Optical Materials 34 (2012) 1310–1313

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Optical Materials

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Optical and electro-optical properties of (Pb,La)TiO3 transparent ceramics

Fernando A. Londoño ⇑, Jose A. Eiras, Ducinei GarciaFerroelectric Ceramics Group, Physics Department, Federal University of São Carlos, Rod. Washington Luiz, km 235, CEP: 13565905, São Carlos, SP, Brazil

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 November 2011Received in revised form 6 February 2012Accepted 9 February 2012Available online 9 March 2012

Keywords:FerroelectricsGuided wavesElectro-optical materialOptical materials

0925-3467/$ - see front matter � 2012 Elsevier B.V. Adoi:10.1016/j.optmat.2012.02.020

⇑ Corresponding author.E-mail address: flondono@df.ufscar.br (F.A. Londoñ

(Pb1�xLax)TiO3 (PLT) ceramics were synthesized by solid-state reaction and densified by hot uniaxialpressed and pressureless sintering in order to reach optical transparency and characterize their elec-tro-optical properties. Dense and transparent PLT ceramic bodies were obtained. In this work, significantchanges in optical properties were observed with post-annealing temperature. The optical qualityattained in the samples allowed the electro-optical coefficient to be determined for this system. Its valuewas comparable to those observed in other solid solutions of the transparent ferroelectric ceramic family.

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1. Introduction

Since their discovery in 1940, ferroelectric ceramics have beenexploited in many applications, such as capacitors, sensors, piezo-electric transducers, electro-optic devices and thermistors [1,2]. Ingeneral, ferroelectric ceramics are opaque, though at certain com-positions, they can be suitably prepared to exhibit optical translu-cency or even transparency. Such materials are called transparentferroelectric ceramics (TFC). The TFC perovskites were discoveredin the late 1960s as a result of a study to find optimal processingand doping parameters for the lead zirconate titanate (PZT) system[3]. In fact, the best known and commercialized TFCs are thosefrom the lanthanum modified lead zirconate titanate (PLZT) solidsolution system [4,5]. This material can display excellent electro-optical properties, suitable for optical waveguides, infrared sen-sors, dynamic random access memories and non-volatile memo-ries. The optical and electro-optical properties of the PLZTtransparent ferroelectric ceramics have been investigated for manydecades, except those with compositions at the Zr- and Ti- richends of the phase diagram, possibly due to the processing andreproducibility problems presented by them. In particular, lantha-num-doped lead titanate (PLT) ferroelectric ceramics haveemerged as highly promising materials for piezo-mechanical andpyroelectric applications, owing to their high electro-mechanicalanisotropy factor and large pyroelectric coefficient along the polar-ization axis, respectively [6,7]. However, only Yamamoto et al.have investigated the entire tetragonal range of La-modified PTceramics as a TFC system [8]. Optical transmittance of 61%, at

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o).

wavelengths above 600 nm, was observed for the compositionwith tetragonality factor c=a of 1.010, in the case of a 300 lm-thickhot-pressed ceramic. However, no electro-optical measurementwas carried out on this system. A number of devices using the elec-tro-optic effect of lead titanate films have been proposed [9]. Thosefilms are interesting for the application to integrate optic compo-nents. Dogheche et al. prepared thin films with high transparencyfor integrated optics. They studied structural and optical propertiesas a function of post-annealing temperature [10].

Therefore, this study was carried out to more completely inves-tigate the optical and electro-optical properties of the (Pb,La)TiO3

system. The process used to fabricate transparent (Pb1�xLax)TiO3

ceramics, with 0.18 6 x 6 0.23, is described and their optical prop-erties are reported. High optical transmittance and a PLZT compat-ible Pockels electro-optical coefficient were found for thepressureless sintered ceramics with 21 mol% of La (PLT21).

2. Experimental procedure

Ceramic powders of formula (Pb1�xLax)TiO3 with 0.18 6x 6 0.23, were prepared by solid-state reaction. The batch formulaallows a x/2 PbO excess, assuming that Pb-site vacancies occur forthe maintenance of the electroneutrality. The starting materialswere PbO (MGK 99% purity), La2O3 (Aldrich >99% purity) andTiO2 (Alfa Aesar 99.8+% purity) were milled in distilled water, for3 h, using zirconium balls as grinding media. The slurry was driedand calcined in a covered alumina crucible at 850 �C, for 3 h. In thefirst trials, MgO molds were used for hot uniaxial pressed in O2

atmosphere followed for 10 h at 1220 �C, and 6 MPa, to compactthe samples for a preliminary material characterization. After hotuniaxial pressed, each ceramic surface was polished to minimize

