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Dielectric relaxation and conductivity in barium salt of 12-tungstophosphoric acid hydrate
Solid State Ionics 147 (2002) 123 – 128 www.elsevier.com/locate/ssi
Dielectric relaxation and conductivity in barium salt of 12-tungstophosphoric acid hydrate M. Davidovic´ a,b,*, T. Cˇajkovski a, D. Cˇajkovski a, V. Likar-Smiljanic´ b, R. Biljic´ c, U.B. Miocˇ d, Z. Nedic´ d a Vincˇa Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Yugoslavia Faculty of Electrical Engineering, University of Belgrade, P.O. Box 816, 11001 Belgrade, Yugoslavia c VTA, Zˇarkovo, 11000 Belgrade, Yugoslavia d Faculty of Physical Chemistry, University of Belgrade, P.O. Box 137, 11001 Belgrade, Yugoslavia
b
Received 30 November 2000; received in revised form 22 November 2001; accepted 21 December 2001
Abstract Barium salt of 12-tungstophosphoric acid (WPA) hydrate BaHPW12O407H2O (BaHWPA7H2O) has been investigated both in the microwave (X-band) region and in the lower frequency (5 Hz – 500 kHz) region. This work is a continuation of our previous investigations on the influence of monovalent and bivalent cations on the dielectric properties of WPA hydrate salts. The real and imaginary parts of the permittivity (eV, eW) as a function of frequency and temperature, were determined in both frequency regions. The observed relaxations were attributed to specific relaxation processes. In the lower frequency range, the relaxation time s = 1.0 10 6 s was assigned to barium ion jumps. In the microwave region, the dielectric relaxation time s = 1.7 10 11 s was attributed to polyatomic ions (H3O + ) and/or H2O molecules reorientations. A phase transition was observed at 330 K. The low value of the activation energy for reorientation in the microwave frequency region is discussed. The d.c. conductivity and its temperature dependence were determined. Measurements were made in the temperature interval from 286 to 353 K. In the upper part of this temperature interval a PTCR effect was observed. D 2002 Published by Elsevier Science B.V. Keywords: Heteropolyacid salts; Protonic conductivity; Permittivity measurements
1. Introduction Heteropolyacids (WPA) as a class of chemical substances with a high catalytic activity have been known for quite a time. Investigations on these substances have been intensified after their high * Corresponding author. Vincˇa Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Yugoslavia. Tel.: +381-11-458222x740; fax: +381-11-344-0100. E-mail address: [email protected] (M. Davidovic´).
conductivity was noticed [1] and applications, such as solid electrolytes became possible. Heteropolyacids are superionic proton conductors having a room temperature conductivity of the order (1 –100) 10 3 S cm 1. Their charge carriers are protons from various protonic entities: OH , H2O, H3O+ , H5O2+ and H+ (H2O)n [2]. Heteropolyacids salts have been less investigated. The influence of monovalent cations on the properties of the corresponding salts have been established by impedance measurements in the RF region, pulsed NMR, IR and
0167-2738/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 0 0 3 - 6
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Raman spectroscopy, neutron scattering and X-ray diffraction [3 –9]. Not so much work has been done on heteropolyacid salts with bivalent ions. Such salts of 12-tungstophosphoric acid hydrates were the main objects of our recent investigations. Dipolar relaxations in the microwave X-band region were observed in CaHWPA13H2O, CuHWPA15H2O and MgHWPA 14H2O [10,11]. Recent studies of Mg2+ and Ca2+ salts of WPA [12] yielded an interesting correlation between the characteristics of cations and the dynamic equilibrium of the different proton species and the hydrated proton entities. In this work, barium salt of tungstophosphoric acid hydrate BaHPW12O407H2O was investigated. Measurements included real and imaginary parts of permittivity (eV(x,T), eW(x,T)) in the microwave X-band as well as in the lower frequency region (5 Hz – 500 kHz). Measurements were carried out in the temperature interval from 286 to 353 K, in which BaHWPA7H2O O is sufficiently stable. A dipolar relaxation was observed in the microwave region and was assigned to a fast reorientation of H3O+ ions and/or H2O molecules. From the temperature dependence of the relaxation time, the activation energy for dipole reorientation was determined. The activation energy for conduction was determined from the temperature dependence of d.c. conductivity.
