P–T phase diagram of PbZr0.52Ti0.48O3 (PZT)

Solid State Sciences 5 (2003) 451–457 www.elsevier.com/locate/ssscie

P –T phase diagram of PbZr0.52Ti0.48 O3 (PZT) J. Rouquette a , J. Haines a,∗ , V. Bornand a , M. Pintard a , Ph. Papet a , B. Bonnet b , F.A. Gorelli c a Laboratoire de physico-chimie de la matière condensée, UMR CNRS 5617, Université Montpellier II sciences et techniques du Languedoc,

cc 003, place Eugène Bataillon, 34095 Montpellier cedex 5, France b Laboratoire des agrégats moléculaires et matériaux inorganiques, UMR CNRS 5072, Université Montpellier II sciences et techniques du Languedoc,

cc 015, place Eugène Bataillon, 34095 Montpellier cedex 5, France c LENS and INFM, Via Nello Carrara 1, 50019 Sesto Fiorentino (Florence), Italy

Received 10 September 2002; received in revised form 18 November 2002; accepted 25 November 2002

Abstract The important piezoelectric material, lead zirconate titanate perovskite PbZr0.52 Ti0.48 O3 (PZT) was investigated as a function of pressure and temperature by Raman spectroscopy. At ambient pressure, the transition between the low-temperature and high-temperature ferroelectric monoclinic phases occurs near 210 K and is followed by transformation to the tetragonal phase at close to 305 K. The critical pressure for the transition to the disordered polar cubic phase increases with decreasing temperature from 5 GPa at 298 K to 9 GPa at 44 K. The ferroelectric monoclinic phases thus have an extended stability range at low temperatures and high pressure. A strong Raman spectrum is obtained for the cubic phase at high pressure over the temperature range between 44 and 298 K indicating the presence of polar nanodomains. A P –T phase diagram of this material is proposed based on the present results.  2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Lead zirconate titanate; PZT; Phase diagram; Raman spectroscopy; Piezoelectricity; Ferroelectricity

1. Introduction Lead-zirconate-titanate PbZr1−x Tix O3 (PZT) solid solutions are the basis of the most-commonly used piezoelectric ceramic materials. The strong piezoelectric response of this material was considered for over 40 years to arise from the coexistence of adjacent ferroelectric tetragonal (FT -space 1 ) and rhombohedral phases (F -space group P 4mm-C4v R 5 group R3m-C3v ) at what is termed the “morphotropic phase boundary” (MPB) for values of x of close to 0.48 [1]. Very recently, a series of new phases have been discovered in PZT, which require both the phase diagram to be updated and the understanding or the origin of the strong piezoelectric response to be revised. The discovery of a first monoclinic HT -space group Cm-C 3 ) near the MPB can provide phase (FM s an explanation for this piezoelectric response as the polar axis in this form can lie anywhere between the pseudocubic [111] and [001] directions [2–5]. The monoclinic phase transforms to the tetragonal form at close to room temperature for x = 0.48 [5]. A transformation to the cubic * Corresponding author.

E-mail addresses: [email protected] (J. Haines), [email protected] (V. Bornand).

1 ¯ paraelectric phase (PC -space group Pm3m-O h ) is observed LT at 667 K [6–8]. A second monoclinic phase [9–11] (FM 4 space group Cc-Cs ) with a doubled c lattice parameter is found below 210 K and is obtained via an antiferrodistortive transition involving octahedral tilting. In spite of the known sensitivity of the dielectric and piezoelectric properties of PZT ceramics, which are mainly governed by domain wall motion, to stress induced by external elastic or electric fields [12–17], little attention has been paid to the effect of pressure on the phase diagram of PbZr0.52Ti0.48 O3 . Early high-pressure, hightemperature dielectric measurements [18,19] up to 0.8 GPa indicate that the dT /dP slope of the FT –PC transition is negative. X-ray diffraction measurements indicate that transformation to a phase of cubic symmetry occurs at 5 GPa at room temperature; however a strong Raman spectrum is observed for this phase [20]. This result is in sharp contrast with the very weak first-order Raman spectrum, which may be expected for the cubic paraelectric phase. Even though weak bands may be observed for the paraelectric phases of solid solutions such as PZT due to partial breakdown of the Raman selection rules arising from a loss of translational symmetry as a result of Zr–Ti positional disorder, the strong observed spectrum of this