F.A. Londoño et al. / Optical Materials 34 (2012) 1310–1313 1311

contamination and thermal treatments in an atmosphere of pureO2 by 5 h, 10 h and 20 h were realized to reach adequate transpar-ency for optical characterization. Even then, it was found that Laconcentration influenced the optical transmittance level, as indi-cated earlier by Yamamoto and co-authors [8]. Hence, only thecomposition x = 0.21, which showed the highest optical transmit-tance among the hot uniaxial pressed samples, was chosen forpressureless firing. Ceramic bodies, 14 mm in diameter and 2 mmthick, were then uniaxially (15 MPa) and isostatically (90 MPa)cold pressed, and fired for 10 h at 1220 �C, in O2 and a suitablePbO-rich atmosphere (using a atmosphere controlled furnace).The apparent density of the fired ceramics was determined bythe Archimedes method. X-ray diffraction (XRD) patterns wasidentified and analyzed, to check phase formation. The ideal den-sity was calculated from the unit cell volume found by XRD analy-sis and the molecular weight from the formula (Pb1�3x/2Lax)TiO3.The microstructure of the ceramics was observed with a scanningelectron microscope (JEOL, JSM-5800 LV). After polishing with3 lm diamond paste, the ceramics were thermally etched in airat 1100 �C, for 1 min. in a sealed alumina crucible. The opticaltransmittance was measured with a spectrophotometer (Micronal,model B582). Transverse-mode electro-optical measurementswere performed on a electrode-coated sample placed betweentwo crossed polarizers, with polarizing planes arranged at �45�to the electric field. The output light intensity for this configurationis represented by Levinson [3]

I ¼ Io sind2

� �¼ Io sin2 p‘

kDnðEÞ

� �ð1Þ

where Io is the incident light intensity, d is the optical retardation, ‘is the path length, k (=632 nm in this case) is the light wavelength,Dn is the birefringence and E is the electrical field intensity.

The linear, r and quadratic, R electro-optic coefficients were ob-tained by adjusts of the equation:

DnðEÞ ¼ �12

n3ðRE2 þ rEÞ ð2Þ

3. Results and discussion

High relative density values were found for (Pb1�xLax)TiO3

ceramics (greater than 96% and 99% of the theoretical crystal den-sity for densified pressureless and uniaxially hot pressed respec-tively). This fact is corroborated by the pore-free microstructure,with average grain size of 8 lm and 10 lm for ceramics obtainedby pressureless and uniaxial hot pressed respectively as shown inFig. 1 for the composition x = 0.21. This high relative density waspossible because of the thermal treatment in oxygen with excessPbO in the batch formula, in the sintering process. With air atmo-

Fig. 1. SEM micrographs of the polished and thermally etched surfaces of (a) hot prestreatment (1220 �C, 5 h + 5 h + 10 h, O2 atmosphere).

sphere alone, densities of �96% can be reached, but with the oxy-gen atmosphere density can reach �99%. Excess PbO cancompensate weight loss (volatilization) during firing, as well asleading to higher densification rates via liquid-phase sintering.When both of these techniques are used, bulk densities of �100%can be achieved, as demonstrated by the high optical transparencyobtained for pressureless sintered PLZT [11,12] and by the resultsfound here for the PLT ceramic system. The X-ray diffraction re-sults revealed the presence of the tetragonal phase with a contin-uous decrease of the tetragonal factor as the La content increasedas reported in the reference [12].

The optical transmittance of (Pb1�xLax)TiO3 uniaxial hot pressedx = 21 ceramics is plotted against wavelength in Fig. 2. The ceramicis transparent from visible to the near-infrared region. For the ther-mal treatment (post-annealing) the ceramics are transparent invisible region and they became completely absorbing around400 nm, indicating an optical absorption edge (known also asabsorption limit) in near UV. This is similar to absorption for mostoxygen-octahedral perovskites [13]. The measurements of trans-mission spectra were extended to 1.1 lm and no absorption wasobserved. As compared with 5 h and 10 h of thermal treatments,the transmittance of ceramic with 20 h of post-annealing is larger(transmittance greater that 40% for wavelength exceeding650 nm). However, it is possible to improve the transmittance(the theoretical transmittance are near to 70%, considering thereflection loss calculated by Fresnel expression for the transparentferroelectric ceramics [14]) with the improvement of ceramicprocessing.

Good transparency was achieved only after 20 h of thermaltreatment probably because of the removal of excess PbO fromthe grain boundaries. The same behavior was encountered forthe other compositions. Hou et al. [15] observed that the excessof PbO will stay at the grain boundary and form a grain boundarylayer. Using appropriate annealing technique, the grain boundaryphase can be eliminated and the clear boundary is formed. Thus,it can be expected that the optimal microstructure after annealingwould favor the improvement of electrical properties. Hence theelimination of the grain boundary by thermal annealing will facil-itate the increase in the dielectric constant as well as the opticaland electro-optical properties of PLT ceramics. However, the trans-mittance was strongly affected by the lanthanum concentration, ascan be seen in Fig. 3, which displays the variation of transmittancewith La content at three different wavelengths (500 nm, 650 nmand 900 nm). Since the Curie temperature (Tc) and the tetragonal-ity factor (not shown here) decrease with increasing La content, itcan be assumed that light scattering due to multiple reflections bythe domain boundaries of randomly-oriented grains is the maincause of the fall in optical transmittance at lower concentrationsof the modifier element [7,12]. When the incident light passes

sed and (b) pressureless sintered (Pb1�xLax)TiO3 ceramics (x = 0.21) after thermal

Fig. 2. Optical transmittance of uniaxial hot pressed (Pb1�xLax)TiO3 (x = 0.21)ceramics after sequential thermal treatment, plotted against wavelength.

