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Dielectric dispersion and Ramanspectroscopy in PbHfO3 single crystalsmodified by addition of Sn ionsI. Jankowska-Sumaraa, A. Majchrowskib, M. Ptakc & M. Mączkac

a Institute of Physics, Pedagogical University of Cracow, Kraków,Polandb Institute of Applied Physics, Military University of Technology,Warszawa, Polandc Institute of Low Temperature and Structure Research, PolishAcademy of Sciences, Wrocław, PolandPublished online: 15 Jan 2015.

To cite this article: I. Jankowska-Sumara, A. Majchrowski, M. Ptak & M. Mączka (2015): Dielectricdispersion and Raman spectroscopy in PbHfO3 single crystals modified by addition of Sn ions, PhaseTransitions: A Multinational Journal, DOI: 10.1080/01411594.2014.986730

To link to this article: http://dx.doi.org/10.1080/01411594.2014.986730

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Dielectric dispersion and Raman spectroscopy in PbHfO3 single

crystals modified by addition of Sn ions

I. Jankowska-Sumaraa*, A. Majchrowskib, M. Ptakc and M. Maczkac

aInstitute of Physics, Pedagogical University of Cracow, Krak�ow, Poland; bInstitute of AppliedPhysics, Military University of Technology, Warszawa, Poland; cInstitute of Low Temperature and

Structure Research, Polish Academy of Sciences, Wroc»aw, Poland

(Received 13 October 2014; accepted 5 November 2014)

Dielectric studies of a PbHf1¡xSnxO3 (with x D 0.025) single crystal undergoing twostructural phase transformations at about 426 and 474 K were performed in broadfrequency and temperature ranges. Phase transitions sequence and their nature wereestablished on the basis of differential scanning calorimetry measurements.Investigations of the complex dielectric permittivity in the frequency range 20 Hz to2 MHz revealed dipolar dielectric dispersion that exists before AFE2�PE phasetransition and is connected with the motion of domain walls. Analysis of temperatureevolution of the first-order Raman light scattering spectra confirmed the sequence ofphase transitions and gave evidence of locally broken cubic symmetry in the sampledue to polar clusters of lower symmetry.

Keywords: antiferroelectrics; phase transitions; dielectric dispersion; Ramanspectroscopy

1. Introduction

The tin-modified lead zirconate titanate Pb(Zr,Sn,Ti)O3 (PZST) has been intensively

investigated in recent years for the electric-field-induced antiferroelectric (AFE) to ferro-

electric (FE) phase transformation, which is also called phase switching.[1�3] The ability

to switch between these two phases is caused by the fact that the free energy of the anti-

ferroelectric crystal is comparable to that of the FE modification. In addition, a large

strain accompanying the phase switching makes PZST a favorable material for applica-

tions in the next generation of actuators. The increasing content of Ti ions stabilizes FE

phase, whereas increasing content of Sn stabilizes AFE arrangement in this solid solution.

The controlled modification of this delicate balance between the ferroelectricity and anti-

ferroelectricity was the motivation to study properties of simple solid solutions based on

PbZrO3:Sn (PZS). Our recent studies revealed that Sn ions are mainly responsible for the

switching mechanism.[4] The reason may lie in a change of the nature of chemical bonds

from the ionic�covalent to more covalent together with increasing tin content, which,

according to the theory presented in [5], promotes the appearance of the FE phase. In the

study it was shown that PZS solid solution undergoes two phase transitions, namely

AFE1�AFE2 between two antiferroelectric phases and from AFE2 to paraelectric (PE)

phase directly or via an intermediate�multicell�cubic phase in the case of high concen-

trations of Sn (AFE2�PE transition). While AFE1 phase is isostructural with pure

PbZrO3 single crystal,[6] the AFE2 phase has incommensurate modulations that could

*Corresponding author. Email: ijsumara@up.krakow.pl

� 2015 Taylor & Francis

Phase Transitions, 2015

http://dx.doi.org/10.1080/01411594.2014.986730

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occur due to competing interactions between transverse acoustic (TA) and softening

ferroelectric transverse optical (TO) modes.[7] This leads to a conclusion that AFE2

phase can be relatively easily triggered to the FE state by application of a relatively low

external electric field. The symmetry of the AFE2 phase is still a matter of debate since

designation of incommensurate structure needs more sophisticated methods to be used.