salt pellets together with the attached golden disc electrodes were mounted between the condenser plates utilizing a spring to provide the necessary pressure. Impedance measurements were made with a HP4800A vector impedance meter. The humidity inside the specimen chamber was adjusted by using some chemicals providing a constant relative humidity and was registered with a remote sensor humidity meter. The degree of hydration of the specimen was checked by thermal measurements viz. DTA and TGA. In order to assure a constant hydration of the specimen, the following measures were undertaken. Pressing of the polycrystalline powder was done under room ambient conditions. Following the pressing, the specimen, together with the waveguide flange, was transferred to a dessicator and stored there one month in an ambient with a relative humidity (RH) of 35%, before the start of measurements. Pressed pellets were mounted into the specimen jig at the room ambient. The assembled specimen holder, with specimen was transferred into the measurement chamber with 35% RH. The specimen was left in this ambient one month before the measurements were started. There is a strong influence of ambient humidity on the conductivity. For a relative humidity change from 25% to 85% the conductivity changes by five orders of magnitude [13].
3. Results and discussion 2. Experimental procedure The Ba2 + salt of 12-tungstophosphoric acid hydrate (BaHWPA7H2O) was made by the procedure described previously [6]. The substance was synthesized in the form of polycrystalline powder. By crushing this powder in an agate mortar fine powder was produced, with an average grain size of 2 mm. For microwave measurements the powder was pressed into a rectangular waveguide flange. The specimens for the lower frequency range were pressed into a die of circular cross section. The disc-shaped specimens had a 13-mm diameter and a thickness between 1.0 and 1.5 mm. The density of the specimens was 4.5 g/cm3. The real and imaginary parts of permittivity in the X-band were determined by measuring the S-parameters with a HP8410C network analyzer. For the lower frequency range (5 Hz –500 kHz) a specimen holder was made, which consisted of a variable air gap condenser. Barium
A dipolar relaxation was observed in the frequency range between 8.0 and 12.0 GHz. Frequency dependencies of the real and imaginary parts of permittivity (eV, eW) of BaHWPA7H2O at room temperature are shown in Fig. 1(a) and (b). Due to the dipolar relaxation, eV drops from the value 9.5 to 7.5. The middle of transition region coincides with the eW maximum. If there is no interaction among dipoles, which is apparently true for most of the proton conductors [14], then for the maximum of eW we have xs = 1. Here, x is angular frequency and s is the Debye relaxation time. The relaxation time at room temperature is s = 1.7 1011 s. When this value of s is compared with relaxation times in the classification table (Table 1 in Ref. [15]), it leads to the conclusion that in BaHWPA7H2O, the dipolar relaxation can be attributed to the fast reorientation of polyatomic ions (H3O+) and/or H2O molecules.
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relaxation time, possible relaxators and their activation energy for reorientations. Experiments were also done in the lower frequency range (5 Hz –500 kHz) in order to get information on long range dynamics in BaHWPA7H2O. Being a polycrystalline material, it is well known [17,18] that these materials exhibit a variety of frequency dependent effects associated with heterogeneities, such as grain boundaries or surface layers in addition to the intrinsic properties. In Fig. 3, frequency dependence of conductivity of barium salt for several temperatures is given. As the temperature rises, conductivity increases as is usual for thermally activated process exhibiting a plateau in the central frequency range. After the temperature passes the phase transition at 330 K, conductivity drops, which means that we have here positive temperature coefficient of resistance (PTCR). This effect is not as nearly large as it is in the BaTiO3, but its mere existence makes it worth investigating. It might be a grain boundary effect, but this remains to be proved in future investigations.
