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Ferroelectricity and polytypism in TlGaSe2 Crystals
Solid State Communications, Vol. 77, No. 6, pp. 453-455, 1991. Printed in Great Britain.
0038-1098/91 $3.00 + .00 Pergamon Press plc
FERROELECTRICITY AND POLYTYPISM IN TIGaSe2 CRYSTALS R.M. Sardarly, O.A. Samedov, I.Sh. Sadykhov, E.I. Mardukhaeva and T.A. Gabibov Department of Radiation Investigations, Academy of Sciences of the Azerbaijan SSR, Baku
(Received 24 July 1990 by P. Wachter) The present report deals with a complex of investigations of the temperature dependences of the static dielectric constant, the spontaneous polarization and the pyroelectric coefficient in different T1GaSe2 crystal modifications. It has been shown that in the temperature region below T,. dielectric hysteresis loops are observed and in this case spontaneous polarization grows according to a linear law and reacts the value of 1.3 x lO-7Ccm 2,
THE FERROELECTRICAL properties of T1GaSe2 crystals were revealed by measuring soft optical modes [1, 2] in submillimetre spectra and on the basis of dielectric measurements performed in earlier studies [3-5]. Further investigation carried out with these crystals disclosed soft optical modes active in Raman spectra [6, 7]. In [8] a special feature was studied in the soft-mode behaviour of a T1GaSe2 crystal with results different from those obtained elsewhere [1, 2], which pointed to dissimilar phase-transition (PT) dynamics in specimens grown under an uncontrolled variation of growth conditions. Neutron diffraction studies conducted on a series of TIGaSe2 crystal specimens [9] revealed two modifications designated as and ft. The s-modification T1GaSe2 specimens experience a PT with a quadrupling of the period of fundamental translation along the C-axis, whereas the fl-modification structure is modulated with a period C = 10C' = 160 A (the C-axis is perpendicular to the layers) at room temperature already. Despite the fact that the critical temperature behaviour of the polar soft mode bears witness to the ferroelectrical nature of the phase transition in T1GaSe2, no hysteresis loops have been revealed in it. TIGaSe2 layer crystals are prone to ambiguous layer joining (polytypism) and compound tends to from different modifications, and besides, they are defective. The specimens under study were grown by the Bridgman method and represented parallelepipeds cut out at right angles to the cleavage planes. A silver paste was used to make the electrodes. The dielectric constant was measured at a frequency of 1 kHz by means of an alternating-current bridge E7-8. Dielectric hysteresis loops were studied with the use of a modified Sawyer-Tower circuit. Measurements were conducted at a frequency of 50Hz. Pyroelectric
studies were carried out by the quasistatic method at a heating rate of 1 K min i. Figure 1 gives the temperature dependence of the real part of the dielectric constant for three T1GaSe2 crystal modifications. The e(T) dependence of one of these modifications, show as curve C in Fig. 1, fails to exhibit any dielectric anomalies in the 100 to 135 K temperature region. However, the second-type specimens show a substantial anomaly in the E(T) dependence (curves A and B in Fig. 1), the dielectric hysteresis loop (inset of Fig. 1) and the pyroelectric reaction (Fig. 2). Now, let us analyze curve A of Fig. 1. The authors of another publication [10], have considered from the standpoint of the phenomenological theory the appearance of an incommensurate phase due to the instability in the polar optical branch at q = K0 and plotted a qualitative trend of the temperature dependence of the dielectric constant near the transition from the incommensurate to a commensurate polar phase. A comparison between the estimated 8(T) dependence obtained in [10] for intrinsic ferroelectrics and our experimental o~(T) curve for T1GaSe2 (curve A) makes it possible to assert that it is just this TIGaSe2 crystal modification which is an intrinsic ferroelectric, whose transition to a polar phase is preceded by an incommensurate phase. This result is also in agreement with the data obtained in [5] for the static dielectric constant of a T1GaSe2 crystal, calculated on the basis of the Leiden-Sax-Tayler relation, the Curie-Weiss law, and experiments aimed at studying soft optical modes in the frequency region from 10 9 to 10" nz. However, sometimes T1GaSe 2 specimens are encountered whose ¢(T) dependence has the shape of curve B in Fig. 1. Such a type of behaviour of the temperature dependence of the real-part of the dielec-
453
454
F E R R O E L E C T R I C I T Y A N D POLYTYPISM IN T1GaSe2 CRYSTALS
T=107.5 K
900
800
_
A;
700, 600
Vol. 77, No. 6
;
1 •
1L / l !
•
' ~
i -
,
'1~
• I
95
/ = / u u r, I I
\~C
100 105
%?
4
T.K
(1) 500 400
d
300
-9O
200 100 -
T
105
110
tl 115
130
Fig. 2. Temperature dependence of the pyroelectric coefficient of a T1GaSe2 crystal.
