Poland b Laboratoire de Physique, University of Bourgogne, Dijon, France c MOPS-CLOES, University of Metz and Supelec,

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1 This article was downloaded by: [Irena Sumara] On: 02 April 2013, At: 14:12 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Ferroelectrics Publication details, including instructions for authors and subscription information: Dielectric spectroscopy of the antiferroelectric PbHfO 3 K. Roleder a, M. Maglione b, M. D. Fontana c, I. Jankowska-Sumara d, G. E. Kugel c & J. Dec a a Institute of Physics, University of Silesia, Katowice, Poland b Laboratoire de Physique, University of Bourgogne, Dijon, France c MOPS-CLOES, University of Metz and Supelec, Metz, France d Cracov Pedagogical University, Kraków, Poland Version of record first published: 09 Mar To cite this article: K. Roleder, M. Maglione, M. D. Fontana, I. Jankowska-Sumara, G. E. Kugel & J. Dec (2000): Dielectric spectroscopy of the antiferroelectric PbHfO 3, Ferroelectrics, 238:1, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be

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3 Ferroekctrics. 2000, Vol. 238, pp Repnnts available directly from the publisher Photmopying permitted by license only OPA (Overseas Publisher5 Association) N.V Published by license under the Gordon and Breach Science Publishers imprint. Printed in Malaysia Dielectric Spectroscopy of the Antiferroelectric PbHf03 K. ROLEDERa, M. MAGLIONEb, M. D. FONTANA', I. JANKOWSKA-SUMARAd, G. E. KUGEL' and J. DECa ahtitute of Physics, University of Silesia, Katowice, Poland, blaboratoire de Physique, University of Bourgogne, Dijon, France, cmops-cloes, University of Metz and Supelec, Metz, France and 'Cracov Pedagogical University, Krakdw, Poland (Received July 12, 1999) Dielectric studies of a PbHfO, (lead hafnate) single crystal undergoing two structural phase transformations at about 163 C (436 K) and 209 C (482 K) were performed in broad frequency and temperature range. Special attention was paid to the paraelectric phase. Investigations of the complex dielectric permittivity in the frequency range 10 Hz to 10 MHz revealed two relaxations of polydispersive and nearly monodispersive (Debye-like) character connected with space charge behaviour. In the frequency range of Hz a polar relaxation mode has been found and related to a disorder in the Pb sublattice. Temperature dependence of the dielectric step of this relaxation corresponds to a sharp anomaly of the &(T) function at the phase transition from the paraelectric to antiferroelectric state. Analysis of temperature evolution of the first order Raman light scattering spectra showed existence of a soft line at this transformation. Crossover of the displacive and order-disorder type of the transformation mechanism is thus postulated. Keywords: antiferroelectric; phase transitions; relaxations IN'I'KO1)UC'I'ION Lead hafnate PbHfO3 single crystal of the perovskite structure undergoes two structural transformations"l: i) at about 163 C from the antiferroelectric A1 (the orthorhombic Pbam12') to the antiferroelectric A2 phase (the orthorhombic phase with still not established space group) and ii) at about 209 C from the antiferroelectric A2 to the paraelectric P phase (cubic Pm3m). In Figure I a temperature dependence of the real part of dielectric permittivity E' measured at lmhz is presented. Distinct jumps and very distinct anomalies are seen on the c '(7) dependence at temperatures corresponding to these main structural transformations. [703]/139

4 140/[704] K. ROLEDER et al. :I, ' +7+c ;,., TWI FIGURE 1 Temperature dependence of the dielectric permittivity. The Al-Az transition possesses distinct thermal hysteresis. In contrast to literature data additional anomalies were observed close to the T, in form of local maximum on cooling and heating (with thermal hysteresis) on the ~'(2') runs (inset in Figure 1). Symptoms of ferroelectric behaviour in this temperature range, just below T,, have been observed. DIELECTRIC RESPONSE IN FREQUENCY RANGE 10'-106Hz Dielectric relaxation investigations were performed using a Hewlett- Packard 4192A Impedance Anaiyser in the frequency range of 10 Hz- I MHz and temperature range C. Real and imaginary parts E' and E" were calculated as E'= ClC, and E"= GhCO where CO is empty cell capacitance. Capacitance C and conduction G were recorded directly from the analyser. In each phase a low-frequency relaxation of Debye character has been observed with characteristic maximum of E"V) function clearly seen in the antiferroelectric phases (Figure 2). At 161'C the relaxation frequency is of the order of 20 Hz. This relaxation -which appears in samples rejuvenated at 5W C-moves with increasing temperature to higher frequency range (about IOOHz at 201OC). Relaxation times well obey the Arrhenius law and the dielectric step - in the order of loo00 - is independent on temperature up to about 300 C. This relaxation process of nearly monodispersive character can come from an accumulation of charges at the vicinity of

