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Dielectric anomaly in valence fluctuating state of Sm3Se4
Journal of Magnetism and Magnetic Materials 76 & 77 (1988) 297-298 North-Holland, A m s t e r d a m
297
D I E L E C T R I C A N O M A L Y IN VALENCE F L U C T U A T I N G S T A T E O F Sm3Se 4 K. BABA a, K. A N D O , A. O C H I A I , Y.S. K W O N , M. SAITO, M. T A N A K A , S. K U N I I , T. S U Z U K I , T. K A S U Y A and T. F U J I M U R A a " Research Institute for Scientific Measurements and Dept. of Physics, Tohoku University, Sendai, Japan The frequency dependence of the dielectric constant and of the electrical conductivity were measured by the Time Domain Spectroscopy (TDS). The temperature dependence of ~(w) shows an anomaly at 305.5 K. From the analogy to the ferroelectric phase transition, our result suggests the occurrence of a charge ordered state associated with an electric dipole order.
The electrical conductivity of Sm3Se 4 is governed by the 4f hopping at higher temperatures and dc resistivity is explained by an activation-type temperature dependence; Ode=P0 exp(Eo/kT ) where p 0 = 3 . 0 x 1 0 - 1 2 £ c m and E 0 = 0 . 1 4 eV. The ac resistivity depends strongly on the frequency with no activation energy at the lowest temperature and gradually close to the dc one at higher temperatures [1]. We measured the complex dielectric constant to clarify this conduction mechanism. It is hard to measure the complex dielectric constant a n d / o r conductivity of Sm3Se 4 precisely at higher temperatures by the usual methods. We therefore measured these electrical properties with the lumped-capacitor time-domain spectroscopy (TDS). The method developed by two of us [2,3] has m a n y advantages summarized as follows. (1) By a computational analysis of one reflected wave from a sample, one can obtain the dielectric constant and conductivity at the frequency range from dc to about 10 GHz. (2) The data acquisition time is shorter than 5 s in our case. (3) Because the dc and ac components of the dielectric constant and conductivity can be determined readily from the analysis of only one wave form, one can measure the electrical properties of the conductive materials like Sm3Se 4 with high accuracy. We measured the properties of Sm3Se 4 in the temperature range from 190 to 355 K by TDS. The single crystal of Sm3Se 4 was grown by the same method as in previous work [1]. Fig. 1 shows the frequency and temperature dependence of the real part of the dielectric constant. A remarkable
anomaly can be seen in Sm3Se 4 at 305.5 K. Fig. 2 shows the frequency dependence of the resisitvity plotted as log100 against 1/T. The activation energy deduced from the straight line of loga00dc is 0.116 eV which agrees well with earlier study [1]. The temperature dependence of the dielectric relaxation time is shown in fig. 3, where an anomaly is also observed at 305.5 K. The dielectric properties of Sm3Se 4 seem to be described by the superposition of two relaxations. One is characterized by the anomaly at 305.5 K and its behavior is the same as the critical slowing down associated with the order-disorder type ferroelectric phase transition [4,5]. This is a some~ " ( 1 0 4) 8
I
I
1
Srn3Se4
7 6 --
.~.
../'2 ,.
4_
../.
•
, H.
•
•
250
".
~0
,,.7
4.1.7
0 O0
I
~, 3 o 5 . 5 K
300
1 3.50
T(K)
Fig. 1. The frequency and temperature dependence of the dielectric constant of Sm 3Se4.
0304-8853/88/$03.50 © Elsevier Science Publishers B.V.
298
K. Baba et al. / Dielectric anomaly in S m 3Se 4
l- o (I O-as)
IOg,oP ( ~ c m )
fs~---
0.4 L
]
de
Sm3Se4
~
O ~2
-
0.4
O
2.8
3.0
3.2
I / T ( IO-3/
.5.4
3.6
K)
I 200
I 250
4 300
I! :350
Y (N) Fig. 3. Temperature dependence of the dielectric relaxation time of Sm~Se4.