1293-2558/03/$ – see front matter  2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. doi:10.1016/S1293-2558(03)00030-X

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phase is similar to those of ferroelectric relaxors [21–27] and arises from the presence of polar nanoregions [28–30]. This second cubic form, tentatively termed “FC ”, appears to have ¯ structure, but is characterized a long-range, average Pm3m by static, symmetry-breaking disorder on a local level due to off-center cation displacements. The present study of PbZr0.52Ti0.48O3 as a function of pressure and temperature by Raman spectroscopy was undertaken in order to further characterize this new cubic phase and to investigate the P –T phase stability of the monoclinic phases, which could be of great interest for the optimization of PZT ceramics and thin films.

2. Experimental PbZr0.52Ti0.48 O3 was prepared by the conventional solid state reaction from high-purity (> 99.9%) oxides via a twostage calcination process. To ensure a better distribution of the B-site cations and limit the formation of parasitic pyrochlore-type phases, ZrO2 and TiO2 were first ballmilled for 4 h in a deionized water medium. The slurry was dried and calcined at 1673 K for 4 h to form the corresponding Zr0.52Ti0.48 O2 mixed oxide. Stoichiometric amounts of lead oxide were added. The fine-grained (< 3 µm) mixtures obtained by attrition milling for 4 h were then calcined in air at 1003 K for 4 h in covered Al2 O3 crucibles to produce perovskite-type materials. To complete the synthesis, as-obtained powders were pressed to form pellets, fired in a PbO-rich environment at 1523 K and ground leading to high-purity, fine-grained powders. The net weight loss during sintering was limited to 1.3%, which can be correlated [31] to a lead vacancy content of approximately 0.3%. The X-ray diffraction data obtained on a θ –2θ diffractometer using Cu Kα1 radiation can be interpreted in terms of a tetragonal cell with a = 4.0395(5) Å and c = 4.1355(7) Å. A degree of peak asymmetry and broadening present can be linked to the presence of a very slight monoclinic distortion and phase coexistence. Micro-Raman mapping of the sample indicated very high homogeneity over a 25.7 × 5.2 µm2 area. Variable-temperature Raman experiments (10–350 K) at ambient pressure were performed using an Oxford Instruments Microstat closed-cycle helium cryostat. The Raman spectra were obtained with a Jobin-Yvon T64000 triple monochromator (double subtractive premonochromator + spectrograph; gratings 1800 lines/mm) equipped with an Olympus microscope and a CCD cooled to 140 K. A 50× objective was used to investigate sample regions with a diameter of about 3 µm. The temperature was measured using a silicon diode. The 647.1 nm line of a krypton-ion laser was chosen for excitation in order to be less sensitive to modes arising from the surface of the powder grains, as PZT samples absorb rather strongly in the blue and green regions of the visible spectrum.

Variable-temperature, high-pressure Raman experiments were performed using a membrane-type diamond anvil cell (DAC). The cell was mounted on a CTI Cryogenics closedcycle helium cryostat. Samples were loaded along with a ruby crystal using argon as a pressure transmitting medium in the 150 µm diameter hole of a stainless steel gasket, which had been preindented to a thickness of about 50 µm. Raman experiments were performed in modified back-scattering geometry using a Jobin Yvon Model U 1000 double monochromator (gratings 1800 lines/mm) and a liquid-nitrogen cooled CCD. The 647.1 nm line of a krypton ion laser was used for excitation. The modified back-scattering geometry consisted of bringing in the incident laser beam at an angle of about 15◦ with respect to both the axes of the DAC and the collecting lens in order to reduce stray light. The temperature was measured using a silicon diode and the pressure was estimated based on the shift of the ruby R1 fluorescence line taking into account both pressure [32] and temperature contributions [33]. Deconvolution of the Raman spectra was performed by fitting the modes to Lorentzians in order to obtain representative peak positions.