Fig. 3. Optical transmittance at three wavelengths plotted against La content of(Pb1�xLax)TiO3 uniaxial hot-pressed and thermal treated (5 h + 5 h + 10 h) ceramics.

Fig. 4. Plot of optical transmittance of pressureless-sintered (Pb1�xLax)TiO3

(x = 0.21) against wavelength, for the conditions: as fired and after thermaltreatment (5 h + 5 h + 10 h).

Fig. 5. Dependence of relative light intensity on the electric field in the transverseelectro-optical measurements on the (Pb1�xLax)TiO3 ceramic obtained by pressure-less sintering.

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through the ceramic, those domain boundaries cause multiplescattering and generate larger scattering losses. On the other hand,PLT ceramics with high concentrations of La exhibited secondphases at the grain boundaries, which were highly deleterious tothe optical transmittance [12]. Thus, it seems that at low La con-centrations (<20mol%), light scattering is mainly due to the influ-ence of ferroelectric domains (higher tetragonality factor), whileat higher concentrations (>22mol%), second-phase scatteringpredominates.

In light of these results, the pressureless sintering study wasperformed only on the (Pb1�xLax)TiO3, x = 0.21, composition, whichshowed the highest optical transmittance among all the uniaxialhot-pressed PLT ceramics. Fig. 4 shows the plot of optical transmit-tance against wavelength for the pressureless sintered PLT(x = 0.21) ceramic. It can be seen that a high optical transmittanceis achieved even in the as-fired material. The thermally treatedsample showed an improvement of only �5% of the optical trans-mittance, in the wavelength range analyzed. However, in thiswork, significant changes in optical properties are observed withpost-annealing temperature. A suggestion is that the PbO has vol-atilized during the annealing as proposed by Wood et al. [16].

Linear and quadratic electro-optical behavior was observed inthe dependence of birefringence on the electric field, in thePLT21 electro-optical data; the fitting of Eq. (1) to these data is rep-resented as a dashed line in Fig. 5. Fig. 5 shows the light variationintensities of PLT21 transparent ceramic obtained by pressurelesssintered as a function of applied transverse electric field. The calcu-lated quadratic, R and linear coefficients, r from of Eq. (2) were0.5 � 10�16 m2/V2 and 1.9 � 10�10 m/V, respectively, at wave-length of 632 nm, at room temperature. These coefficients are sim-ilar to those found for the PLZT 9.5/65/35 [R = 1.5 � 10�16 m2/V2]and PLZT 12/40/60 [r = 1.0 � 10�10 m/V] ceramics [5]. The r valueof PLT21 is around that of LiNbO3 commonly used for electro-opticwaveguiding devices. In order to increase the electro-optical coef-ficient of PLT21 to the level of commercially used ceramics, studiesare in process and will be presented in a future paper.

4. Conclusion

Transparent ferroelectric ceramics of the system (Pb1�xLax)TiO3,with x = 0.21, were produced by uniaxial hot pressed and pressure-less sintering. Several short thermal annealing steps are necessary

F.A. Londoño et al. / Optical Materials 34 (2012) 1310–1313 1313

to reach high optical transmittance in the case of the hot-pressedsamples, in order to reduce the light scattering effect due to thePbO layer present at the grain boundaries. Besides the numberthe steps, the temperature, the time and the atmosphere (PbOand O2) of the annealing procedure must satisfy the ideal conditionsfor PbO volatilization combined with the grain accommodation, toavoid microstructural defects. By contrast, the pressureless sinter-ing conditions applied to the system required no further thermaltreatment, proving to be suitable for the preparation of transparentPLT ceramics in a single step. Nevertheless, this is the first investi-gation of optical and electro-optical coefficients in PLT ceramics,showing mixed linear and nonlinear electro-optical behavior, beingthe case that evaluations of the better optical and electro-opticalproperties in function of process ceramic are in progress. However,we can estimate with this work that PLT21 ceramic can be used forelectro-optical devices in optical waveguiding for example.

Acknowledgements

To CAPES, FAPESP and CNPq for the financial support. To Dr. Y.P. Mascarenhas (São Carlos Physics Institute at the University SãoPaulo), for the use of the XRD laboratory facilities. And to Mr. Fran-cisco José Picon for the technical support.

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