa, an abrupt change of the overall shape of rðTÞ occurs and the anomaly in rðTÞ associated with the magnetic ordering is absent. At 3.2 GPa, the reduction of the electronic scattering already happens at about 100 K, which is related to the instability of the valence of Eu ions. For higher pressures, the temperature at which the fluctuation from a 2+ state to a
ARTICLE IN PRESS G. Dionicio et al. / Physica B 378–380 (2006) 724–725
300
700
2.6 3.2 4.3
600
5.0 5.5 6.4 7.3
5 GPa
FWHM 200
T (K)
(µΩcm)
TN
T (K) 40
0
150
300
x
~2+ PM
(µΩcm)
100
Tx
~3+ NON-MAG EuCo2Ge2
TN
3.2 GPa 510
EuCo2Ge2
2+ / 3 +
T (K)
60 T
540
500
0
/T (a.u.)
P (GPa) 0.76 1.54
400
725
AFM 100
200
0
300
T (K) Fig. 1. Temperature dependence of the electrical resistivity of EuCo2 Ge2 at selected pressures. The inset shows the anomaly observed in rðTÞ at 3.2 GPa.
3+ state occurs is even higher. It is to be mentioned that for p45 GPa, the fluctuation of the ions already occurs at room temperature. A tiny anomaly, labelled as T X , was observed at about 50 K at 3.2 GPa, but its nature is not clear at present (inset of Fig. 1). A tentative (T; p) phase diagram for EuCo2 Ge2 is shown in Fig. 2. Upon applying pressure up to 2.6 GPa, T N can be determined unambiguously and it increases roughly by 3 K/GPa in this pressure range. The absence of a sharp or a hysteretic transition in rðTÞ for p43:2 GPa suggests a temperature- and pressure-induced continuous valence transition from a high-temperature Eu2þ state to a lowtemperature Eu3þ state. The gradual and continuous crossover is represented by the shaded area in Fig. 2. The shaded area was defined from the first derivative of rðTÞ, as it is shown in the inset of Fig. 2. Thus, it is possible to divide the phase diagram in three areas: (i) in the AFM and PM regions the Eu ions are divalent while (ii) in the low temperature and high pressure non-magnetic region the Eu ions of EuCo2 Ge2 are trivalent. (iii) In the shaded area, the Eu ions fluctuate between these two mentioned valence states.
0
2
4 p (GPa)
6
8
Fig. 2. (T; p) Phase diagram of EuCo2 Ge2 . At low pressures, a secondorder phase transition from a paramagnetic (PM) state to an AFM state occurs upon cooling while a continuous crossover (shaded area) to a nonmagnetic (NON-MAG) state takes place for p43:2 GPa. The inset shows the maximum in qr=qT at 5 GPa. The full-width at half-maximum (FWHM) of qr=qT at different pressures was used to define the shaded area.
Based on the abrupt disappearance of the AFM-order at 2:6 GPaopc o3:2 GPa, one can suspect the presence of a first-order transition on the ground state as a function of pressure, as denoted by the dotted line in Fig. 2. The absence of a well-defined temperature-dependent firstorder valence transition for p4pc , as usually observed in this kind of systems, is therefore surprising and a specific feature of EuCo2 Ge2 . It may be possible that the first-order transition occurs only in a narrow pressure range above pc and the anomaly at T X may be a reminiscence of it. References [1] [2] [3] [4] [5] [6]
Fournes, et al., Z. Phys. B: Condens. Matter 75 (1989) 501. H. Wada, et al., Phys. Rev. 59 (1999) 1141. Z. Hossain, et al., J. Magn. Magn. Mater. 272–276 (2004) 2352. C.U. Segre, et al., Phys. Rev. Lett. 49 (1982) 1947. Z. Hossain, et al., J. Magn. Magn. Mater. 264 (2003) 142. B. Biereckhoven, J. Wittig, J. Phys. E 21 (1988) 841.
Physica B 378–380 (2006) 724–725 www.elsevier.com/locate/physb
Temperature- and pressure-induced valence transition in EuCo2 Ge2 G. Dionicioa,, H. Wilhelma, Z. Hossainb, C. Geibela a
Max Planck Institute for Chemical Physics of Solids, No¨tnitzer Str. 40, 01187 Dresden, Germany b Department of Physics, Indian Institute of Technology, Kanpur-208016, India
Abstract The temperature dependence of the electrical resistivity rðTÞ of EuCo2 Ge2 was measured up to 7 GPa in the temperature range 4 KoTo300 K. At ambient pressure, the magnetic Eu2þ moments order antiferromagnetically below T N ¼ 23 K. Upon applying pressure, T N increases monotonously. For pressures higher than 3 GPa, the change of the overall shape of rðTÞ suggests a temperatureand pressure-induced continuous valence transition from a high-temperature Eu2þ state to a low-temperature Eu3þ state. r 2006 Elsevier B.V. All rights reserved. PACS: 74.62.Fj; 75; 75.30.Mb; 74.62.c Keywords: EuCo2 Ge2 ; Resistivity; Valence transition; High pressure
Among the intermetallic compounds that crystallize in the tetragonal ThCr2 Si2 structure, those which contain Eu certainly are of interest since their electronic configuration is sensitive to chemical substitution and pressure (p). The Eu ions can adopt either a magnetic (S ¼ 72Þ Eu2þ (4f 7 ) configuration or a non-magnetic (J ¼ 0Þ Eu3þ (4f 6 ) configuration. Thus, while EuCo2 Si2 , e.g., a stable nonmagnetic state [1], EuNi2 Ge2 orders antiferromagnetically below 45 K. In the latter compound, pressures higher than 1.5 GPa destabilizes the Eu2þ state and a isostructural firstorder transition towards a non-magnetic Eu3þ occurs. An average reduction of the volume of the unit cell of EuNi2 Ge2 can be achieved with a partial substitution of Ge with Si, which has an equivalent effect to pressure in this compound [2,3]. In quite a number of compounds, one observes a continuous evolution from a divalent state at high-temperature to a trivalent state at low-temperature with an extended valence fluctuating regime in between [1,4]. Studies at ambient pressure performed on single-crystalline samples of EuCo2 Ge2 reveal that the Eu ions are divalent. Magnetic susceptibility, resistivity and specific Corresponding author. Tel.: +49 351 4646 3129; fax: +49 351 4646 3119. E-mail address: [email protected] (G. Dionicio).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.258