C
I *1 100
~ 0K
I
I
I
I
120
125
130
135
T.K Fig. 1. Temperature dependence of the dielectric constant of a T1GaSe2 crystal, measured at a frequency of 1 kHz. The temperature dependence of the spontaneous polarization of the TIGaSe2 crystal is shown in the inset. tric constant is typical of a non-intrinsic ferroelectric with an incommensurate phase [11]. The qualitative shape of the d ( T ) dependence for a non-intrinsic ferroelectric at "nonpolar-incommensurate-polar" phase transitions, estimated in [10], features the type of curve B in Fig. I in [12] a numerical calculation of the 8 ( T ) dependence is presented for an incommensurate phase. The best fitting of the estimated and experimental g ( T ) dependence made it possible to obtain the temperature of transition from a highly symmetrical to an incommensurate phase. According to [12], this temperature is in the vicinity of 300K. Slight g ( T ) anomalies typical of nonpolar-incommensurate phase transitions in these specimens are revealed at 130 K. An investigation of polarization properties has shown that in the temperature region below T,, dielectric hysteresis loops are observed, which points to the ferroelectric nature of the low-temperature phase of this crystal, and in this case spontaneous polarization grows according to a linear law and reacts the value of 1.3 x 1 0 - 7 C c m -2. At these temperatures the coercive field value is ~ 40 kV m ]. The shape of the dielectric loop and the temperature trend of spontaneous polarization are depicted in the inset of Fig. 1. The linear growth of
spontaneous polarization in the temperature region from 103 to 107K and its low value are typical of non-intrinsic ferroelectric second-order phase transitions. This result is in agreement with the data obtained in a neutron diffraction study carried out earlier [9]. The existence of spontaneous polarization in TIGaSe2 enabled us to carry out a pyroelectric study. Pyroelectric measurements were conducted with specimens prepolarized in an external electric field. A 50 kV m ] electric field was applied to the specimens at a temperature of 230K and then the crystal was cooled in the field to 90 K. There upon the field was removed, the specimen was short-circuited and measurements were made under heating. Figure 2 depicts the temperature dependence of the pyroelectric coefficient ?(T) over the whole temperature range under study, including the incommensurate phase. The maximum value of the pyroelectric coefficient is at 107.5K, whereas at 105K an " a r m " is observed. The existence of two special features in the 7(T) dependence of the T1GaSe2 crystal points to the fact that one of these features is associated with ferroelectric polarization and the other one with the appearance of a pyroelectric charge due to a lattice distortion.
REFERENCES i.
A.A. Volkov, U.G. Goncharov, G.V. Kozlov, S.P. Lebedev, A.M. Prokhorov, R.A. Aliev & K.R. Allakhverdiev, Zh. Eksper. Teor. Fiz., Pisma 37, 517 (1983).
Vol. 77, No. 6 2. 3. 4. 5. 6. 7.
FERROELECTRICITY AND POLYTYPISM IN TIGaSe2 CRYSTALS
A.A. Volkov, U.G. Goncharov, G.V. Kozlov, K.R. Allakhverdiev & R.M. Sardarly, Fiz. Tverd. Tela 26, 1754 (1984). R.A. Aliev, K.R. Allakhverdiev, A.U. Baranov, H.R. Ivanov & R.M. Sardarly, Fiz. Tverd. Tela 26, 1271 (1984). W. Paprotny & J. Grigas, Ferroelectrics 65, 201 (1985). U. Banis, A. Brilingas, J. Grigas & G. Guseinov, Fiz. Tverd. Tela 29, 3324 (1987). R.A. Aliev & K.R. Allakhverdiev, Preprint No. 256 Instityt Fiziki Akad. Nauk Azerb. SSR, Baku (1987). B.S. Kulbydjev, L.M. Rabkin, V.I. Morgushev & U. Uzuk, Fiz. Tverd. Tela 30, 195 (1988).
8. 9. 10. 11. 12.
455
A.A. Volkov, U.G. Goncharov, G.V. Kozlov & R.M. Sardarly, Zh. Eksper. Teor. Fiz., Pisma 39, 293 (1984). S.B. Vakhryshev, B.E. Kvatkovsky, N.M. Okyneva, K.R. Allakhverdiev & R.M. Sardarly, Preprint No. 886, FTI, Leningrad (1984). B.A. Strykov & A.P. Levanuk, Fizicheski Osnovi Segnetoelectricheskih Javleni v Kristallah, Nauka, Moskva (1983). B,A. Strukov, I. Yesy & V.M. Arytunova, Zh. Eksper. Teor. Fiz., Pisma 35, 424 (1982). F.M. Gashimzade, B.R. Gadjuev, K.R. Allakhverdiev & R.M. Sardarly, Fiz. Tverd. Tela 27, 2286 (1985).
