Ferroelectrics Infrared spectroscopy of lead zirconate ...

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Infrared spectroscopy of lead zirconate single crystal, ceramics and films a

a

Tetyana Ostapchuk , Jan Petzelt , Vladimir a

a

b

Železný , Stanislav Kamba , Barbara malič , b

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Maria Kosec , Laila Čakare , Krystian Roleder c

& Jan Dec

c

a

Institute of Physics, Czech Academy of Science , Na Slovance 2, 18221, Prague 8, Czech Republic b

Jozef Stefan Institute, Jamova 39 , 1001, Ljubljana, Slovenia c

Institute of Physics, Silesian University , Uniwersytecka 4, l40-007, Katowice, Poland Published online: 26 Oct 2011.

To cite this article: Tetyana Ostapchuk , Jan Petzelt , Vladimir Železný , Stanislav Kamba , Barbara malič , Maria Kosec , Laila Čakare , Krystian Roleder & Jan Dec (2000) Infrared spectroscopy of lead zirconate single crystal, ceramics and films, Ferroelectrics, 239:1, 109-115, DOI: 10.1080/00150190008213312 To link to this article: http://dx.doi.org/10.1080/00150190008213312

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Ferroelectrics, 2000, Vol. 239, pp. 109-115 Reprints available directly from the publisher Photocopying permitted by license only

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Infrared Spectroscopy of Lead Zirconate Single Crystal, Ceramics and Films TETYANA OSTAPCHUK~,JAN PETZELT~,VLADIMIR AELEZN+ a, STANISLAV KAMBA~,BARBARA MALI^^, MARIA KOSEC~, LAILA eAKAREb, KRYSTIAN ROLEDER' and JAN DECC 'Institute of Physics, Czech Academy of Science, Na Slovance 2, 18221 Prague 8, Czech Republic, bJozef Stefan Institute, Jamova 39, 1001 Ljubljana, Slovenia; and 'Institute of Physics, Silesian University, Uniwersytecka 4, 4G007 Katowice, Poland (Received July 12, 1999)

Infrared reflection and transmission measurements on PbZrO, single crystal, ceramic and thin films in the 1&600 K temperature range enabled us to determine the dielectric response function and discuss the dynamic origin of the antiferroelectric transition. The strong dielectric anomaly is caused by an overdamped mode in the 10 cm-I range which is assigned to anharmonic hopping of Pb atoms whereas the phonon modes contribute only by - 100 to the relative permittivity value. Thin films show somewhat weaker and more smeared phonon spectra and lower permittivity values. Keywords: antiferroelectric transition; polar phonons; PbZrO, crystal; PbZrO3 ceramics; PbZrO, films

INTRODUCTION PbZr03 (PZ), as the end member of the famous PZT system, is the first known antiferroelectric ' I ' (space group Pbam, u:,, 2 4 ) "I with a first order phase transition into a simple cubic perovskite structure at Tc c 508 K. In some samples a narrow intermediate ferroelectric phase was detected close below Tc 13.41 The transition at Tc is revealed by a strong

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TETYANA OSTAPCHUK et al.

anomaly in permittivity which in the para-phase of single crystals obeys a Curie-Weiss law ~ ( 0=)C/T-To with C = 1 5 lo5 K and To r 472 K and 300 below Tc Is] The drops sharply from almost 5000 down to paraelectric dielectric anomaly undergoes a strong relaxational dispersion in the 1-100 MHZ range, which indicates an order-disorder mechanism of the transition 15' This is also supported by Raman scattering data 14s61, where no strong mode softening was observed below Tc Above Tc the first-order Raman spectra are forbidden by symmetry so that no clear conclusion about the paraelectric phonon softening can be drawn. On the other hand, earlier microwave studies at 37 GHz on ceramics show still a very pronounced dielectric anomaly of the Curie-Weiss type above Tc l7], which clearly show that the main critical dynamics occurs above this frequency As sufficiently large single crystals are now available and previous far IR data are available only at room temperature on ceramics, we decided to carry out the first IR measurements on single crystals as well as on recently available PZ thin films Iyl and bulk ceramics for comparison to clarify the dynamic origin of the phase transition.

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EXPERIMENT Polydomain flux-grown single crystals of the shape of thin planeparallel plates were used for reflection as well as for transmission IR measurements with a Bruker IFS 1 13v Fourier transform spectrometer in the temperature range 10-600 K and frequency range 20-650 cm" Thin films of various thicknesses (690-1630 nm) were deposited by the sol-gel method 19' directly on sapphire substrate The FIR transmission spectra (20-250 cm-') were taken with the same spectrometer in the temperature range 300-580 K Reflectivity measurements on a polished dense ceramics were also performed in the temperature range 10-580 K. Preliminary room temperature transmission measurements in the submillimetre range 10-17 cm" were performed also using the monochromatic backward-wave-oscillator spectrometer

RESULTS AND EVALUATION In Figure 1 we present the PZ single-crystal reflection spectra at selected temperatures Our sample was not completely opaque as indicated by the transmission spectrum at 10 K This influences the

