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Dielectric relaxation in Rochelle salt in non-parallel electric fields
Solid State communications, Vol. 89, No. 4, pp. 393-395, 1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/94 $6.00 + .00
Pergamon
0038-1098(93)E0019-T D I E L E C T R I C R E L A X A T I O N IN R O C H E L L E SALT IN N O N - P A R A L L E L E L E C T R I C FIELDS L. Kalisz, B. Fugiel and J. Ziolo Institute of Physics, Silesian University, Uniwersytecka 4, 40-007 Katowice, Poland
(Received 2 April 1993; in revised form 6 September 1993 by P. Burlet)
Results are presented of dielectric measurements for non-parallel configuration of constant external electric field and measuring electric field. Large values of relaxation time of the order of several hours were obtained and also Curie-Weiss like behaviour of this time. The influence of non-parallel constant electric field on the hysteresis loop is given.
IT IS G E N E R A L L Y understood that the electric field influences upon the critical dielectric properties of ferroelectrics. However, up to now the majority of experimental investigations have been made in fields parallel to the ferroelectric axis. The works [1-3] belong to the not many papers where the dielectric measurements for non-parallel configuration of constant external electric field E and measuring electric field Em have been performed for TGS and LATGS. No similar investigations for Rochelle Salt are known to us. The realization of such measurements were the aim of our present paper. The starting point for our investigations was the statement that the influence of the electric field E non-parallel to the ferroelectric axis in Rochelle Salt strongly depends on time. The time The when the electric permittivity achieves its equilibrium value after change of external electric field E non-parallel to ferroelectric axis is longer than the corresponding time -rp in the case of external field parallel to this axis. Moreover, what is more interesting, values of rnp can be of the order of several hours even in the paraelectric phase! Hence, for the realization of the aim of our paper the measurements of relaxation effects were needed. These effects have been indeed investigated in [4, 8] but without applying of non-parallel fields. In the present paper the problem of relaxation in Rochelle Salt in non-parallel electric fields close to the upper critical point is examined. The corresponding experimental data are set out. Measurement circuits are shown in Fig. l(a, b) together with the corresponding classical time dependences of capacity proportional to permittivity (different slightly from our experimental results). Particularly noteworthy is the silver paste electrodes system on a sample of cube
shape with edge of 9 m m as shown in Fig. 2. The capacity of measurement capacitor with the sample and the hysteresis loop were measured with the help of smaller electrodes (w_~ 4mm, see Fig. 2). The amplitude and frequency of the bridge measuring voltage were Um= 1 V and f = 1 kHz, respectively. The constant voltage U was connected to larger electrodes so that an electric field not parallel to the ferroelectric axis (see Fig. 2) was applied. As indicated on Fig. 1 two measurement stages can be distinguished. In the first a constant electric field E was applied to the larger pair of electrodes [Fig. l(a)]. During the second stage the relaxation after disconnection of field E was observed [Fig. l(b)]. This last effect was the subject of special consideration. In Fig. 3 are shown the time dependencies of ( C 0 - C ) / ( C o - C') in semi-logarithmic scale for various temperatures T both above [Fig. 3(a)] and below [Fig. 3(b)] the upper critical point T_~ 25°C. For experimental description the ratio ( C o - C)/ (Co - C') was used such as to obtain unity for time t = 0. The dependences in Fig. 3 were obtained for E = 0 as in Fig. 1(b) after earlier reaching saturation in non-zero field E for U = 1500 V as in Fig. 1(a), and afterwards disconnecting voltage (i.e. electric field E) U = 1500V. A slight deviation from linearity is observed, especially for temperatures close to the critical point. The origin of this effect is unknown. However, for In [(Co - C)/(Co - C')] > - 1 / 2 the slope -1/~- [cf. formula on Fig. l(b)] connected with the "initial" relaxation time -r can be determined with acceptable accuracy. The temperature dependence of 7 is presented in Fig. 4 while on the insert is the Curie-Weiss like behaviour of T-1. The most interesting results are unexpectedly large values of r
393
394
D I E L E C T R I C R E L A X A T I O N IN R O C H E L L E SALT
C
(a)
I. .. ,." . . ,':.'..
To= 24.97oc
• ...::.?....