Additional help in understanding the phenomenon occurring in Pb(Zr,Sn)O3 solid solu-

tion can be achieved by extending the research to a related, but different and more com-

plex system of PbHfO3 modified with Sn ions. Sn ions promote the stabilization of an

intermediate phase that is believed to be ‘missed’ in pure PbZrO3.[8] In contrast to pure

PbZrO3, pure PbHfO3 undergoes two phase transformations between the PE and AFE2

phase at 485 K and between the AFE2 and AFE1 phases at 436 K.[9�12]

The objective of this study is thus to determine the influence of partial substitutions of

Hf4C ions by isovalent Sn4C ions in the perovskite-type crystalline structure of PbHfO3.

As a consequence of this substitution, a modification of the phase transition temperatures

occurring in PbHf1¡xSn x O3 (PHS) solid solution, caused by the cation replacement, is

reported. The characteristic parameters of phase transitions were determined by thermal

analysis (differential scanning calorimetry � DSC), optical studies (Raman scattering),

and dielectric spectroscopy, over wide temperature and frequency ranges.

2. Experimental setup

Lead hafnate single crystals doped with tin were grown by means of spontaneous crystalli-

zation from high-temperature solution. Pb3O4�B2O3 mixture was used as the solvent. The

starting composition of the melt used in our experiments was the same as in [11], only

PbO content was recounted to Pb3O4. The growth was carried out in a Pt crucible, covered

with a Pt lid diminishing evaporation of the solvent, which was placed in a one-zone

resistance furnace controlled with Eurotherm 2704 microprocessor regulator. Cooling or

heating rate of the melt could be changed in a very broad range, from 300 to 0.1 K/h,

depending on the phase of the crystallization process. In the initial phase of the process,

the furnace was heated up to 1453 K, measured under the bottom of the crucible and soak-

ing lasted 10 h to enable complete dissolving of PbHfO3 in the melt. In the next step, the

temperature was lowered at a rate of 2 K/h down to 1173 K. The remaining melt was

decanted and the furnace was cooled down to room temperature at a rate of 10 K/h. The

as-grown PbHfO3:Sn single crystals were etched in diluted acetic acid to remove

solidified solvent.

The chemical composition was verified from energy-dispersive spectrometry meas-

urements using the scanning electron microscope Hitachi S-4700. To describe the basic

physical properties of crystals under the study, the parameters like dielectric constant e,specific heat cp, domain structure and Raman spectra were measured and used to establish

the sequence of phase transitions and to determine the nature of these transitions. The

measurements were provided in wide temperature range from 150 to 550 K.

The electrical properties were provided in the frequency range from 20 Hz to 2 MHz

with the E4980A Precision LCR Meter. Single crystal in the form of a thin plate with

dimensions 1.6 £ 0.7 £ 0.3 mm3 was used for the measurement. The opposite faces of the

crystal were electroded using cold silver paste. During the measurements, weak electric

field amplitudes (Eo»2 V/cm) were applied to the sample to ensure the linear response. A

homemade furnace was used to heat the samples from room temperature to 620 K. Heating

and cooling rates were set to 0.2 K/min. The temperature protocols required by experiment

were managed using a Lake Shore Model 331 temperature controller.

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The specific heat measurements were made using the Netzsch DSC F3 Maia calorime-

ter in the temperature range from 120 to 820 K under argon atmosphere at the flow rate

30 mL/min. The specimen consisting of a piece of single crystal of average mass 20 mg

was placed in an alumina crucible. The data were collected both on heating and cooling

processes. The temperature changes were performed at a rate of 0.5 K/min.

The Raman spectra were measured using the Renishaw InVia Raman spectrometer

equipped with confocal DM 2500 Leica optical microscope, with long-working distance

objective (20x), a thermoelectrically cooled charge coupled device (CCD) as a detector

and a diode laser operating at 830 nm. This setup allowed us to measure Raman spectra

in the 20�1000 cm¡1 range. The resolution of spectra was set to 2 cm¡1. The sample’s

temperature was held by the high-precision Linkam TMS600 temperature stage combined

with the Raman imaging system.