5 DIELECTRIC SPECTROSCOPY [705]/141 8OOO- :< E >t ; E.., charges at the electrodes and thermal diffusion tends to oppose this accumulation. That is why the dielectric step related to the relaxation decreases with temperature just above 300 C. With increasing temperature the E values go to huge values for the lowest frequencies and another poiydispersive Debye-like relaxation process appears. Maximum of the ~ (n first shifts up to about 10 Hz at 340 C and then turns back to lower frequencies. This effect is related with response of mobile space charges in the bulk of sample to external alternating electric fields. Temperature evolution of both relaxations is shown in Figure 3. E I 3- \ Paraelectric Paraelectric phase P I relamban seen in he mti!ermei&ic phases FIGURE 3 I 10 lo2 10. loo 10 lo2 103 [khz1 Temperature e\ evolutions folutions of low-frequency relaxations However some deviation in the shape of Debye-like ~ (0 function, i.e. a local maximum at the lowest frequencies (below 6 Hz), has been observed. This unexpected maximum seems to indicate electrochemical

6 142/[706] K. ROLEDER et al. processes taking place in the perovskite materials which are influenced by thermal diffusion in particular at temperatures above 300 C. DIE1411:CTRIC RESPONSE IN FREQUENCY RANGE Hz The measurements of the dielectric dispersion in the radio frequency range were performed in temperature range 2O-3OO0C using a Hewlett- Paccard 4191A impedance analyser. As in low frequency range in this layout the sample was considered to be a lossy capacitor with lumpedcircuit capacitance C and conductance G. The impedance characteristics were converted directly into the real part E and imaginary E part of the dielectric permittivity E*=E -ie. In Figure 4a is shown an example of the recorded frequency dispersion of the E and E in the P phase and in the neighbourhood of the phase transition point T,.. These runs are characteristic for dipolar dispersion E U) and absorption ~ cf) in an external field. Appearance of the relaxation above T, is also proved by the ~ (7) dependence measured at 4 MHz: a distinct anomaly of the dielectric losses has been found in between the OoC temperatures (Figure4b). This type of relaxation was observed not only in the P phase but also in the A, and A2 phases of the polydomain single crystal n - b px&ecinc pase E ED mOm ax, ZM 2a frequency (MHz) Tpcl Figure 4 (a) Example of relaxation with a fit to the equation (1) and (b) E (T) function measured at 4MHz around Tc point (P-A2 phase transformation). The E V; r) and E V; T) dependencies were analysed taking into account the Cole-Cole behaviour given by the complex relation:

7 DIELECTRIC SPECTROSCOPY [707]/143 &*(0)=&inf + (1 + A& where AE is the dielectric step and ~~~f the permittivity being responsible for all processes at frequencies higher than the mechanism under consideration. r= f;' is the mean relaxation time and w is the angular frequency (w = 25cf). The parameter a is a measure of a departure from the ideal Debye response. The obtained &*(f; r) dependencies were satisfactorily fitted to the Cole-Cole relation and the relaxation parameters were calculated. In the whole temperature range the parameter a is small and does not exceed value Temperature dependence of the relaxation frequency fr is shown in Figure 5. Attention should be paid to a distinct difference in values of relaxation frequencies on cooling and heating above T,. This nearly monodispersive dipolar-like relaxation may indicate an order-disorder mechanism of the antiferroelectric transformation in the lead hafnate. However thef,t) dependence in the paraelectric phase is not similar to those monotonic dependencies found in ferroelectric materials and antiferroelectric lead zirconate.i6] Instead, unexpected different fr( T) runs on cooling and heating have been observed. 0 TPCI FIGURE 5 Temperature dependence of relaxation frequency fr in the paraelectric phase (lines are guides for the eyes). Temperature dependence of the dielectric step A& obtained from the fits of the experimental data to the Cole-Cole relation is presented in Figure 6. It clearly resembles the run of the low frequency dielectric permittivity around T, for the PbHfO,. It is noteworthy to point out that not only the shape of AIE(7J but also the values of the AE step well correspond to &'(TI function recorded at frequency 106Hz (Figure I).