0038-1098/91 $3.00 + .00 Pergamon Press plc
FERROELECTRICITY AND POLYTYPISM IN TIGaSe2 CRYSTALS R.M. Sardarly, O.A. Samedov, I.Sh. Sadykhov, E.I. Mardukhaeva and T.A. Gabibov Department of Radiation Investigations, Academy of Sciences of the Azerbaijan SSR, Baku
(Received 24 July 1990 by P. Wachter) The present report deals with a complex of investigations of the temperature dependences of the static dielectric constant, the spontaneous polarization and the pyroelectric coefficient in different T1GaSe2 crystal modifications. It has been shown that in the temperature region below T,. dielectric hysteresis loops are observed and in this case spontaneous polarization grows according to a linear law and reacts the value of 1.3 x lO-7Ccm 2,
THE FERROELECTRICAL properties of T1GaSe2 crystals were revealed by measuring soft optical modes [1, 2] in submillimetre spectra and on the basis of dielectric measurements performed in earlier studies [3-5]. Further investigation carried out with these crystals disclosed soft optical modes active in Raman spectra [6, 7]. In [8] a special feature was studied in the soft-mode behaviour of a T1GaSe2 crystal with results different from those obtained elsewhere [1, 2], which pointed to dissimilar phase-transition (PT) dynamics in specimens grown under an uncontrolled variation of growth conditions. Neutron diffraction studies conducted on a series of TIGaSe2 crystal specimens [9] revealed two modifications designated as and ft. The s-modification T1GaSe2 specimens experience a PT with a quadrupling of the period of fundamental translation along the C-axis, whereas the fl-modification structure is modulated with a period C = 10C' = 160 A (the C-axis is perpendicular to the layers) at room temperature already. Despite the fact that the critical temperature behaviour of the polar soft mode bears witness to the ferroelectrical nature of the phase transition in T1GaSe2, no hysteresis loops have been revealed in it. TIGaSe2 layer crystals are prone to ambiguous layer joining (polytypism) and compound tends to from different modifications, and besides, they are defective. The specimens under study were grown by the Bridgman method and represented parallelepipeds cut out at right angles to the cleavage planes. A silver paste was used to make the electrodes. The dielectric constant was measured at a frequency of 1 kHz by means of an alternating-current bridge E7-8. Dielectric hysteresis loops were studied with the use of a modified Sawyer-Tower circuit. Measurements were conducted at a frequency of 50Hz. Pyroelectric
studies were carried out by the quasistatic method at a heating rate of 1 K min i. Figure 1 gives the temperature dependence of the real part of the dielectric constant for three T1GaSe2 crystal modifications. The e(T) dependence of one of these modifications, show as curve C in Fig. 1, fails to exhibit any dielectric anomalies in the 100 to 135 K temperature region. However, the second-type specimens show a substantial anomaly in the E(T) dependence (curves A and B in Fig. 1), the dielectric hysteresis loop (inset of Fig. 1) and the pyroelectric reaction (Fig. 2). Now, let us analyze curve A of Fig. 1. The authors of another publication [10], have considered from the standpoint of the phenomenological theory the appearance of an incommensurate phase due to the instability in the polar optical branch at q = K0 and plotted a qualitative trend of the temperature dependence of the dielectric constant near the transition from the incommensurate to a commensurate polar phase. A comparison between the estimated 8(T) dependence obtained in [10] for intrinsic ferroelectrics and our experimental o~(T) curve for T1GaSe2 (curve A) makes it possible to assert that it is just this TIGaSe2 crystal modification which is an intrinsic ferroelectric, whose transition to a polar phase is preceded by an incommensurate phase. This result is also in agreement with the data obtained in [5] for the static dielectric constant of a T1GaSe2 crystal, calculated on the basis of the Leiden-Sax-Tayler relation, the Curie-Weiss law, and experiments aimed at studying soft optical modes in the frequency region from 10 9 to 10" nz. However, sometimes T1GaSe 2 specimens are encountered whose ¢(T) dependence has the shape of curve B in Fig. 1. Such a type of behaviour of the temperature dependence of the real-part of the dielec-
453
454
F E R R O E L E C T R I C I T Y A N D POLYTYPISM IN T1GaSe2 CRYSTALS
T=107.5 K
900
800
_
A;
700, 600
Vol. 77, No. 6
;
1 •
1L / l !
•
' ~
i -
,
'1~
• I
95
/ = / u u r, I I
\~C
100 105
%?
4
T.K
(1) 500 400
d
300
-9O
200 100 -
T
105
110
tl 115
130
Fig. 2. Temperature dependence of the pyroelectric coefficient of a T1GaSe2 crystal.