3. Specific heat

In order to obtain the phase transition temperatures, DSC measurements were made in the

temperature range where phase transitions were expected. In DSC measurements, back-

ground was carefully subtracted off using interpolated values far below and far above the

apparent transition temperature. The anomalies observed as peaks on cp(T) dependences

have the size and shape that are typical for the first-order phase transitions (they are large

and narrowly centered). Figure 1 shows the temperature dependence of excess specific

heat in a PHS crystal on heating (red line) and on cooling (blue line)

While the high-temperature phase transition PE�AFE2 occurs at about 474 K with a

small thermal hysteresis of the order of 0.5 K, the difference in lower phase transition

temperatures on cooling and heating between two antiferroelectric phases AFE2�AFE1

is almost 3 K. The values of transition energies and excess entropy were calculated by the

Δ

ΔΔ

Δ

Δ

Figure 1. Plot of anomalous part of cp(T) � the excess specific heat near the phase transitions forPHS single crystal (red line � on heating, blue line � on cooling).

Phase Transitions 3

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integration of DCp (T) with

DE ¼Z

DcpðTÞdT ; DS

ZDcpT

dT (1)

The experimental curves integrated over the range of the phase transitions and results

were included in Figure 1. The transition energy at TC on heating was estimated to be

2.31 J/g (985 J/mol) and the entropy of transition to 0.011 J/(g�K) (eq. 4.6 J/(mol�K)).These values indicate that the phase transition is of order�disorder character. The transi-

tion energy of the lower transition is approximately 1 J/g (426 J/mol) and the excess

entropy is 0.004 J/(g�K) (eq. 1.7 J/(mol�K)), and these values indicate the displacive char-acter of the phase transition. Displacive character of AFE1�AFE2 phase transition is in

good agreement with our previous studies of Raman scattering for pure PbHfO3.[11]

4. Dielectric measurements

The e0(T)-dependences for PHS are shown in Figure 2. The results are basically similar to

those quoted for pure PbHfO3 (PH) in the literature [9�12] and consistent with DSC

measurements. For the PHS sample, dielectric measurements evidenced a slight shift (of

about 10 K down) and extension of the transient AFE2 phase toward lower temperatures

in comparison with pure PH. The PE�AFE2 phase transition takes place at 474 K with a

sharp divergence in e0 and only a negligible thermal hysteresis (about 0.5 K). A small but

clear additional anomaly close to PE�AFE2 transition in the range of just a few degrees

was observed as a local maximum with distinct thermal hysteresis. A dielectric dispersion

is associated with this anomaly. The plots of e0(f) for a few chosen temperatures are pre-

sented in Figure 3. The plots presented in Figure 3 show a single relaxation process which

is well described by the following Cole�Cole formula:

eðvÞ ¼ e1 þ e0 ¡ e11þ ðivtÞ1¡a

(2)

ε

Figure 2. Temperature dependences of the real part of e0(T) for different frequencies of PHS singlecrystal. The inset shows more precisely a temperature range in which dielectric dispersion exists.

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where a determines the width of the relaxation times around the mean value of t. The val-

ues of a (a »0.35�0.38) indicate a significantly polydispersive character of the observed

dispersion. The calculated values of relaxation times (10¡5 s) suggest that this dispersion

can be associated with the domain wall motion near the AFE2�PE (or IM) phase

transition.

It is also worth to mention that for pure PH single crystals, a well-saturated hysteresis

loop, however, with small spontaneous polarization (Ps »0.1 mC/cm2), was observed in

the region of an analogous anomaly.[13] No other dielectric dispersion in the frequency

range between 20 Hz and 2 MHz was observed in any other phase.

5. Symmetry discussion and Raman scattering measurements

At room temperature, PH is characterized [10] by orthorhombic symmetry with the space

group Pbam. The low-temperature symmetry for PHS�AFE1 phase is the same as in

pure PH crystals. The group-theoretical analysis by considering the site symmetry for the

G-point phonons of orthorhombic PHS indicates that the following 60 modes are Raman-

active among 120 zone-center phonon modes in the Pbam structure:[14] GRaman D16Ag C 16B1g C14B2g C14B3g D 60. The structure of the AFE2 phase was first reported

to be tetragonal.[8] In a recent study,[15,16] there were reported superstructure reflec-

tions, most straightforwardly understood as a sign of 5ap £ 5ap £ 8cp AFE super cell,

which is very different from the much more simple orthorhombic AFE1 structure, that is,p2ap £ 2

p2ap £ 2cp. Later several studies reported ambiguous and contradictive

results, tentatively due to the limitations of the powder diffraction that was used. For

example, Kupriyanov [15] reported that AFE2 phase is orthorhombic Cmm2 (or Bmm2)

similar to the orthorhombic phase of BaTiO3 below 278 K; however, this space group

possesses polar axis. He also reported that at 453 K, the orthorhombic structure changes

to a tetragonal one (P4mm). On the other hand, it is known that in PbZrO3 single crystal,

ε

εετ α

τ

Figure 3. Dielectric dispersion at chosen temperatures of the range indicated in Figure 2. The dot-ted curves represent the best fits to the Cole�Cole relation (2). The inset shows the temperaturedependence of the relaxation time t.