Fig. 1. Frequency dependence of (a) real part eV and (b) imaginary part eW of the complex permittivity e* for BaHWPA7H2O. Relaxation time s = 1.7 10 11 s.
The frequency dependencies of eV and eW in the microwave region have been obtained at several temperatures in the interval from 286 to 353 K. The relaxation times are plotted in an log s vs. (1000/T) diagram shown in Fig. 2. The plotted points follow fairly satisfactory the Arrhenius law, up to the phase transition point (330 K). Then a sudden change in slope occurs, which results in two activation energies E a1 = 3.4 10 3 and E a2 = 6.5 10 4 eV. These rather small values of activation energies might be due to the local field effects, caused by substitution of oxonium ion by cations (Li+ , Na+ , K+ , Ba2+ ) [16]. Dielectric relaxation experiments at microwave frequencies have given us some information such as
Fig. 2. Temperature dependence of relaxation time. Activation energies found for the two reorientation relaxations in BaHWPA7H2O are Ea1 = 3.4 10 3 and Ea2 = 6.5 10 4 eV.
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In Fig. 5, d.c. conductivity data are plotted in an Arrhenius diagram. The data obey the Arrhenius law in the lower temperature region. The estimated activation energy for conduction is Ea = 0.27 eV. At higher temperatures, conductivity data deviate from the Arrhenius straight line after passing the phase transition point. Analyzing the TGA and DTA curves in the temperature interval from 303 to 523 K, one can see that water is leaving BaHWPA7H2O in distinct steps: at 330, 361, 408 and 435 K. After a temperature of 523 K, the sample completely dehydrated. The loss of water up to 348 K is one molecule. The step at 330 Fig. 3. Frequency dependence of conductivity at four temperatures. Frequency range 50 Hz – 500 kHz. Note the onset of the PTCR effect at 330 K.
In the description of dielectric relaxations, a generalized Debye model due to Cole and Cole [19] is used with frequency-dependent complex permittivity e* presented (taking into account interactions among the dipoles) by the well known expression e*ðxÞ e1 1 ¼ ; es e1 1 þ ðixsÞ1a where e1 and es are the limiting values of e* (x) as x approaches 1 and 0 respectively; s is the Debye relaxation time and a is an empirical ‘‘broadness’’ parameter (0 V a V 1) showing the degree of departure from the Debye model for which a = 0 [19,20]. For the eW maximum we have xs = 1. In Fig. 4, a Cole – Cole diagram for 300 K temperature is shown. After subtraction of the conductivity losses a depressed semicircle with its center below the real axis is obtained. This means that in our system there is some interaction among dipoles. The relaxation time is s = 1.0 106 s. This relaxation time can be attributed to barium ions jumps. If we compare this result with Mg2+ salt of WPA [11], we notice that Ba2 + ion has a longer relaxation time than Mg2+ ion due to the mass effect. For barium salt we get a = 0.25 at room temperature. When the temperature increases, the relaxation time becomes shorter and for Ba2+ ion at 330 K it is s = 5.4 107 s. As activation energy for Ba2+ ions jumps we obtained, Ea = 0.19 eV.
Fig. 4. Cole – Cole diagram for BaHWPA7H2O at T = 300 K. Full line—spectrum; dotted line—deconvolution curves; solid circle— selected frequencies. Relaxation time s = 1.0 10 6 s.
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dipoles, polarization decreases and eV and eW diminish. For frequencies higher than 3 kHz, in the temperature interval where our experiments were done, dipoles are not sufficiently free to follow the changing electric field, and therefore eV and eW remain practically constant. In addition to the above simple explanation, there remain many questions concerning the nature of the phase transition. Questions like ferro-para transition, Curie temperature, etc. demand a thorough analysis of this problem. Wide range impedance spectroscopy measurements with data analysis using various formalisms might be the right way to continue this investigation [18].
Fig. 5. Temperature dependence of d.c. conductivity. Conductivity activation energy Ea = 0.27 eV. Note the onset of the PTCR effect at 330 K.