3. Results and discussion LT –monoclinic 3.1. Temperature-induced monoclinic FM HT HT FM and monoclinic FM –tetragonal FT phase transitions

The ambient-temperature Raman spectrum of PbZr0.52Ti0.48O3 consists of several broad lines, each of which consists of several subpeaks [34,35]. Previous X-ray and neutron diffraction studies indicate that the monoclinic to tetragonal phase transition in PbZr0.52Ti0.48O3 occurs just above room temperature [5,36]. At these temperatures, it is very difficult to distinguish the monoclinic and tetragonal phases by diffraction techniques due to the extremely small differences in cell constants. The vibrational spectrum of PbZr0.52Ti0.48 O3 at 298 K can thus be interpreted in terms of monoclinic Cm-Cs3 symmetry. Group theory predicts 12 Raman active modes of which 7 are of A and 5 are of A symmetry. It has been noted that the situation is in fact much more complicated [37] and that in local regions, the symmetry may be different and the unit cell larger. As a consequence, more Raman modes than those predicted by theory are observed for PZT, in particular near the MPB. In addition, in these polar materials, strong LO (longitudinal optic)–TO (transverse optic) splitting is present. The evolution of the Raman spectrum of PbZr0.52Ti0.48 O3 in the low-frequency, 50–400 cm−1 , region is shown in Fig. 1. Twenty-seven vibrational modes (13A + 14A ) are expected for the low temperature form. Due to the close relationship between the two monoclinic structures, most of these modes have similar frequencies to those observed for HT form. Gradual decreases are observed with increasthe FM ing temperature in the intensities of the Raman modes at 180 cm−1 and 360 cm−1 and the higher-frequency mode at

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Fig. 1. Raman spectra of PbZr0.52 Ti0.48 O3 as a function of temperature.

450 cm−1 , which are characteristic of the low-temperature form and can be linked to the doubled unit cell. It can be noted that the mode at 360 cm−1 lies at a similar frequency to those calculated for the BO6 rotation vibrations in PbSc1/2Nb1/2O3 (PSN) and PbSc1/2 Ta1/2 O3 (PST) [25] and thus can be linked to the tilting of the BO6 octahedra in the low-temperature phase. In the above calculations, the frequency of this mode was found to be independent of the nature of the B cation. The above three modes are no longer LT –F HT transition at 210 K. An inobserved above the FM M crease in the line widths of the Raman modes occurs and the spectral line shapes become more symmetric between 300 and 310 K indicating the transformation to the tetragonal phase. This latter is what would be expected due to the lower number of predicted Raman-active modes for the tetragonal form (3A1 + B1 + 4E). 3.2. Pressure-induced monoclinic to cubic phase transitions The transition to the cubic phase was found to occur at 5 GPa at room temperature and is characterized by the disappearance of the Raman peaks in the 110–145 cm−1 region, Figs. 2 and 3, and a decrease in the intensities of the modes between 450 and 650 cm−1 . A strong Raman spectrum remains above 5 GPa and was interpreted in terms of the