Phase Transitions 5

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Sn ions promote appearance of incommensurate phase below TC, so the symmetry of the

AFE2 phase in PHS single crystal can also be disturbed by the incommensurability.[4]

Thus, the exact symmetry of the AFE2 phase is still controversial and the exact number

of vibrational modes as yet remains indeterminate. High-temperature phase of both PH

and PHS is cubic in macroscopic sense with Pm3m space group and 15 vibrational modes:

Gvib ¼ 4Fð3Þ1u CF

ð3Þ2u . However, according to the selection rules, none of these modes are

Raman-active. Nevertheless, it was reported that micro-polar regions may arise in para-

electric phase, that can locally break cubic symmetry and induce Raman spectra for-

bidden by selection rules.[4]

Raman scattering data of the orthorhombic PHS were obtained by employing back-

scattering geometries at temperature range from room temperature to 535 K, and the

results through the phase transitions are presented in Figure 4. In the same figure, the

results of fitting of the room-temperature Raman spectrum using a sum of Lorentzian

bands are presented.

The main changes in the Raman scattering spectra in the vicinity of the phase transi-

tion between AFE1 and AFE2 phases are as follows:

� The two highest intensity low-frequency lines at 41 and 43 cm¡1 shift to lower fre-

quency and coalescence into one band.

� A mode at about 88 cm¡1 is activated.

� The modes at 122 and 264 cm¡1 soften.

A few degrees before the phase transition from AFE2 to PE spectral line changes its

shape forming a diffused broad shoulder. This change can be connected with the transi-

tion to the intermediate (IM) tetragonal FE phase as reported by Kupriyanov [15], or

some other kind of intermediate polar phase connected with the dipolar dispersion

observed in dielectric measurements.

Figure 4. Evolution of the Raman spectra for PHS in the AFE1, AFE2, IM and PE phases. The fit-ting of Lorentzians (color lines) of the experimental spectrum measured at ambient temperature isalso included in the plot.

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Above the phase transition to PE phase, a broad band extending up to 400 cm¡1

appears. The intensity of this structure is temperature-dependent and remains in the PE

phase at least up to 535 K. The fact that Raman spectra above TC do not disappear sug-

gests the existence of the stable (static) regions of non-centrosymmetric (polar) properties

at temperatures far above TC that break selection rules and induce first-order Raman scat-

tering. In a number of recent reports,[e.g., 17,18] it was theoretically confirmed that the

existence of local instabilities in the PE phase is a common feature for all perovskite crys-

tals, both ferroelectrics and antiferroelectrics, and is an intrinsic effect based on the short

wavelength structural instability that is related to the long wavelength polar soft mode

and driven by the temperature-dependent fluctuating dipole moments. A similar situation

was already observed in PZS single crystals. More detailed analysis of Raman spectra in

PHS solid solution concerning scattering in the PE phase will be provided elsewhere.

6. Conclusions

This paper reports the DSC, dielectric and Raman scattering data pertaining to PbHfO3

single crystals modified with small amount of Sn ions. The aim of this study was to deter-

mine the modifications in the phase transition sequence by the incorporation of foreign

ions (Sn4C) replacing Hf4C in the lattice. In our recent study performed for the similar

antiferroelectric compound PbZrO3, it was shown that the role of Sn ions was to stabilize

antiferroelectric phase below TC. The same was expected to obtain for PHS although

some publications [15,16] reported the possibility of transient FE phase occurrence in

pure PbHfO3. For this reason we added a small amount of Sn ions (2.5 mol%). Results

presented in this paper clearly show that the phase transitions are shifted by about 10 K

down with respect to those observed for pure PbHfO3. On the basis of Raman scattering

measurements, it was found that the possibility of the transient FE phase appearance is

limited to a very thin temperature range just below TC (less than 5 K), while in the paper

by Kupriyanov [15], this range was established to be of about 40 K.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This research was financially supported by the National Science Centre within the project number1955/B/H03/2011/40, and co-financed by the European Regional Development Fund under theInfrastructure and Environment Programme [grant number UDA-POIS.13.01-023/09-00].

