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The ferroelectric characteristics in Fe-Ti-O system
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Solid State Communications, Vol. 92, No. 10, pp. 831-833, 1994 Elsevier Science Ltd Printed in Great Britain. 0038-1098/94 $7.00 + .00
Pergamon
0038-1098(94)00600-8 THE FERROELECTRIC CHARACTERISTICS IN F e - T i - O SYSTEM R.P. Viswanath* Department of Chemistry, Indian Institute of Technology, Madras 600 036, India and A.T. Seshadri Department of Physics, Indian Institute of Technology, Madras 600 036, India
(Received 29 April 1994; in revised form 19 July 1994 by A.A. Maradudin) Dielectric constant (e') and electrical conductivity (tr) have been determined over a wide range of temperature (300-650 K) for the synthetic ilmenite stabilised by non-magnetic bivalent metal ion. The material exhibits ferroelectric characteristics and the Curie temperature is found to be around 600 K. The conductivity graph (log tr vs 1/T) shows two linear regimes with a change in slope around 560 K. The results are analysed. Keywords: A. transitions
ferroelectrics,
B. chemical synthesis, D.
1. I N T R O D U C T I O N FERROELECTRICITY is known to be exhibited by a group of solids wherein, a spontaneous polarization of electric dipoles takes place. Just as in the case of ferromagnetism in ferroelectricity also, after a particular temperature (To) the self-polarization disappears and the value of dielectric constant decreases rapidly to a paraelectric state around this Tc. This phenomenon is explained on the basis of either the mobility of hydrogen bonds (e.g. KH2PO4) [1] or on account of change in the crystal structure (e.g. BaTiO3) [2]. In BaTiO 3, the central Ti 4+ ion being very small is able to polarize to a large extent with respect to a particular lattice and thus the spontaneous polarisation is observed. Above 120°C (To), the crystal structure changes from tetragonal to cubic accompanied by the disappearance in the ferroelectric behaviour. Ilmenite, a naturally occurring ore of titanium has the composition of FeTiO3 with a rhombohedral crystal structure and having interesting magnetic excitations [3]. Thus, the ilmenite is related to BaTiO3 and hence this can be expected to have ferroelectric properties.
* To whom all correspondence should be sent.
phase
However, there is no evidence in the literature to the best of authors' knowledge to support this hypothesis. In the natural ilmenite the impurities present in the ore might not have allowed the spontaneous polarisation. On the contrary, properly stabilized synthetic ilmenite, like the synthetic perovskite (CaTiO3) can be expected to exhibit ferroelectric properties. An attempt has been made in our laboratory to prepare the FeTiO3, by the incipient wet impregnation technique, having a small amount of non-magnetic divalent metal ion for obtaining optimum crystal structure. Dielectric and electrical conductivity properties of this material have been studied and the ferroelectric property of the material is reported in this study. 2. EXPERIMENTAL
2.1. Dielectric measurement Dielectric constant (e') and dielectric loss (e") were determined for the material in the form of a pellet l0 mm in diameter and 3 mm in thickness from the capacitance measurements using a G R 1620A capacitance bridge for the four frequencies (1 KHz, 2KHz, 5KHz and 10KHz) as a function of temperature (300-650 K).
831
832
THE FERROELECTRIC CHARACTERISTICS IN F e - T i - O SYSTEM Vol. 92, No. 10
2.2. Resistivity measurement For the same pellet d.c. electrical conductivity (a) was measured over the temperature range 300-650 K. tl
3. RESULTS AND DISCUSSION
~9
The chemical analysis, of the material having 0.2 mol% of CuO, for the iron content indicates that of the total iron (34.1%) as much as 32.2% remains as Fe 2+ in the fresh sample and in the used solid it is 32.4%. This indicates that the change in the concentration of Fe 2÷ is very minimal and is very much within the limits of experimental error. In Table 1 the XRD data for the material prepared in the laboratory, along with the synthetic ilmenite, is given. The experimental sample had in addition some more lines (not very strong) which were not resolvable. From this it can be safely concluded, that the material under investigation has a structure similar to that of ilmenite having a rhombohedral structure. Figure 1 shows a plot of log (or) vs l I T for the material. It can be seen from the graph, that there are two linear regions with a change in slope around 560 K. In Fig. 2, the variation of dielectric constant, at different frequencies, as a function of temperature is given. It can be seen that at higher temperatures, there occurs a spontaneous polarization up to a particular temperature (580K) and afterwards the dielectric constant of the material shows a sharp decrease. In all the four frequencies studied a similar trend is observed and Tc remains almost unaffected and independent of the frequency.