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presence of polar nanoregions in this phase. Such spectra are typically observed for ferroelectric relaxors [21–27]. Two new bands were observed in the high-pressure phase at 80 and 360 cm−1 . Similar bands are observed for AB1−x Bx O3 ferroelectric relaxors such as PbSc1/2Nb1/2 O3 (PSN) and PbSc1/2Ta1/2 O3 (PST) at ambient pressure [38]. The higherfrequency band is also present in PbMg1/3Nb2/3O3 (PMN) at high pressure [27]. Normal-mode calculations for PSN [25] indicate that the observation of both modes is related to off-center lead displacements. The lower-frequency mode of F2g symmetry is lead localized and arises from cell doubling 5 ¯ to give Fm3m–O h symmetry on a local level (i.e. nanosized clusters). The higher-frequency BO6 rotation mode of F2u symmetry is also linked to O vibrations of the Pb–O bonds and is observed in the Raman spectrum due to dynamically off-centered fluctuations resulting from electronphonon coupling. Although considerable disorder is present in the monoclinic phase, there is a change from the overall long-range order in the ferroelectric phase to short range order in the high-pressure phase. The actual length scale of these phenomena has not yet been determined and will condition the existence of relaxor-like behavior at high pressure. It is known that pressure induced a crossover between classic ferroelectric and relaxor behavior in lanthanum-doped PZT (PLZT). In these systems, pressure reduces the correlation length among nanodomains and relaxor behavior is enhanced [39,40]. Under ambient conditions in the monoclinic phase, the disorder present, characterized by the activation of optical phonons at off-center k-points in the Brillouin zone [41], already places PbZr0.52Ti0.48 O3 in an intermediate position between systems, in which the ferroelectric lattice distortions are almost entirely indexed by the single Γ point (i.e. BaTiO3 , FT -PbZr0.50Ti0.50O3 , FR PbZr0.54Ti0.46O3 ), and a system such as PSN, which under certain conditions exhibits relaxor behavior. The behavior of the tetragonal A1 (TO) Raman mode near 135 cm−1 of PZT compositions close to the MPB is distinct from the that observed for the tetragonal phase for 0.60
x
1 [37]. The frequency of this mode can be related to the spontaneous polarization Ps and the spontaneous tetragonal strain, z1/2 = (c/a − 1)1/2 , where a and c are the tetragonal cell constants. This is demonstrated by the high-temperature behavior of tetragonal PbTiO3 [42,43], for which the frequency of the A1 (TO) mode, ωA1 (TO), Ps and z1/2 all decrease with increasing temperature and disappear at the transition to the cubic phase. These correlations break down close to the MPB. Whereas z1/2 decreases with x up to transition to the monoclinic phase, ωA1 (TO) only decreases slightly down to x = 0.60 and then remains constant with further decreases in x [37]. This can be related to the symmetry change and the modification to the direction of polarization. The difference with respect to tetragonal PbTiO3 is even more marked at high pressure. Whereas ωA1 (TO) decreases and exhibits the expected soft-mode behavior at the second-order FT –PC transition in PbTiO3 [44], the equivalent A mode in monoclinic PbZr0.52Ti0.48 O3 hardens with

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Fig. 2. Raman spectra of PbZr0.52 Ti0.48 O3 as a function of pressure at 44 K, 154 K and 298 K.

pressure. This hardening and the observation that the firstorder Raman spectrum does not disappear above 5 GPa indicate that the nature of the high pressure phase and the transition mechanism are entirely different in the latter case and that both displacive and order–disorder processes are involved. Based on these Raman results, the high-pressure cubic form of PbZr0.52Ti0.48 O3 is polar on the nanoscale level and is tentatively termed phase “FC ”. There are many possible sources of local polarization in PZT [20], such as off-center shifts of the Pb, Zr, or Ti cations and/or different local environments around Zr and Ti. The effect of pressure on the low-temperature monoclinic LT was investigated by Raman scattering at 44 K and phase FM 154 K. The Raman bands are sharper at 44 K and 154 K than those obtained at room temperature due to the increase in phonon life time with decreasing temperature. As at room temperature, the Raman mode near 240 cm−1 , which can be related to the monoclinic or rhombohedral phases increases in intensity as a function of pressure, whereas the intensity of the modes between 450 and 650 cm−1 and in the 110– HT –“F ” 145 cm−1 region decreases. Again as for the FM C LT transition at 5 GPa and 298 K, the FM –“FC ” transition can be detected by the disappearance of the latter Raman modes. LT –“F ” transition was found to occur at close to 7.5 The FM C GPa at 154 K and close to 9 GPa at 44 K. The disappearance of the mode near 280 cm−1 can also be clearly observed at the phase transition at 9 GPa and 44 K due to the narrower line widths, Fig. 2. In contrast to the spectra at 298 K, the lead-localized mode at 80 cm−1 and BO6 rotation mode at 360 cm−1 are observed in the monoclinic phase beginning at low pressure. It can be noted that, in this case, the monoclinic cell is HT cell and that these already doubled with respect to the FM