K corresponds to the temperature where there is a change of slope in the Arrhenius diagram in Fig. 2 and a deviation from the Arrhenius plot in Fig. 5. Therefore, one can conclude that even a small loss of water leads to structural changes, that is to the phase transformation, because it changes dynamic equilibrium between protonic species. In Fig. 6(a) and (b), we present the temperature dependence of real and imaginary parts of permittivity eV and eW for several values of frequency in the interval from 500 Hz to 100 kHz. At lower frequencies, from 500 Hz to about 3 kHz both eV and eW rise with the temperature and then pass through the maximum. For frequencies higher than 3 kHz there is practically no temperature dependence. Such a behavior of eV and eW can be rather simply explained by using arguments for temperature dependence of orientational polarization. In the left part of the diagram the temperature rise makes dipoles more and more free to orient themselves parallel to the applied field. As the temperature goes further up, thermal motion in the crystal is inhibiting the orientation of the
Fig. 6. Temperature dependence of (a) real part eV and (b) imaginary part eW of permittivity in the frequency interval from 500 Hz to 100 kHz. Note the drop in eV and eW from 330 K onwards.
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4. Conclusions In this work, barium salt of WPA was investigated by dielectric measurements in the microwave region and in the lower frequency region (5 Hz – 500 kHz). A dielectric relaxation was observed in the microwave region and attributed to H3O+ ions and/or H2O molecules reorientation. The low value for the activation energy of the reorientation processes suggest the influence of local field effects due to substituted cations. A PTCR effect was observed in the lower frequency region at the temperature 330 K. The relaxation time of 1.0 106 s for ionic jumps was attributed to Ba2+ ions. There is some interaction among the dipoles, which leads to a shape factor a = 0.25. Activation energy for Ba2+ ion jumps is 0.19 eV. Temperature dependence of permittivity at low frequencies exhibits a phase transition. Presently, it is not clear if this is a ferroelectric – paraelectric transition.
Acknowledgements This work has been partly supported by the Ministry for Science and Technology of the Republic of Serbia, Grant no. 01E15. The authors wish to thank Prof. Ph. Colomban and Prof. A. Djordjevic´ for valuable suggestions and discussions, and Mrs. Lj. Novakovic´ for performing TGA measurements.
References [1] O. Nakamura, T. Ogino, T. Kodama, Solid State Ionics 3 – 4 (1981) 347.
[2] M. Davidovic´, U.B. Miocˇ, Mater. Sci. Forum 214 (1996) 99, Trans. Tech. Publications, Switzerland. [3] E.A. Ukshe, L.S. Leonova, A.I. Korosteleva, Solid State Ionics 36 (1989) 219. [4] V. Steinberg, L. Erofeev, A. Korosteleva, L. Leonova, E. Ukshe, Elektrokhimiya 24 (1988) 954. [5] N. Tjapkin, M. Davidovic´, Ph. Colomban, U. Miocˇ, Solid State Ionics 61 (1993) 179. [6] U.B. Miocˇ, M. Davidovic´, B.R. Stanisavljevic´, M.R. Nedic´, Z.P. Nedic´, S. Uskokovic´, J. Serb. Chem. Soc. 61 (1996) 759. [7] T. Cˇajkovski, M. Davidovic´, D. Cˇajkovski, V. Likar-Smiljanic´, R. Biljic´, U. Miocˇ, Z. Nedic´, Mater. Sci. Forum 282 – 283 (1998) 193, Trans. Tech. Publications, Switzerland. [8] S.D. Mikhailenko, S. Kaliaguine, J.B. Moffat, Solid State Ionics 99 (1997) 281. [9] N. Essayem, Y.Y. Tong, H. Jobic, J.C. Vedrine, Appl. Catal., A 194 – 195 (2000) 109. [10] T. Cˇajkovski, M. Davidovic´, D. Cˇajkovski, V. Likar-Smiljanic´, R. Biljic´, U. Miocˇ, V. Stamenkovic´, Solid State Phenom. 