I .... t
l
•
.
f~
0
0
VOLTAGE SOURCE
(a)
". "-i-.
~-
"1,
°
-
C
;~;xlSOC
METER
]
C
(b)
o C-C'--(CaC'RI-~w)
I
.::• ••. 5!717-...• rD • "''. I._| • "•°..', r..)
t 0
Fig. 1. Measurement circuits: (a) for measurements when the non-parallel constant electric field E is applied and (b) for measurements after switching off the field. Corresponding time dependences are also presented.
• . "•
ELEC'TRODES FOR C MEASUREMENTS
AXIS CONSTANT VOLTAGE V ELECTRODES
Fig. 2. Ferroelectric crystal of Rochelle Salt in the form of cube in non-parallel electric fields• Electrodes configuration relative to the crystallographic coordinates system is also presented.
s
(h)
i
(b)
".
• ' °
, ".
I-
G
V
:\ r-.-I
found even in the paraelectric phase• Hence, it is difficult to interpret these experimental data with the help of mechanisms associated with domain structure. The Curie-Weiss behaviour of r vs T is also noteworthy• Certainly for T = Tc a maximum was detected in temperature dependence of relaxation time in Rochelle Salt [8] but values of 7- were much less than those obtained by us. However, in bridge measurements described in [8] no constant electric
? to-' t
T¢= 24.97°C
¢"'1
C"
FERROELECTRIC
Vol. 89, No. 4
o
-I
2414oC"
o
i
. "24.740(2
i to t (h)
~1..94oc
6
Fig. 3. Time dependences of In [(Co - C)/(Co - C')] after disconnection of larger electrodes constant voltage U = 1500V, where C O is the capacity of measurement capacitor when saturation is reached, C = capacity of this capacitor after time t, C ' = capacity of the same capacitor for t = 0, i.e. at the moment when field is switched off: (a) above the upper critical point and (b) below this point in Rochelle Salt. field was used. The following intervals were used for relaxation times measured by our method with nonparallel field: in the temperatures range 1.5°C > T - Tc > 0.2°C r rises from about 2-21 h and in the range I°C > T c - T > 0.1°C r rises from about 6-24 h (Fig. 4). The dependences presented on Figs. 3 and 4 were obtained, as already mentioned, after switching off the voltage 1500V. On Fig. 5 are given the saturation values C' of capacity C in fields E [see Fig. l(a)] for voltage: 0V ( C ' = Co), 300V and 1500 V, at various temperatures. The influence of non-parallel constant electric field on hysteresis loop is also interesting. When a nonparallel field is applied the spontaneous polarization (measured with the help of the smaller electrodes
Vol. 89, No. 4
DIELECTRIC RELAXATION IN R O C H E L L E SALT
24
zo/,- (h-,). •
3
VS.
z[ . . T (0c)
16
1
.~
60
."
20
"".
395
."
5O
-'"
,
o. " " 40
¢D
30
't~
°
'
o
5
10
15
20
25
t (h)
T (oc) Fig. 4. Temperature dependence of relaxation time T close to the upper critical point in Rochelle Salt. Insert: Curie-Weiss like behaviour of ~--] vs temperature T. shown on Fig. 2) vanishes gradually with time and after switching off the field appears just lately when capacity reaches saturation stage. On Fig. 6 are shown the time dependence of capacity of capacitor with the sample and the corresponding hysteresis loops, both observed after disconnecting the non-parallel field (U = 1500 V). The aim of the present paper is to report certain experimental results for discussion. Up to now we have not found any satisfactory interpretation of these data. However, we think that their publication in the present form could prove of interest because some new results concerning values of relaxation time T, its temperature dependence and also hysteresis loop in non-parallel constant external electric field
Fig. 6. Time dependence of capacity of measurement capacitor with sample and hysteresis loop, both observed after switching off the non-parallel constant electric field, i.e. voltage U -- 1500 V [see Fig. 1(b)]. are presented. These data can give answers to the following questions: what rate of temperature changes must be used during eventual future temperature dependences measurements in nonparallel electric fields, what is the order of the strength of the non-parallel saturation electric field (Fig. 5) and also how can the parameters of the hysteresis loop be changed by the external non-parallel electric field (Fig. 6). The results presented above can also be treated as the starting point for not only experimental but also theoretical investigations concerning for example relaxation mechanisms with long relaxation times, we understand, not observed up to now in the paraelectric phase. Acknowledgements - - The authors wish to express their thanks to Dr Alicja Ratuszna for making a crystallographic analysis of Rochelle Salt crystals. This work is supported by the Scientific Committee in Poland. REFERENCES
i
~1~
1.