modes, which are related to off-center lead displacements and octahedral tilting respectively, can be expected to be active. These modes are enhanced as a function of increasing pressure or decreasing temperature. Strong Raman spectra are present for the cubic phase at low temperature. The types of vibrations involved in these modes can be proposed by analogy to cubic PSN [25], Table 1, for which calculations were performed based on 5 ¯ Fm3m–O h local symmetry and in which disorder-induced breakdown of the Raman selection rules was also present (in the absence of disorder, only the F2g modes would be Raman active for PSN). The cubic symmetry is supported by the presence of up to three components in the monoclinic phase, which merge to give triply-degenerate modes in the cubic 5 ¯ phase, Fig. 3. In order to have Fm3m–O h local symmetry as proposed in relaxor systems [21,23,25], there must be 1:1 site order of the B-cations over the scale of a few unit cells. This cannot be confirmed for PZT and local cell doubling may be due to short-range ordering of the offcenter cation displacements resulting in a symmetry lower 5 ¯ than Fm3m–O h . It is not known whether octahedral tilting LT phase. These may play a role as is the case in the FM spectra, Fig. 2, gradually weaken in intensity with further increases in pressure as was the case at 298 K and could be an indication of a gradual diffuse transition to a paraelectric state over an extended pressure range. Based on the observed Raman intensities, the critical pressure for such a diffuse transition would increase with decreasing temperature. Upon comparing spectra, obtained at the highest pressure reached at the three different temperatures, a gradual weakening of the spectrum with increasing temperature is also observed, which could be expected to continue at higher temperatures also leading to a paraelectric state. In both cases, this process

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Fig. 4. P –T phase diagram of PbZr0.52 Ti0.48 O3 . The present Raman scattering results are indicated by the symbols (Q, F, ") and (2) corresponds to the dielectric measurements of Pisarski [18,19]. The Raman scattering results obtained at ambient pressure as a function of temperature and those at high pressure and ambient temperature are in good agreement with the results obtained using various complementary techniques (X-ray diffraction [5], neutron diffraction [36] and dielectric measurements [9] as a function of temperature, high-pressure X-ray diffraction [20]).

Fig. 3. Pressure dependence of the Raman modes in PbZr0.52 Ti0.48 O3 at 44 K, 154 K and 298 K. The critical pressures are indicated by the dashed lines.

corresponds to either a temperature- or pressure-induced decrease in the correlation length among nanodomains. 3.3. Pressure-temperature phase diagram of PbZr0.52Ti0.48 O3 The ambient pressure X-ray diffraction [5], neutron diffraction [36], Raman scattering [35,45] and dielectric measurements [9] as a function of temperature along with the high-pressure X-ray diffraction results [20] and the