61 – 62 (1998) 279, Scitec Publications, Switzerland. [11] M. Davidovic´, T. Cˇajkovski, D. Cˇajkovski, V. Likar-Smiljanic´, R. Biljic´, U. Miocˇ, Z. Nedic´, Solid State Ionics 125 (1999) 411. [12] U.B. Miocˇ, M.R. Todorovic´, Ph. Colomban, Z.P. Nedic´, S.M. Uskokovic´, I.D. Borcˇic´, Solid State Ionics 125 (1999) 425. [13] T. Cˇajkovski, D. Cˇajkovski, V. Likar-Smiljanic´, M. Davidovic´, R. Biljic´, U. Miocˇ, Z. Nedic´, in press.. [14] J.C. Badot, A. Fourier-Lamer, T. Mhiri, Ph. Colomban, Solid State Ionics 46 (1991) 151. [15] J.C. Badot, N. Baffier, A. Fourier-Lamer, Ph. Colomban, Solid State Ionics 28 – 30 (1988) 1617. [16] Ph. Colomban, J.C. Badot, in: Ph. Colomban (Ed.), Proton Conductors, Cambridge Univ. Press, Cambridge, 1992, pp. 389 – 406, Chap. 25. [17] T. Takeuchi, E. Betourke, M. Tabuchi, Y. Kobayashi, A. Coats, F. Morrison, D.C. Sinclair, A.R. West, J. Mater. Sci. 34 (1999) 917. [18] N. Hirose, A.R. West, J. Am. Ceram. Soc. 79 (1996) 1633. [19] K.S. Cole, R.H. Cole, J. Chem. Phys. 9 (1941) 341. [20] P. Debye, Polar Molecules, Dover, New York, 1945.
Dielectric relaxation and conductivity in barium salt of 12-tungstophosphoric acid hydrate M. Davidovic´ a,b,*, T. Cˇajkovski a, D. Cˇajkovski a, V. Likar-Smiljanic´ b, R. Biljic´ c, U.B. Miocˇ d, Z. Nedic´ d a Vincˇa Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Yugoslavia Faculty of Electrical Engineering, University of Belgrade, P.O. Box 816, 11001 Belgrade, Yugoslavia c VTA, Zˇarkovo, 11000 Belgrade, Yugoslavia d Faculty of Physical Chemistry, University of Belgrade, P.O. Box 137, 11001 Belgrade, Yugoslavia
b
Received 30 November 2000; received in revised form 22 November 2001; accepted 21 December 2001
Abstract Barium salt of 12-tungstophosphoric acid (WPA) hydrate BaHPW12O407H2O (BaHWPA7H2O) has been investigated both in the microwave (X-band) region and in the lower frequency (5 Hz – 500 kHz) region. This work is a continuation of our previous investigations on the influence of monovalent and bivalent cations on the dielectric properties of WPA hydrate salts. The real and imaginary parts of the permittivity (eV, eW) as a function of frequency and temperature, were determined in both frequency regions. The observed relaxations were attributed to specific relaxation processes. In the lower frequency range, the relaxation time s = 1.0 10 6 s was assigned to barium ion jumps. In the microwave region, the dielectric relaxation time s = 1.7 10 11 s was attributed to polyatomic ions (H3O + ) and/or H2O molecules reorientations. A phase transition was observed at 330 K. The low value of the activation energy for reorientation in the microwave frequency region is discussed. The d.c. conductivity and its temperature dependence were determined. Measurements were made in the temperature interval from 286 to 353 K. In the upper part of this temperature interval a PTCR effect was observed. D 2002 Published by Elsevier Science B.V. Keywords: Heteropolyacid salts; Protonic conductivity; Permittivity measurements
1. Introduction Heteropolyacids (WPA) as a class of chemical substances with a high catalytic activity have been known for quite a time. Investigations on these substances have been intensified after their high * Corresponding author. Vincˇa Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Yugoslavia. Tel.: +381-11-458222x740; fax: +381-11-344-0100. E-mail address: [email protected] (M. Davidovic´).