Fig. 2. Electric dc and ac resisitivities of Sm3Se4 at several frequencies as a function of the reciprocal temperature. what mysterious p r o b l e m , because no sign of the charge o r d e r i n g has been o b s e r v e d in Sm3S4 a n d Sm3Se 4, which is in c o n t r a s t to the fact that the i s o m o r p h o u s c o m p o u n d Eu3S 4 has charge o r d e r ing below a b o u t 186 K [6]. T h e a n o m a l y at 305.5 K of Sm3Se a suggests the occurrence of a charge ordering. W e therefore e x a m i n e d the s y m m e t r y change of crystal structure by C o n v e r g e n t - B e a m Electron D i f f r a c t i o n ( C B E D ) with an accuracy of 10 3 in the lattice p a r a m e t e r at 100 K. T h e Th3P4-type crystal structure of Sm3Se4 is piezoelectric a m o n g cubic symmetries. If a ferroelectric transition occurs at 305.5 K, it m u s t cause a structure change from cubic to some lower p y r o electric symmetries. However, all the diffraction p a t t e r n s in (111), (110) and (111) c o n c l u d e that the p o i n t s y m m e t r y at 100 K is j u s t the same as that in the high t e m p e r a t u r e p h a s e with Ia3 d of Th3P4-type structure. N o crystal structure change was detected within the accuracy of this C B E D m e t h o d . These results m a y suggest the possibility of the charge o r d e r i n g without crystal d e f o r m a t i o n at 305.5 K. This p r o b l e m is still o p e n a n d m o r e
detailed studies are necessary. A n o t h e r m a j o r part of the dielectric r e l a x a t i o n process is characterized by a D e b y e - t y p e process. The electric p o l a r i z a t i o n of this r e l a x a t i o n is p r o b a b l y caused by the hopp i n g m o t i o n of 4f electrons. A d e t a i l e d study is in progress. T h e time d o m a i n s p e c t r o s c o p y is a p r o m i s i n g m e a s u r i n g m e t h o d for investigating the electrical p r o p e r t i e s of dielectrics. This work shows its validity also in the research of the c o n d u c t i v e materials such as Sm3Se 4. References
[1] A. Tamaki, T. Goto, S. KuniL T. Suzuki, T. Fujimura and T. Kasuya, J. Phys. C 18 (1985) 5849. [2] K. Baba and T. Fujimura, Japan. J. Appl. Phys. 24 (1985) 487. [3] K. Baba, T. Yamakami and T. Fujimura, Japan. J. Appl. Phys. 24 (1985) 1532. [4] A. Tamaki, T. Fujimura and K. Kamiyoshi, J. Chem. Phys. 65 (1976) 4867. [5] E. Kanda, A. Tamaki and T. Fujimura. J. Phys. C 15 (1982) 3401. [6] R. Port, G. Gflntherodt, W. Wichelhaus, M. Ohl and H. Bach, Phys. Rev. B 27 (1983) 359.
297
D I E L E C T R I C A N O M A L Y IN VALENCE F L U C T U A T I N G S T A T E O F Sm3Se 4 K. BABA a, K. A N D O , A. O C H I A I , Y.S. K W O N , M. SAITO, M. T A N A K A , S. K U N I I , T. S U Z U K I , T. K A S U Y A and T. F U J I M U R A a " Research Institute for Scientific Measurements and Dept. of Physics, Tohoku University, Sendai, Japan The frequency dependence of the dielectric constant and of the electrical conductivity were measured by the Time Domain Spectroscopy (TDS). The temperature dependence of ~(w) shows an anomaly at 305.5 K. From the analogy to the ferroelectric phase transition, our result suggests the occurrence of a charge ordered state associated with an electric dipole order.
The electrical conductivity of Sm3Se 4 is governed by the 4f hopping at higher temperatures and dc resistivity is explained by an activation-type temperature dependence; Ode=P0 exp(Eo/kT ) where p 0 = 3 . 0 x 1 0 - 1 2 £ c m and E 0 = 0 . 1 4 eV. The ac resistivity depends strongly on the frequency with no activation energy at the lowest temperature and gradually close to the dc one at higher temperatures [1]. We measured the complex dielectric constant to clarify this conduction mechanism. It is hard to measure the complex dielectric constant a n d / o r conductivity of Sm3Se 4 precisely at higher temperatures by the usual methods. We therefore measured these electrical properties with the lumped-capacitor time-domain spectroscopy (TDS). The method developed by two of us [2,3] has m a n y advantages summarized as follows. (1) By a computational analysis of one reflected wave from a sample, one can obtain the dielectric constant and conductivity at the frequency range from dc to about 10 GHz. (2) The data acquisition time is shorter than 5 s in our case. (3) Because the dc and ac components of the dielectric constant and conductivity can be determined readily from the analysis of only one wave form, one can measure the electrical properties of the conductive materials like Sm3Se 4 with high accuracy. We measured the properties of Sm3Se 4 in the temperature range from 190 to 355 K by TDS. The single crystal of Sm3Se 4 was grown by the same method as in previous work [1]. Fig. 1 shows the frequency and temperature dependence of the real part of the dielectric constant. A remarkable
anomaly can be seen in Sm3Se 4 at 305.5 K. Fig. 2 shows the frequency dependence of the resisitvity plotted as log100 against 1/T. The activation energy deduced from the straight line of loga00dc is 0.116 eV which agrees well with earlier study [1]. The temperature dependence of the dielectric relaxation time is shown in fig. 3, where an anomaly is also observed at 305.5 K. The dielectric properties of Sm3Se 4 seem to be described by the superposition of two relaxations. One is characterized by the anomaly at 305.5 K and its behavior is the same as the critical slowing down associated with the order-disorder type ferroelectric phase transition [4,5]. This is a some~ " ( 1 0 4) 8
I
I
1
Srn3Se4
7 6 --
.~.