•
T O e
~
•
'
•
•
•
•
m
•
e
o
•
o
2.
o
300V • • .
o
o
•• ' •
ov o
~
•
3.
1500V ~
•
•
•
•
••
•
•
•
t
~
o
•
•
e
,
T(°C) Fig. 5. Temperature dependences of saturation values of capacity of measurement capacitor with Rochelle Salt sample when non-parallel constant electric fields are applied [see Fig. 1(a)]. Values of larger electrodes voltage are given.
4. 5. 6. 7. 8.
S. Stoyanov, M.P. Michailov & J. Stankowska, Acta Phys. Polon. A65, 141 (1984). J. Stankowska & T. Jasifski, Acta Phys. Polon. A67, 1059 (1985). T. Jasifski & J. Stankowska, Acta Universitatis Wratislaviensis No. 1084, p. 105 (1988). H.G. Unruh & H.E. Miiser, Z. Angew. Physik XIV, 121 (1962). H.G. Unruh, Z. Angew. Physik XVI, 315 (1963). H.E. Mfiser, Z. Physik 184, 105 (1965). F. Sandy & R.V. Jones, Phys. Rev. 168, 481 (1968). R. Ramirez, C. Prieto & J.A. Gonzaio, Acta Phys. Polon. A72, 659 (1987).
Pergamon
0038-1098(93)E0019-T D I E L E C T R I C R E L A X A T I O N IN R O C H E L L E SALT IN N O N - P A R A L L E L E L E C T R I C FIELDS L. Kalisz, B. Fugiel and J. Ziolo Institute of Physics, Silesian University, Uniwersytecka 4, 40-007 Katowice, Poland
(Received 2 April 1993; in revised form 6 September 1993 by P. Burlet)
Results are presented of dielectric measurements for non-parallel configuration of constant external electric field and measuring electric field. Large values of relaxation time of the order of several hours were obtained and also Curie-Weiss like behaviour of this time. The influence of non-parallel constant electric field on the hysteresis loop is given.
IT IS G E N E R A L L Y understood that the electric field influences upon the critical dielectric properties of ferroelectrics. However, up to now the majority of experimental investigations have been made in fields parallel to the ferroelectric axis. The works [1-3] belong to the not many papers where the dielectric measurements for non-parallel configuration of constant external electric field E and measuring electric field Em have been performed for TGS and LATGS. No similar investigations for Rochelle Salt are known to us. The realization of such measurements were the aim of our present paper. The starting point for our investigations was the statement that the influence of the electric field E non-parallel to the ferroelectric axis in Rochelle Salt strongly depends on time. The time The when the electric permittivity achieves its equilibrium value after change of external electric field E non-parallel to ferroelectric axis is longer than the corresponding time -rp in the case of external field parallel to this axis. Moreover, what is more interesting, values of rnp can be of the order of several hours even in the paraelectric phase! Hence, for the realization of the aim of our paper the measurements of relaxation effects were needed. These effects have been indeed investigated in [4, 8] but without applying of non-parallel fields. In the present paper the problem of relaxation in Rochelle Salt in non-parallel electric fields close to the upper critical point is examined. The corresponding experimental data are set out. Measurement circuits are shown in Fig. l(a, b) together with the corresponding classical time dependences of capacity proportional to permittivity (different slightly from our experimental results). Particularly noteworthy is the silver paste electrodes system on a sample of cube