conductivity was noticed [1] and applications, such as solid electrolytes became possible. Heteropolyacids are superionic proton conductors having a room temperature conductivity of the order (1 –100) 10 3 S cm 1. Their charge carriers are protons from various protonic entities: OH , H2O, H3O+ , H5O2+ and H+ (H2O)n [2]. Heteropolyacids salts have been less investigated. The influence of monovalent cations on the properties of the corresponding salts have been established by impedance measurements in the RF region, pulsed NMR, IR and
0167-2738/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 0 0 3 - 6
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Raman spectroscopy, neutron scattering and X-ray diffraction [3 –9]. Not so much work has been done on heteropolyacid salts with bivalent ions. Such salts of 12-tungstophosphoric acid hydrates were the main objects of our recent investigations. Dipolar relaxations in the microwave X-band region were observed in CaHWPA13H2O, CuHWPA15H2O and MgHWPA 14H2O [10,11]. Recent studies of Mg2+ and Ca2+ salts of WPA [12] yielded an interesting correlation between the characteristics of cations and the dynamic equilibrium of the different proton species and the hydrated proton entities. In this work, barium salt of tungstophosphoric acid hydrate BaHPW12O407H2O was investigated. Measurements included real and imaginary parts of permittivity (eV(x,T), eW(x,T)) in the microwave X-band as well as in the lower frequency region (5 Hz – 500 kHz). Measurements were carried out in the temperature interval from 286 to 353 K, in which BaHWPA7H2O O is sufficiently stable. A dipolar relaxation was observed in the microwave region and was assigned to a fast reorientation of H3O+ ions and/or H2O molecules. From the temperature dependence of the relaxation time, the activation energy for dipole reorientation was determined. The activation energy for conduction was determined from the temperature dependence of d.c. conductivity.
salt pellets together with the attached golden disc electrodes were mounted between the condenser plates utilizing a spring to provide the necessary pressure. Impedance measurements were made with a HP4800A vector impedance meter. The humidity inside the specimen chamber was adjusted by using some chemicals providing a constant relative humidity and was registered with a remote sensor humidity meter. The degree of hydration of the specimen was checked by thermal measurements viz. DTA and TGA. In order to assure a constant hydration of the specimen, the following measures were undertaken. Pressing of the polycrystalline powder was done under room ambient conditions. Following the pressing, the specimen, together with the waveguide flange, was transferred to a dessicator and stored there one month in an ambient with a relative humidity (RH) of 35%, before the start of measurements. Pressed pellets were mounted into the specimen jig at the room ambient. The assembled specimen holder, with specimen was transferred into the measurement chamber with 35% RH. The specimen was left in this ambient one month before the measurements were started. There is a strong influence of ambient humidity on the conductivity. For a relative humidity change from 25% to 85% the conductivity changes by five orders of magnitude [13].
3. Results and discussion 2. Experimental procedure The Ba2 + salt of 12-tungstophosphoric acid hydrate (BaHWPA7H2O) was made by the procedure described previously [6]. The substance was synthesized in the form of polycrystalline powder. By crushing this powder in an agate mortar fine powder was produced, with an average grain size of 2 mm. For microwave measurements the powder was pressed into a rectangular waveguide flange. The specimens for the lower frequency range were pressed into a die of circular cross section. The disc-shaped specimens had a 13-mm diameter and a thickness between 1.0 and 1.5 mm. The density of the specimens was 4.5 g/cm3. The real and imaginary parts of permittivity in the X-band were determined by measuring the S-parameters with a HP8410C network analyzer. For the lower frequency range (5 Hz –500 kHz) a specimen holder was made, which consisted of a variable air gap condenser. Barium
A dipolar relaxation was observed in the frequency range between 8.0 and 12.0 GHz. Frequency dependencies of the real and imaginary parts of permittivity (eV, eW) of BaHWPA7H2O at room temperature are shown in Fig. 1(a) and (b). Due to the dipolar relaxation, eV drops from the value 9.5 to 7.5. The middle of transition region coincides with the eW maximum. If there is no interaction among dipoles, which is apparently true for most of the proton conductors [14], then for the maximum of eW we have xs = 1. Here, x is angular frequency and s is the Debye relaxation time. The relaxation time at room temperature is s = 1.7 1011 s. When this value of s is compared with relaxation times in the classification table (Table 1 in Ref. [15]), it leads to the conclusion that in BaHWPA7H2O, the dipolar relaxation can be attributed to the fast reorientation of polyatomic ions (H3O+) and/or H2O molecules.