../'2 ,.
4_
../.
•
, H.
•
•
250
".
~0
,,.7
4.1.7
0 O0
I
~, 3 o 5 . 5 K
300
1 3.50
T(K)
Fig. 1. The frequency and temperature dependence of the dielectric constant of Sm 3Se4.
0304-8853/88/$03.50 © Elsevier Science Publishers B.V.
298
K. Baba et al. / Dielectric anomaly in S m 3Se 4
l- o (I O-as)
IOg,oP ( ~ c m )
fs~---
0.4 L
]
de
Sm3Se4
~
O ~2
-
0.4
O
2.8
3.0
3.2
I / T ( IO-3/
.5.4
3.6
K)
I 200
I 250
4 300
I! :350
Y (N) Fig. 3. Temperature dependence of the dielectric relaxation time of Sm~Se4.
Fig. 2. Electric dc and ac resisitivities of Sm3Se4 at several frequencies as a function of the reciprocal temperature. what mysterious p r o b l e m , because no sign of the charge o r d e r i n g has been o b s e r v e d in Sm3S4 a n d Sm3Se 4, which is in c o n t r a s t to the fact that the i s o m o r p h o u s c o m p o u n d Eu3S 4 has charge o r d e r ing below a b o u t 186 K [6]. T h e a n o m a l y at 305.5 K of Sm3Se a suggests the occurrence of a charge ordering. W e therefore e x a m i n e d the s y m m e t r y change of crystal structure by C o n v e r g e n t - B e a m Electron D i f f r a c t i o n ( C B E D ) with an accuracy of 10 3 in the lattice p a r a m e t e r at 100 K. T h e Th3P4-type crystal structure of Sm3Se4 is piezoelectric a m o n g cubic symmetries. If a ferroelectric transition occurs at 305.5 K, it m u s t cause a structure change from cubic to some lower p y r o electric symmetries. However, all the diffraction p a t t e r n s in (111), (110) and (111) c o n c l u d e that the p o i n t s y m m e t r y at 100 K is j u s t the same as that in the high t e m p e r a t u r e p h a s e with Ia3 d of Th3P4-type structure. N o crystal structure change was detected within the accuracy of this C B E D m e t h o d . These results m a y suggest the possibility of the charge o r d e r i n g without crystal d e f o r m a t i o n at 305.5 K. This p r o b l e m is still o p e n a n d m o r e
detailed studies are necessary. A n o t h e r m a j o r part of the dielectric r e l a x a t i o n process is characterized by a D e b y e - t y p e process. The electric p o l a r i z a t i o n of this r e l a x a t i o n is p r o b a b l y caused by the hopp i n g m o t i o n of 4f electrons. A d e t a i l e d study is in progress. T h e time d o m a i n s p e c t r o s c o p y is a p r o m i s i n g m e a s u r i n g m e t h o d for investigating the electrical p r o p e r t i e s of dielectrics. This work shows its validity also in the research of the c o n d u c t i v e materials such as Sm3Se 4. References
[1] A. Tamaki, T. Goto, S. KuniL T. Suzuki, T. Fujimura and T. Kasuya, J. Phys. C 18 (1985) 5849. [2] K. Baba and T. Fujimura, Japan. J. Appl. Phys. 24 (1985) 487. [3] K. Baba, T. Yamakami and T. Fujimura, Japan. J. Appl. Phys. 24 (1985) 1532. [4] A. Tamaki, T. Fujimura and K. Kamiyoshi, J. Chem. Phys. 65 (1976) 4867. [5] E. Kanda, A. Tamaki and T. Fujimura. J. Phys. C 15 (1982) 3401. [6] R. Port, G. Gflntherodt, W. Wichelhaus, M. Ohl and H. Bach, Phys. Rev. B 27 (1983) 359.