shape with edge of 9 m m as shown in Fig. 2. The capacity of measurement capacitor with the sample and the hysteresis loop were measured with the help of smaller electrodes (w_~ 4mm, see Fig. 2). The amplitude and frequency of the bridge measuring voltage were Um= 1 V and f = 1 kHz, respectively. The constant voltage U was connected to larger electrodes so that an electric field not parallel to the ferroelectric axis (see Fig. 2) was applied. As indicated on Fig. 1 two measurement stages can be distinguished. In the first a constant electric field E was applied to the larger pair of electrodes [Fig. l(a)]. During the second stage the relaxation after disconnection of field E was observed [Fig. l(b)]. This last effect was the subject of special consideration. In Fig. 3 are shown the time dependencies of ( C 0 - C ) / ( C o - C') in semi-logarithmic scale for various temperatures T both above [Fig. 3(a)] and below [Fig. 3(b)] the upper critical point T_~ 25°C. For experimental description the ratio ( C o - C)/ (Co - C') was used such as to obtain unity for time t = 0. The dependences in Fig. 3 were obtained for E = 0 as in Fig. 1(b) after earlier reaching saturation in non-zero field E for U = 1500 V as in Fig. 1(a), and afterwards disconnecting voltage (i.e. electric field E) U = 1500V. A slight deviation from linearity is observed, especially for temperatures close to the critical point. The origin of this effect is unknown. However, for In [(Co - C)/(Co - C')] > - 1 / 2 the slope -1/~- [cf. formula on Fig. l(b)] connected with the "initial" relaxation time -r can be determined with acceptable accuracy. The temperature dependence of 7 is presented in Fig. 4 while on the insert is the Curie-Weiss like behaviour of T-1. The most interesting results are unexpectedly large values of r
393
394
D I E L E C T R I C R E L A X A T I O N IN R O C H E L L E SALT
C
(a)
I. .. ,." . . ,':.'..
To= 24.97oc
• ...::.?....
I .... t
l
•
.
f~
0
0
VOLTAGE SOURCE
(a)
". "-i-.
~-
"1,
°
-
C
;~;xlSOC
METER
]
C
(b)
o C-C'--(CaC'RI-~w)
I
.::• ••. 5!717-...• rD • "''. I._| • "•°..', r..)
t 0
Fig. 1. Measurement circuits: (a) for measurements when the non-parallel constant electric field E is applied and (b) for measurements after switching off the field. Corresponding time dependences are also presented.
• . "•
ELEC'TRODES FOR C MEASUREMENTS
AXIS CONSTANT VOLTAGE V ELECTRODES
Fig. 2. Ferroelectric crystal of Rochelle Salt in the form of cube in non-parallel electric fields• Electrodes configuration relative to the crystallographic coordinates system is also presented.
s
(h)
i
(b)
".
• ' °
, ".
I-
G
V
:\ r-.-I
found even in the paraelectric phase• Hence, it is difficult to interpret these experimental data with the help of mechanisms associated with domain structure. The Curie-Weiss behaviour of r vs T is also noteworthy• Certainly for T = Tc a maximum was detected in temperature dependence of relaxation time in Rochelle Salt [8] but values of 7- were much less than those obtained by us. However, in bridge measurements described in [8] no constant electric
? to-' t
T¢= 24.97°C
¢"'1
C"
FERROELECTRIC
Vol. 89, No. 4
o
-I
2414oC"
o
i
. "24.740(2
i to t (h)
~1..94oc
6
Fig. 3. Time dependences of In [(Co - C)/(Co - C')] after disconnection of larger electrodes constant voltage U = 1500V, where C O is the capacity of measurement capacitor when saturation is reached, C = capacity of this capacitor after time t, C ' = capacity of the same capacitor for t = 0, i.e. at the moment when field is switched off: (a) above the upper critical point and (b) below this point in Rochelle Salt. field was used. The following intervals were used for relaxation times measured by our method with nonparallel field: in the temperatures range 1.5°C > T - Tc > 0.2°C r rises from about 2-21 h and in the range I°C > T c - T > 0.1°C r rises from about 6-24 h (Fig. 4). The dependences presented on Figs. 3 and 4 were obtained, as already mentioned, after switching off the voltage 1500V. On Fig. 5 are given the saturation values C' of capacity C in fields E [see Fig. l(a)] for voltage: 0V ( C ' = Co), 300V and 1500 V, at various temperatures. The influence of non-parallel constant electric field on hysteresis loop is also interesting. When a nonparallel field is applied the spontaneous polarization (measured with the help of the smaller electrodes
Vol. 89, No. 4
DIELECTRIC RELAXATION IN R O C H E L L E SALT
24
zo/,- (h-,). •
3
VS.