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relaxation time, possible relaxators and their activation energy for reorientations. Experiments were also done in the lower frequency range (5 Hz –500 kHz) in order to get information on long range dynamics in BaHWPA7H2O. Being a polycrystalline material, it is well known [17,18] that these materials exhibit a variety of frequency dependent effects associated with heterogeneities, such as grain boundaries or surface layers in addition to the intrinsic properties. In Fig. 3, frequency dependence of conductivity of barium salt for several temperatures is given. As the temperature rises, conductivity increases as is usual for thermally activated process exhibiting a plateau in the central frequency range. After the temperature passes the phase transition at 330 K, conductivity drops, which means that we have here positive temperature coefficient of resistance (PTCR). This effect is not as nearly large as it is in the BaTiO3, but its mere existence makes it worth investigating. It might be a grain boundary effect, but this remains to be proved in future investigations.
Fig. 1. Frequency dependence of (a) real part eV and (b) imaginary part eW of the complex permittivity e* for BaHWPA7H2O. Relaxation time s = 1.7 10 11 s.
The frequency dependencies of eV and eW in the microwave region have been obtained at several temperatures in the interval from 286 to 353 K. The relaxation times are plotted in an log s vs. (1000/T) diagram shown in Fig. 2. The plotted points follow fairly satisfactory the Arrhenius law, up to the phase transition point (330 K). Then a sudden change in slope occurs, which results in two activation energies E a1 = 3.4 10 3 and E a2 = 6.5 10 4 eV. These rather small values of activation energies might be due to the local field effects, caused by substitution of oxonium ion by cations (Li+ , Na+ , K+ , Ba2+ ) [16]. Dielectric relaxation experiments at microwave frequencies have given us some information such as
Fig. 2. Temperature dependence of relaxation time. Activation energies found for the two reorientation relaxations in BaHWPA7H2O are Ea1 = 3.4 10 3 and Ea2 = 6.5 10 4 eV.
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In Fig. 5, d.c. conductivity data are plotted in an Arrhenius diagram. The data obey the Arrhenius law in the lower temperature region. The estimated activation energy for conduction is Ea = 0.27 eV. At higher temperatures, conductivity data deviate from the Arrhenius straight line after passing the phase transition point. Analyzing the TGA and DTA curves in the temperature interval from 303 to 523 K, one can see that water is leaving BaHWPA7H2O in distinct steps: at 330, 361, 408 and 435 K. After a temperature of 523 K, the sample completely dehydrated. The loss of water up to 348 K is one molecule. The step at 330 Fig. 3. Frequency dependence of conductivity at four temperatures. Frequency range 50 Hz – 500 kHz. Note the onset of the PTCR effect at 330 K.