measurements confirm that the Eu2þ ions orders antiferromagnetically (AFM) below T N ¼ 23 K [5]. Considering that a non-magnetic Eu3þ state can be achieved by a complete substitution of Ge with Si, the system is an ideal candidate to study the destabilization of the Eu valence state with pressure. We present the temperature dependence of electrical resistivity of EuCo2 Ge2 measured up to 7 GPa in the temperature range 4 KoTo300 K. The measurements were performed on single crystals grown using a self-flux method [5]. The experiment was carried out in a Bridgmantype of pressure cell with lead as the pressure gauge [6]. The temperature dependence of electrical resistivity rðTÞ of EuCo2 Ge2 at selected pressures is shown in Fig. 1. For pp2:6 GPa, rðTÞ is typical of a well-localized Eu-system having a linear temperature-dependence of rðTÞ in the paramagnetic state (PM) followed by a sudden drop at ToT N . The onset of long-range order causes a reduction of the scattering of the conduction electrons on the magnetic sites. For pressures higher than 3 GPa, an abrupt change of the overall shape of rðTÞ occurs and the anomaly in rðTÞ associated with the magnetic ordering is absent. At 3.2 GPa, the reduction of the electronic scattering already happens at about 100 K, which is related to the instability of the valence of Eu ions. For higher pressures, the temperature at which the fluctuation from a 2+ state to a
ARTICLE IN PRESS G. Dionicio et al. / Physica B 378–380 (2006) 724–725
300
700
2.6 3.2 4.3
600
5.0 5.5 6.4 7.3
5 GPa
FWHM 200
T (K)
(µΩcm)
TN
T (K) 40
0
150
300
x
~2+ PM
(µΩcm)
100
Tx
~3+ NON-MAG EuCo2Ge2
TN
3.2 GPa 510
EuCo2Ge2
2+ / 3 +
T (K)
60 T
540
500
0
/T (a.u.)
P (GPa) 0.76 1.54
400
725
AFM 100
200
0
300
T (K) Fig. 1. Temperature dependence of the electrical resistivity of EuCo2 Ge2 at selected pressures. The inset shows the anomaly observed in rðTÞ at 3.2 GPa.
3+ state occurs is even higher. It is to be mentioned that for p45 GPa, the fluctuation of the ions already occurs at room temperature. A tiny anomaly, labelled as T X , was observed at about 50 K at 3.2 GPa, but its nature is not clear at present (inset of Fig. 1). A tentative (T; p) phase diagram for EuCo2 Ge2 is shown in Fig. 2. Upon applying pressure up to 2.6 GPa, T N can be determined unambiguously and it increases roughly by 3 K/GPa in this pressure range. The absence of a sharp or a hysteretic transition in rðTÞ for p43:2 GPa suggests a temperature- and pressure-induced continuous valence transition from a high-temperature Eu2þ state to a lowtemperature Eu3þ state. The gradual and continuous crossover is represented by the shaded area in Fig. 2. The shaded area was defined from the first derivative of rðTÞ, as it is shown in the inset of Fig. 2. Thus, it is possible to divide the phase diagram in three areas: (i) in the AFM and PM regions the Eu ions are divalent while (ii) in the low temperature and high pressure non-magnetic region the Eu ions of EuCo2 Ge2 are trivalent. (iii) In the shaded area, the Eu ions fluctuate between these two mentioned valence states.
0
2
4 p (GPa)
6
8
Fig. 2. (T; p) Phase diagram of EuCo2 Ge2 . At low pressures, a secondorder phase transition from a paramagnetic (PM) state to an AFM state occurs upon cooling while a continuous crossover (shaded area) to a nonmagnetic (NON-MAG) state takes place for p43:2 GPa. The inset shows the maximum in qr=qT at 5 GPa. The full-width at half-maximum (FWHM) of qr=qT at different pressures was used to define the shaded area.
Based on the abrupt disappearance of the AFM-order at 2:6 GPaopc o3:2 GPa, one can suspect the presence of a first-order transition on the ground state as a function of pressure, as denoted by the dotted line in Fig. 2. The absence of a well-defined temperature-dependent firstorder valence transition for p4pc , as usually observed in this kind of systems, is therefore surprising and a specific feature of EuCo2 Ge2 . It may be possible that the first-order transition occurs only in a narrow pressure range above pc and the anomaly at T X may be a reminiscence of it. References [1] [2] [3] [4] [5] [6]
Fournes, et al., Z. Phys. B: Condens. Matter 75 (1989) 501. H. Wada, et al., Phys. Rev. 59 (1999) 1141. Z. Hossain, et al., J. Magn. Magn. Mater. 272–276 (2004) 2352. C.U. Segre, et al., Phys. Rev. Lett. 49 (1982) 1947. Z. Hossain, et al., J. Magn. Magn. Mater. 264 (2003) 142. B. Biereckhoven, J. Wittig, J. Phys. E 21 (1988) 841.