z[ . . T (0c)
16
1
.~
60
."
20
"".
395
."
5O
-'"
,
o. " " 40
¢D
30
't~
°
'
o
5
10
15
20
25
t (h)
T (oc) Fig. 4. Temperature dependence of relaxation time T close to the upper critical point in Rochelle Salt. Insert: Curie-Weiss like behaviour of ~--] vs temperature T. shown on Fig. 2) vanishes gradually with time and after switching off the field appears just lately when capacity reaches saturation stage. On Fig. 6 are shown the time dependence of capacity of capacitor with the sample and the corresponding hysteresis loops, both observed after disconnecting the non-parallel field (U = 1500 V). The aim of the present paper is to report certain experimental results for discussion. Up to now we have not found any satisfactory interpretation of these data. However, we think that their publication in the present form could prove of interest because some new results concerning values of relaxation time T, its temperature dependence and also hysteresis loop in non-parallel constant external electric field
Fig. 6. Time dependence of capacity of measurement capacitor with sample and hysteresis loop, both observed after switching off the non-parallel constant electric field, i.e. voltage U -- 1500 V [see Fig. 1(b)]. are presented. These data can give answers to the following questions: what rate of temperature changes must be used during eventual future temperature dependences measurements in nonparallel electric fields, what is the order of the strength of the non-parallel saturation electric field (Fig. 5) and also how can the parameters of the hysteresis loop be changed by the external non-parallel electric field (Fig. 6). The results presented above can also be treated as the starting point for not only experimental but also theoretical investigations concerning for example relaxation mechanisms with long relaxation times, we understand, not observed up to now in the paraelectric phase. Acknowledgements - - The authors wish to express their thanks to Dr Alicja Ratuszna for making a crystallographic analysis of Rochelle Salt crystals. This work is supported by the Scientific Committee in Poland. REFERENCES
i
~1~
1.
•
T O e
~
•
'
•
•
•
•
m
•
e
o
•
o
2.
o
300V • • .
o
o
•• ' •
ov o
~
•
3.
1500V ~
•
•
•
•
••
•
•
•
t
~
o
•
•
e
,
T(°C) Fig. 5. Temperature dependences of saturation values of capacity of measurement capacitor with Rochelle Salt sample when non-parallel constant electric fields are applied [see Fig. 1(a)]. Values of larger electrodes voltage are given.
4. 5. 6. 7. 8.
S. Stoyanov, M.P. Michailov & J. Stankowska, Acta Phys. Polon. A65, 141 (1984). J. Stankowska & T. Jasifski, Acta Phys. Polon. A67, 1059 (1985). T. Jasifski & J. Stankowska, Acta Universitatis Wratislaviensis No. 1084, p. 105 (1988). H.G. Unruh & H.E. Miiser, Z. Angew. Physik XIV, 121 (1962). H.G. Unruh, Z. Angew. Physik XVI, 315 (1963). H.E. Mfiser, Z. Physik 184, 105 (1965). F. Sandy & R.V. Jones, Phys. Rev. 168, 481 (1968). R. Ramirez, C. Prieto & J.A. Gonzaio, Acta Phys. Polon. A72, 659 (1987).