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t 5 73
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i?
19
21
23
I s
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29
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Fig. 1. Conductivity of the sample as a function of temperature (log cr vs 1/T). At room temperature the material has a very high dielectric constant (975 at 1 KHz). When the sample is heated, after a slight initial decrease in the value of the dielectric constant, one observes a slow increase in the value up to 475 K and then the increase is very rapid and reaches a maximum around 580-600K before a final drop in the value. This trend is almost similar in all the four frequencies studied. F
Table 1. XRD pattern of the material (Fe-Ti-O system). The intensities of the lines are given in parentheses Ilmenite*
Our sample
3.73 (50) 2.74 (100) 2.54 (85) 2.23 (70) 1.86 (85) 1.72 (100) 1.63 (50) 1.50 (85) 1.34 (70) 1.27 (60)
3.69 (50) 2.70 (100) 2.52 (80) 2.20 (61) 1.84 (62) 1.695 (80) 1.655 (45) 1.486 (71) 1.36 (50) 1.264 (50)
1.18 (60)
1.18 (56)
1.15 (70) 1.12 (70) 1.07 (70)
1.134 (51) 1.12 (60) 1.07 (42)
* Powder Diffraction Standards. File No. 3-781.
~31S'C
Fig. 2. Variation of dielectric constant (e') with temperature at different frequencies.
Vol. 92, No. 10 THE FERROELECTRIC CHARACTERISTICS IN F e - T i - O SYSTEM
TP
I -!
Fig. 3. Ferroelectric hysteresis loop for the material at 300 K.
The spontaneous polarization up to a critical temperature and then the sudden drop in dielectric constant is typical for any ferroelectric material. The well known ferroelectric material BaTiO3, a perovskite, changes from tetragonal crystalline structure to cubic form at 400 K. The cubic form does not have any ferroelectricity while the tetragonal lattice has strong ferroelectric properties associated with the polarizability of the Ti 4+ ions with respect to a particular lattice position. The BaTiO 3 has two other transitions (though small, both are ferroelectric) around 275 and 180K, where the
833
change in the crystalline form is from [4] 273 K 183 K tetragonal, ' orthorhombic, ' rhombohedral. Similarly, the present material FeTiO3 having a rhombohedral structure can be expected to have crystalline transitions. Thus, in the conductivity measurement one finds a change in the slope, probably indicating such a crystalline transition (580K). Around the same temperature one finds that the material changes from ferroelectric to paraelectric state. In Fig. 3 the hysteresis loop is shown for the material at 300 K using the Sawyer-Tower circuit [5], which confirms that the material under investigation is ferroelectric. Further work is in progress to evaluate the parameters of ferroelectricity and temperature dependence of the ferroelectric properties of the material. REFERENCES 1. 2. 3. 4. 5.
R. Blinc, Advances in Magnetic Resonance, Vol. 3 (Edited by J.S. Waugh). Academic Press, New York (1968). J. Villain & S. Aubry, Phys. Status Solidi 33, 337 (1969). H. Kato, Y. Yamaguchi, M. Yamada, S. Funahashi, Y. Nakagawa & H. Takei, J. Phys. C19, 6993 (1986). C.N.R. Rao and K.J. Rao, Phase Transitions in Solids, p. 311. McGraw-Hill, London (1978). M.M. Berkens & Th. Kwaaitaal, J. Phys. El6, 516 (1983).