References

[1] Pan W, Zhang Q, Bahala A, Cross LE. Field-forced antiferroelectric-to-ferroelectric switchingin modified lead zirconate titanate stannate ceramics. J Am Ceram Soc. 1989;72:571�578.

[2] Fuith A, Kabelka H, Birks E, Shebanovs L, Sternberg A. Thermodynamic properties at thephase transition of Pb(Zr,Sn,Ti)O3 solid solutions. Ferroelectrics. 2000;237:153�159.

[3] Wang L, Li Q, Xue L, Liang X. Effect of Ti4C:Sn4C ratio on the phase transition and electricproperties of PLZST antiferroelectric ceramics. J Mater Sci. 2007;42:7397�7401.

[4] Jankowska-Sumara I. Antiferroelectric phase transitions in single crystals PbZrO3:Sn re-vis-ited. Phase Transit. 2014;87:687�728.

[5] Thomann H. A covalency model of ferroic phase transitions in perovskites. Ferroelectrics.1987;73:183�199.

Phase Transitions 7

Dow

nloa

ded

by [

I. J

anko

wsk

a-Su

mar

a] a

t 12:

28 2

0 Ja

nuar

y 20

15

[6] Glazer AM, Roleder K, Dec J. Structure and disorder in single-crystal lead zirconate, PbZrO3.Acta Cryst. 1993;B49:846�852.

[7] Viehland D, Forst D, Xu Z, and Li JF. Incommensurately modulated polar structures in anti-ferroelectric Sn-modified lead zirconate titanate: the modulated structure and its influences onelectrically induced polarizations and strains. J Am Ceram Soc. 1995;78:2101�2112.

[8] Tagantsev AK, Vaideeswaran K, Vakhrushev SB, Filimonov AV, Burkovsky RG, ShaganovA, Andronikova D, Rudskoy AI, Baron AQR, Uchiyama H, Chernyshov D, Bosak A, Ujma Z,Roleder K, Majchrowski A, Ko J-H, Setter N. The origin of antiferroelectricity in PbZrO3.Nat Commun. 2013;4:2229.

[9] Shirane G and Pepinsky R. Phase transitions in antiferroelectric PbHfO3. Phys Rev.1953;91:812�815.

[10] Corker DL, Glazer AM, Kaminsky RW, Whatmore RW, Dec J, Roleder K. Investigation intothe crystal structure of the perovskite lead hafnate, PbHfO3. Acta Cryst. 1998;B54:18�28.

[11] Jankowska-Sumara I, Kugel GE, Roleder K and Dec J. Raman scattering in pure and Ti-dopedPbHfO3 antiferroelectric crystals. J Phys Condens Matter. 1995;7:3957�3972.

[12] Kwapuli�nski J, Pawe»czyk M, Dec J. On the Pb thermal vibrations in PbHfO3. J Phys CondensMatter. 1994;6:4655�4660.

[13] Bussmann-Holder A, Ko J-H, Majchrowski A, Soszy�nski A, Roleder K. Phase transitions andinterrelated instabilities in PbHfO3 single crystals, submitted to J Phys Condens Matter.

[14] Ostapchuk T, Petzelt J, Zelezny V, Kamba S, Bovtun V, Porokhonskyy V, Pashkin A, KuzelP, Glinchuk MD, Bykov IP, Gorshunov B, Dressel M. Polar phonons and central mode in anti-ferroelectric PbZrO3 ceramics. J Phys Condens Matter. 2001;13:2677�2689.

[15] Kupriyanov MF, Petrovich EV, Dutova EV, Kabirovet YuV. Sequence of phase transitions inPbHfO3. Crystallogr Rep. 2012;57:205�207.

[16] Fujishita H, Ishikawa Y, Ogawaguchi A, Kato K, Nishibori E, Takata M, Skata M. A study ofstructures and order parameters in antiferroelectric PbHfO3 by synchrotron radiation. J PhysSoc Jpn. 2005;74:2743�2747.

[17] Ko J-H, G�orny M, Majchrowski A, Roleder K, Bussmann-Holder A. Mode softening, precur-sor phenomena, and intermediate phases in PbZrO3. Phys Rev B. 2013;87:184110.

[18] Bussmann-Holder A, Ko J-H, Majchrowski A, G�orny M, Roleder K. Precursor dynamics,incipient ferroelectricity and huge anharmonicity in antiferroelectric lead zirconate PbZrO3. JPhys Condens Matter. 2013;25:212202.

8 I. Jankowska-Sumara et al.

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