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INFRARED SPECTROSCOPY OF LEAD ZIRCONATE

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500

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FIGURE 1. FIR reflectivity spectra of PbZr03 crystal are given at selected temperatures under and above antiferroelectric phase transition. Transmittance spectrum at 10 K is given for the demonstration of the sample transparency. reflectance somewhat at the low-frequency and around 150 and 450 cm-'. This was taken into account when fitting our spectra The reflectivity fits using a generalized 4-parameter oscillator model for T 2 300 K are also shown in Figure 1. At room temperature, the permittivity BWO submillimetre transmission measurement has indicated an additional broad absorption peak near 10 cm", which was included into our fits In contributes by about A E ~:50 to the relative low-frequency permittivity so that our static value is EO 160 At higher temperatures, but below Tc, the parameters of this overdamped mode

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TETYANA OSTAPCHUK et al.

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FIGURE 2. Real and imaginary parts of dielectric function at selected temperatures, obtained from the fit of TR and submillimetre data. were taken constant, but the lowest two phonon modes show partial softening. This is seen from Figure 2, which shows the dielectric spectra calculated from our fit, as well as from Figure 3, where all the transverse mode frequencies are plotted as a hnction of temperature Above Tc, our reflectivity spectra show an increase at the lowfrequency end, which indicates a strong overdamped mode below our spectral region whose parameters cannot be unambiguously determined

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60 40

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FIGURE 3. Temperature dependence of transverse optic modes in bulk PbZrOs crystal

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Wavenumber (cm")

FIGURE 4. Transmission ratio spectra of thin film plus substrate sample and bare substrate. Thickness of the film is 1630 nm.

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Therefore we have adjusted its strength to fit the microwave permittivity data I' This makes our fit unambiguous (see Figures 2 and 3) Our reflectivity measurements on PZ ceramics show very similar spectra, slightly more smeared at the low-frequency end They are practically identical to those recently measured with PLZT 2/95/5 ceramics ""I. In Figure 4 we show the transmission spectra of a thin PZ film (d=1630 nm) obtained by dividing the film plus substrate transmission by the bare substrate transmission. Also these spectra are very similar to those of PLZT 2/95/5, recently published "".

DISCUSSION As there is no essential difference between the IR spectra of thin single crystal and bulk ceramics, we shall further compare only the spectra of single crystal and thin films. The factor-group analysis in the antiferroelectric phase yields ' 'I rv,hr= 16A6+12 A , + 1 6 B ~ , + 1 2 B i , + 1 4 B ~ , + I 8 B ~ , +14B3,+18B~u So, as much as 45 modes can be expected in the 1R spectra of a multidomain or polycrystalline PZ sample. Al low temperatures we have revealed 26 modes in a satisfactory agreement with the prediction. The spectra in thin films (Figure 4) reveal only the stronger modes with much higher effective damping, compared to that in bulk sample The low-frequency permittivity evaluated from the fit to our thin-film data is E" = 70 at room temperature which is appreciably lower than in bulk samples. The bulk Sam le value is definitely higher than that previously I . ' reported at 500 MHz P S, indicating that the relaxation revealed in the I00 MHz range "I, might be somewhat weaker. Let us discuss the low-frequency dynamics in the paraelectric phase From Figure 3 it follows that the lowest polar phonon mode near 35 cm" shows a slight hardening on increasing the temperature above Tc. On the other hand, no appreciable hardening of this mode is seen from thin-film spectra, as also discussed in the case of PLZT 2/95/5 ( ' ( ' . i i l . Even if the situation is not completely clear, this phonon mode together with all higher frequency modes contribute only about 100 to the permittivity value, almost independently of temperature The whole microwave dielectric anomaly is therefore caused by the overdamped excitation in the 10 cm-' range, which confirms the order-disorder origin of the transition suggested earlier ' I The situation is very similar to that observed in PLZT 2/95/5 "I' whose chemical composition differs only slightly from PZ. Below Tc this excitation dies out as evidenced by the '

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strong decrease in the microwave permittivity, but some traces of its weak presence remain down to room temperature, as evidenced from our submillimetre data. The microscopic origin of the overdamped excitation is most probably the dynamic disorder in lead ['I. It was previously suggested also as the origin of the 10' Hz relaxation 15]. The situation reminds very much that in PbScl!zTal,z03, where also two distinct dispersion regions in 10' and 10" Hz range were recently revealed in the paraelectric phase The higher-frequency one was assigned to local Pb hopping and the lower-frequency one to collective polar clusters dynamics. The existence of such clusters in PZ was recently revealed also from TEM studies (I4], therefore we suggest a similar assignment for PZ. More detailed submillimetre and microwave studies are in progress to confirm this interpretation in a more quantitative way. Acknowledgements

The authors thanks B Gorshunov for performing the RT sub-mm measurements The work was supported by the Grant agency of the Czech Republic (project No 202/98/1282), Grant agency of the AS CR (projects Nos A1010918 and A1010828) and Ministry of Education of the Czech Republic (project COST OC 5 14 10)

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