Solid State Communications, Vol. 92, No. 10, pp. 831-833, 1994 Elsevier Science Ltd Printed in Great Britain. 0038-1098/94 $7.00 + .00
Pergamon
0038-1098(94)00600-8 THE FERROELECTRIC CHARACTERISTICS IN F e - T i - O SYSTEM R.P. Viswanath* Department of Chemistry, Indian Institute of Technology, Madras 600 036, India and A.T. Seshadri Department of Physics, Indian Institute of Technology, Madras 600 036, India
(Received 29 April 1994; in revised form 19 July 1994 by A.A. Maradudin) Dielectric constant (e') and electrical conductivity (tr) have been determined over a wide range of temperature (300-650 K) for the synthetic ilmenite stabilised by non-magnetic bivalent metal ion. The material exhibits ferroelectric characteristics and the Curie temperature is found to be around 600 K. The conductivity graph (log tr vs 1/T) shows two linear regimes with a change in slope around 560 K. The results are analysed. Keywords: A. transitions
ferroelectrics,
B. chemical synthesis, D.
1. I N T R O D U C T I O N FERROELECTRICITY is known to be exhibited by a group of solids wherein, a spontaneous polarization of electric dipoles takes place. Just as in the case of ferromagnetism in ferroelectricity also, after a particular temperature (To) the self-polarization disappears and the value of dielectric constant decreases rapidly to a paraelectric state around this Tc. This phenomenon is explained on the basis of either the mobility of hydrogen bonds (e.g. KH2PO4) [1] or on account of change in the crystal structure (e.g. BaTiO3) [2]. In BaTiO 3, the central Ti 4+ ion being very small is able to polarize to a large extent with respect to a particular lattice and thus the spontaneous polarisation is observed. Above 120°C (To), the crystal structure changes from tetragonal to cubic accompanied by the disappearance in the ferroelectric behaviour. Ilmenite, a naturally occurring ore of titanium has the composition of FeTiO3 with a rhombohedral crystal structure and having interesting magnetic excitations [3]. Thus, the ilmenite is related to BaTiO3 and hence this can be expected to have ferroelectric properties.
* To whom all correspondence should be sent.
phase
However, there is no evidence in the literature to the best of authors' knowledge to support this hypothesis. In the natural ilmenite the impurities present in the ore might not have allowed the spontaneous polarisation. On the contrary, properly stabilized synthetic ilmenite, like the synthetic perovskite (CaTiO3) can be expected to exhibit ferroelectric properties. An attempt has been made in our laboratory to prepare the FeTiO3, by the incipient wet impregnation technique, having a small amount of non-magnetic divalent metal ion for obtaining optimum crystal structure. Dielectric and electrical conductivity properties of this material have been studied and the ferroelectric property of the material is reported in this study. 2. EXPERIMENTAL
2.1. Dielectric measurement Dielectric constant (e') and dielectric loss (e") were determined for the material in the form of a pellet l0 mm in diameter and 3 mm in thickness from the capacitance measurements using a G R 1620A capacitance bridge for the four frequencies (1 KHz, 2KHz, 5KHz and 10KHz) as a function of temperature (300-650 K).
831
832
THE FERROELECTRIC CHARACTERISTICS IN F e - T i - O SYSTEM Vol. 92, No. 10
2.2. Resistivity measurement For the same pellet d.c. electrical conductivity (a) was measured over the temperature range 300-650 K. tl
3. RESULTS AND DISCUSSION
~9
The chemical analysis, of the material having 0.2 mol% of CuO, for the iron content indicates that of the total iron (34.1%) as much as 32.2% remains as Fe 2+ in the fresh sample and in the used solid it is 32.4%. This indicates that the change in the concentration of Fe 2÷ is very minimal and is very much within the limits of experimental error. In Table 1 the XRD data for the material prepared in the laboratory, along with the synthetic ilmenite, is given. The experimental sample had in addition some more lines (not very strong) which were not resolvable. From this it can be safely concluded, that the material under investigation has a structure similar to that of ilmenite having a rhombohedral structure. Figure 1 shows a plot of log (or) vs l I T for the material. It can be seen from the graph, that there are two linear regions with a change in slope around 560 K. In Fig. 2, the variation of dielectric constant, at different frequencies, as a function of temperature is given. It can be seen that at higher temperatures, there occurs a spontaneous polarization up to a particular temperature (580K) and afterwards the dielectric constant of the material shows a sharp decrease. In all the four frequencies studied a similar trend is observed and Tc remains almost unaffected and independent of the frequency.