In the description of dielectric relaxations, a generalized Debye model due to Cole and Cole [19] is used with frequency-dependent complex permittivity e* presented (taking into account interactions among the dipoles) by the well known expression e*ðxÞ e1 1 ¼ ; es e1 1 þ ðixsÞ1a where e1 and es are the limiting values of e* (x) as x approaches 1 and 0 respectively; s is the Debye relaxation time and a is an empirical ‘‘broadness’’ parameter (0 V a V 1) showing the degree of departure from the Debye model for which a = 0 [19,20]. For the eW maximum we have xs = 1. In Fig. 4, a Cole – Cole diagram for 300 K temperature is shown. After subtraction of the conductivity losses a depressed semicircle with its center below the real axis is obtained. This means that in our system there is some interaction among dipoles. The relaxation time is s = 1.0 106 s. This relaxation time can be attributed to barium ions jumps. If we compare this result with Mg2+ salt of WPA [11], we notice that Ba2 + ion has a longer relaxation time than Mg2+ ion due to the mass effect. For barium salt we get a = 0.25 at room temperature. When the temperature increases, the relaxation time becomes shorter and for Ba2+ ion at 330 K it is s = 5.4 107 s. As activation energy for Ba2+ ions jumps we obtained, Ea = 0.19 eV.
Fig. 4. Cole – Cole diagram for BaHWPA7H2O at T = 300 K. Full line—spectrum; dotted line—deconvolution curves; solid circle— selected frequencies. Relaxation time s = 1.0 10 6 s.
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dipoles, polarization decreases and eV and eW diminish. For frequencies higher than 3 kHz, in the temperature interval where our experiments were done, dipoles are not sufficiently free to follow the changing electric field, and therefore eV and eW remain practically constant. In addition to the above simple explanation, there remain many questions concerning the nature of the phase transition. Questions like ferro-para transition, Curie temperature, etc. demand a thorough analysis of this problem. Wide range impedance spectroscopy measurements with data analysis using various formalisms might be the right way to continue this investigation [18].
Fig. 5. Temperature dependence of d.c. conductivity. Conductivity activation energy Ea = 0.27 eV. Note the onset of the PTCR effect at 330 K.
K corresponds to the temperature where there is a change of slope in the Arrhenius diagram in Fig. 2 and a deviation from the Arrhenius plot in Fig. 5. Therefore, one can conclude that even a small loss of water leads to structural changes, that is to the phase transformation, because it changes dynamic equilibrium between protonic species. In Fig. 6(a) and (b), we present the temperature dependence of real and imaginary parts of permittivity eV and eW for several values of frequency in the interval from 500 Hz to 100 kHz. At lower frequencies, from 500 Hz to about 3 kHz both eV and eW rise with the temperature and then pass through the maximum. For frequencies higher than 3 kHz there is practically no temperature dependence. Such a behavior of eV and eW can be rather simply explained by using arguments for temperature dependence of orientational polarization. In the left part of the diagram the temperature rise makes dipoles more and more free to orient themselves parallel to the applied field. As the temperature goes further up, thermal motion in the crystal is inhibiting the orientation of the
Fig. 6. Temperature dependence of (a) real part eV and (b) imaginary part eW of permittivity in the frequency interval from 500 Hz to 100 kHz. Note the drop in eV and eW from 330 K onwards.
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4. Conclusions In this work, barium salt of WPA was investigated by dielectric measurements in the microwave region and in the lower frequency region (5 Hz – 500 kHz). A dielectric relaxation was observed in the microwave region and attributed to H3O+ ions and/or H2O molecules reorientation. The low value for the activation energy of the reorientation processes suggest the influence of local field effects due to substituted cations. A PTCR effect was observed in the lower frequency region at the temperature 330 K. The relaxation time of 1.0 106 s for ionic jumps was attributed to Ba2+ ions. There is some interaction among the dipoles, which leads to a shape factor a = 0.25. Activation energy for Ba2+ ion jumps is 0.19 eV. Temperature dependence of permittivity at low frequencies exhibits a phase transition. Presently, it is not clear if this is a ferroelectric – paraelectric transition.
Acknowledgements This work has been partly supported by the Ministry for Science and Technology of the Republic of Serbia, Grant no. 01E15. The authors wish to thank Prof. Ph. Colomban and Prof. A. Djordjevic´ for valuable suggestions and discussions, and Mrs. Lj. Novakovic´ for performing TGA measurements.
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