C
I *1 100
~ 0K
I
I
I
I
120
125
130
135
T.K Fig. 1. Temperature dependence of the dielectric constant of a T1GaSe2 crystal, measured at a frequency of 1 kHz. The temperature dependence of the spontaneous polarization of the TIGaSe2 crystal is shown in the inset. tric constant is typical of a non-intrinsic ferroelectric with an incommensurate phase [11]. The qualitative shape of the d ( T ) dependence for a non-intrinsic ferroelectric at "nonpolar-incommensurate-polar" phase transitions, estimated in [10], features the type of curve B in Fig. I in [12] a numerical calculation of the 8 ( T ) dependence is presented for an incommensurate phase. The best fitting of the estimated and experimental g ( T ) dependence made it possible to obtain the temperature of transition from a highly symmetrical to an incommensurate phase. According to [12], this temperature is in the vicinity of 300K. Slight g ( T ) anomalies typical of nonpolar-incommensurate phase transitions in these specimens are revealed at 130 K. An investigation of polarization properties has shown that in the temperature region below T,, dielectric hysteresis loops are observed, which points to the ferroelectric nature of the low-temperature phase of this crystal, and in this case spontaneous polarization grows according to a linear law and reacts the value of 1.3 x 1 0 - 7 C c m -2. At these temperatures the coercive field value is ~ 40 kV m ]. The shape of the dielectric loop and the temperature trend of spontaneous polarization are depicted in the inset of Fig. 1. The linear growth of
spontaneous polarization in the temperature region from 103 to 107K and its low value are typical of non-intrinsic ferroelectric second-order phase transitions. This result is in agreement with the data obtained in a neutron diffraction study carried out earlier [9]. The existence of spontaneous polarization in TIGaSe2 enabled us to carry out a pyroelectric study. Pyroelectric measurements were conducted with specimens prepolarized in an external electric field. A 50 kV m ] electric field was applied to the specimens at a temperature of 230K and then the crystal was cooled in the field to 90 K. There upon the field was removed, the specimen was short-circuited and measurements were made under heating. Figure 2 depicts the temperature dependence of the pyroelectric coefficient ?(T) over the whole temperature range under study, including the incommensurate phase. The maximum value of the pyroelectric coefficient is at 107.5K, whereas at 105K an " a r m " is observed. The existence of two special features in the 7(T) dependence of the T1GaSe2 crystal points to the fact that one of these features is associated with ferroelectric polarization and the other one with the appearance of a pyroelectric charge due to a lattice distortion.
REFERENCES i.
A.A. Volkov, U.G. Goncharov, G.V. Kozlov, S.P. Lebedev, A.M. Prokhorov, R.A. Aliev & K.R. Allakhverdiev, Zh. Eksper. Teor. Fiz., Pisma 37, 517 (1983).
Vol. 77, No. 6 2. 3. 4. 5. 6. 7.
FERROELECTRICITY AND POLYTYPISM IN TIGaSe2 CRYSTALS
A.A. Volkov, U.G. Goncharov, G.V. Kozlov, K.R. Allakhverdiev & R.M. Sardarly, Fiz. Tverd. Tela 26, 1754 (1984). R.A. Aliev, K.R. Allakhverdiev, A.U. Baranov, H.R. Ivanov & R.M. Sardarly, Fiz. Tverd. Tela 26, 1271 (1984). W. Paprotny & J. Grigas, Ferroelectrics 65, 201 (1985). U. Banis, A. Brilingas, J. Grigas & G. Guseinov, Fiz. Tverd. Tela 29, 3324 (1987). R.A. Aliev & K.R. Allakhverdiev, Preprint No. 256 Instityt Fiziki Akad. Nauk Azerb. SSR, Baku (1987). B.S. Kulbydjev, L.M. Rabkin, V.I. Morgushev & U. Uzuk, Fiz. Tverd. Tela 30, 195 (1988).
8. 9. 10. 11. 12.
455
A.A. Volkov, U.G. Goncharov, G.V. Kozlov & R.M. Sardarly, Zh. Eksper. Teor. Fiz., Pisma 39, 293 (1984). S.B. Vakhryshev, B.E. Kvatkovsky, N.M. Okyneva, K.R. Allakhverdiev & R.M. Sardarly, Preprint No. 886, FTI, Leningrad (1984). B.A. Strykov & A.P. Levanuk, Fizicheski Osnovi Segnetoelectricheskih Javleni v Kristallah, Nauka, Moskva (1983). B,A. Strukov, I. Yesy & V.M. Arytunova, Zh. Eksper. Teor. Fiz., Pisma 35, 424 (1982). F.M. Gashimzade, B.R. Gadjuev, K.R. Allakhverdiev & R.M. Sardarly, Fiz. Tverd. Tela 27, 2286 (1985).