~7
t 5 73
~9
i~
i?
19
21
23
I s
?'7
29
3~
Fig. 1. Conductivity of the sample as a function of temperature (log cr vs 1/T). At room temperature the material has a very high dielectric constant (975 at 1 KHz). When the sample is heated, after a slight initial decrease in the value of the dielectric constant, one observes a slow increase in the value up to 475 K and then the increase is very rapid and reaches a maximum around 580-600K before a final drop in the value. This trend is almost similar in all the four frequencies studied. F
Table 1. XRD pattern of the material (Fe-Ti-O system). The intensities of the lines are given in parentheses Ilmenite*
Our sample
3.73 (50) 2.74 (100) 2.54 (85) 2.23 (70) 1.86 (85) 1.72 (100) 1.63 (50) 1.50 (85) 1.34 (70) 1.27 (60)
3.69 (50) 2.70 (100) 2.52 (80) 2.20 (61) 1.84 (62) 1.695 (80) 1.655 (45) 1.486 (71) 1.36 (50) 1.264 (50)
1.18 (60)
1.18 (56)
1.15 (70) 1.12 (70) 1.07 (70)
1.134 (51) 1.12 (60) 1.07 (42)
* Powder Diffraction Standards. File No. 3-781.
~31S'C
Fig. 2. Variation of dielectric constant (e') with temperature at different frequencies.
Vol. 92, No. 10 THE FERROELECTRIC CHARACTERISTICS IN F e - T i - O SYSTEM
TP
I -!
Fig. 3. Ferroelectric hysteresis loop for the material at 300 K.
The spontaneous polarization up to a critical temperature and then the sudden drop in dielectric constant is typical for any ferroelectric material. The well known ferroelectric material BaTiO3, a perovskite, changes from tetragonal crystalline structure to cubic form at 400 K. The cubic form does not have any ferroelectricity while the tetragonal lattice has strong ferroelectric properties associated with the polarizability of the Ti 4+ ions with respect to a particular lattice position. The BaTiO 3 has two other transitions (though small, both are ferroelectric) around 275 and 180K, where the
833
change in the crystalline form is from [4] 273 K 183 K tetragonal, ' orthorhombic, ' rhombohedral. Similarly, the present material FeTiO3 having a rhombohedral structure can be expected to have crystalline transitions. Thus, in the conductivity measurement one finds a change in the slope, probably indicating such a crystalline transition (580K). Around the same temperature one finds that the material changes from ferroelectric to paraelectric state. In Fig. 3 the hysteresis loop is shown for the material at 300 K using the Sawyer-Tower circuit [5], which confirms that the material under investigation is ferroelectric. Further work is in progress to evaluate the parameters of ferroelectricity and temperature dependence of the ferroelectric properties of the material. REFERENCES 1. 2. 3. 4. 5.
R. Blinc, Advances in Magnetic Resonance, Vol. 3 (Edited by J.S. Waugh). Academic Press, New York (1968). J. Villain & S. Aubry, Phys. Status Solidi 33, 337 (1969). H. Kato, Y. Yamaguchi, M. Yamada, S. Funahashi, Y. Nakagawa & H. Takei, J. Phys. C19, 6993 (1986). C.N.R. Rao and K.J. Rao, Phase Transitions in Solids, p. 311. McGraw-Hill, London (1978). M.M. Berkens & Th. Kwaaitaal, J. Phys. El6, 516 (1983).