8 144/[708] K. ROLEDER el al. This means that the relaxational process observed is dominant in the low frequency dielectric behaviour within the whole investigated temperature range. In the P phase the value of the is rather high and depends on temperature with local anomaly on &,,I(T) function corresponding to minimum of thefr(('l) function detected on cooling in the same temperature range (see inset in Figure 6). Contribution of lattice phonons to the.clnf value can be evaluated through the Lyddane-Sachs-Teller (LST) relation."] From the temperature independent TO and LO phonons frequencies characterising the lead perovskite crystals in the cubic phase, the phonon (and electronic) contribution was estimated to have value of the order of 30. In fact values of the found in the paraelectric phase are much higher. This means that the E,,,~ obtained from the dielectric measurements cannot correspond only to the lattice contribution and there are additional phenomena between the relaxational process appearing in the range of lo6-10'hz and the lowest frequency phonons which would have a contribution to low frequency dielectric response of the PbHf03. A n, % Tc I.,..., I,.,. I V0C1 FIGURE 6 Dielectric step for relaxation mode as a function of temperature. Inset shows temperature dependence of the ~,,,f above T, (on cooling). As a consequence the relaxation in lo6 - lo9 Hz is not responsible for the whole temperature behaviour of the dielectric permittivity in the paraelectric phase of the lead hafnate. The mechanism of the P-A2 phase transition seems to be mostly of order-disorder type and could be related mainly, as in the case of PbZrO3,"' to a disorder in the Pb sublattice.

9 DIELECTRIC SPECTROSCOPY [709]/145 DIELECTRIC RESPONSE IN THE FREQUENCY RANGE 101'-1012 Hz As in many lead perovskites the first order Raman spectra in the paraelectric phase, in form of a broad shoulder below 100cm", was recorded up to very high temperatures (Figure7a).[*l It can be interpreted as a defect-induced phonon line which clearly softens approaching the T, from the high temperature side. The experimentally recorded spectrum was analysed in terms of damped-harmonic oscillator model using the lorentzian-like function for the phonon line. For the quasielastic part of spectrum the Debye-like function, originating from a relaxational behaviour, was used. Assuming that the Raman spectra consist of the sum of one relaxator and several oscillators the expression for scattered intensity was adjusted to the following function: f where Si, u,, are respectively the oscillator strength, frequency and damping of ith phonon. S, and describe the strength and relaxation rate of a relaxator. The terms n(w) + 1 and n(o) are the Bose - Einstein 2? d I 3! 1'"" 5000 Paraelectric phare b) C irequency(cm-') FIGURE 7 a) Evolution of Raman spectra in the P phase which were fitted to equation (2) and b) temperature dependence of frequency of soft line w, (with fitted curve) and ratio of damping 35 to frequency o~.

10 146/[710] K. ROLEDER et al, factors and correspond to the Stokes and anti-stokes scattering, respectively. From this equation the frequency, damping and intensity for a mode w, could be determined as function of temperature. Calculations show that this mode clearly softens and its temperature dependence obeys the relation (us)-' = [A(T- TO)].' + (a).' 19' with a= 198 cm-' and To lying near -200 C i.e. far below T,. (Figurc 7b). At T, mainly the parameters of the relaxation mode appearing in the range of lo6- lo9 Hz change distinctly. This co-existence of the relaxation mode and soft lattice phonon leads to conclusion that the structural transformation between the paraelectric and antiferroelectric phases in the lead hafnate can be described as the crossover of the displacive and order-disorder type of phase transition. ACKNOWLEDGEMENT. This paper was partially supported by Polish Committee for Scientific Research (KBN). References [l] J. Kwapulihski, M. Pawekzyk, J. Dec, J. Phys.: CondensedMatter; 6,4655, (1994). [2] D. L. Corker, A. M. Glazer, W. Kaminsky, R. W. Whatmore, J. Dec, K. Roleder, Acta Cryst., B54, 18, (1998). [3] R. Coelho, Physics ofdielectrics,elsevier Scientific Publishing Company, (1979). [4] A. K. Jonsher, Universal relaxation law, Chelsea Dielectric Press, (1996). [5] 1. Jankowska-Sumara, K. Roleder, J. Dec, S. Miga, Ferroelectrics 193, 149, (1997). [6] K. Roleder, M. Maglione, M. D. Fontana, J. Dec, J. Phys.: Condensed Mutrer; 8, 10669, (1996). 171 R. H. Lyddane, R. G. Sachs, E.Teller, Phys. Rev., 59,673 (1941). [8] I. Jankowska-Sumara, G. E. Kugel, K. Roleder, J. Dec J. Phys.: Condensed Mutter, 7, 3957, (1995). 191 J. Petzelt, G. V. Kozlov, A. A. Volkov, Ferroelectrics, 73, 101, (1987).