variable temperature-variable pressure dielectric [18,19] and Raman data can be used to construct a P –T phase diagram of PbZr0.52Ti0.48O3 , Fig. 4. Very recently, some doubt has LT phase [46,47]. The been cast on the stability of the FM present Raman results are consistent with the presence of a second monoclinic phase below 210 K and are thus in agreement with dielectric measurements and neutron diffraction results [9–11]. The Raman spectra obtained at the lowest temperatures cannot be interpreted as arising HT phase with minor amounts from the presence of the FM LT phase as of either the FRLT phase as in [46] or the FM in [47]. The FT –PC phase boundary up to 0.8 GPa was shown to be negative by dielectric measurements [18,19]. HT and F LT The boundary between the low pressure FT , FM M ferroelectric phases and the disordered cubic “FC ” phase is obtained from X-ray diffraction data [20] and the present Raman results. Based on the decrease in the intensity of the Raman spectra of the cubic phase, both with increasing pressure and increasing temperature, a hypothetical “FC ”– PC phase boundary is proposed. In addition to the presence of the two cubic phases, an important feature of this phase diagram is the extended pressure range of stability of the LT – ferroelectric monoclinic phases. The slope of the FM HT phase boundary may be positive. A weak feature near FM 360 cm−1 , which can be related to octahedral tilting, was present in the Raman spectrum of the monoclinic phase at room temperature just prior to the phase transition to the cubic phase. This would be consistent with theoretical calculations for rhombohedral PZT [48], which indicate that octahedral tilting is favored by the application of high pressure. More thermal energy would thus be necessary HT structure. at high pressure to obtain the undistorted FM LT phase is in fact present It is not certain whether the FM at room temperature and high pressure as the length scale of any ordering of the octahedral tilts has not yet been

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Table 1 Raman data (cm−1 ) for the monoclinic and cubic phases of PbZr0.52 Ti0.48 O3 LT FM 44 K 0.1 MPa

LT FM 154 K 0.1 MPa

HT FM 298 K 0.1 MPa

“FC ” 44 K 10.7 GPa

“FC ” 154 K 12 GPa

“FC ” 298 K 11.1 GPa

748 620 545 504 450

745 610 541 508 460

748 600

750 650 570 510

750 640 575 519

760 640 568 516

P2 Eg sym B–O stretching or P3 F1u asym B–O stretching P4 P5 F2g sym O–B–O bending P5 F2g sym O–B–O bending

430

430

435

P6 F1u asym O–B–O bending

398 360 320 276 253 239 207 180 145 119 111

401 358 319 276 254 238 207 178 144 121 114

406 380 334

379 333

373 330

P7 F2u BO6 rotation P8 = P 7

244 187

242 188

243 199

P9 F2u B-localized P9 F1g BO6 rotation

510

328 275 252 224 204

Vibrational modea

P10 F1u BO6 translation

143 126 112 87

85

86

P11 F2g Pb-localized

a Calculated vibrational modes in cubic PSN [25]. The modes of the cubic phase will split into up to three A or A components in monoclinic symmetry, HT phase can also be related to the known E(TO), E(LO), A (TO), A (LO) and B + E modes of F which may be shifted in frequency. The modes of the FM T 1 1 1 PbTiO3 [49] with the appropriate monoclinic splittings [45,50].

determined. The monoclinic phases, which give rise to the strong piezoelectric response of this material, are found to be particularly stable with respect to hydrostatic stress. This result may be useful in the design and optimization of thin film and ceramic devices based on this composition.

and R. Bini, L. Ulivi and M. Santoro for useful discussions. We would also like to thank J.L. Sauvajol for providing access and assistance with the variable-temperature, micro-Raman experiments at the Groupe d’Exploitation des Techniques Avancées (GETA, Département de Physique, Université Montpellier II).

4. Conclusion References PbZr0.52Ti0.48 O3 was investigated by Raman spectroscopy as a function of temperature and pressure. The transition between ferroelectric monoclinic phases occurs near 210 K and the transformation to the tetragonal phase is observed at close to 305 K at ambient pressure. The critical pressure for the transition to the cubic phase increases with decreasing temperature from 5 GPa at 298 K to 9 GPa at 44 K. A strong Raman spectrum is obtained for the cubic phase at high pressure over this temperature range indicating the presence of polar nanodomains. This cubic phase, due to the presence of static polar disorder, is distinct from the paraelectric phase observed at high temperature. These results have enabled a P –T phase diagram of PbZr0.52Ti0.48O3 to be constructed. This phase diagram is characterized by the presence of two cubic phases, the paraelectric PC form and the disordered polar “FC ” form, and the extended range of stability of the monoclinic forms at high pressure.

Acknowledgements We would like to thank the European Union for funding the work at LENS under Contract No. HPRICT1999-00111

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