Domain Reversal and Wall Structure of 180°°°°Ferroelectric Domains in LiTaO3 Crystals

Venkatraman Gopalan*, K. Kitamura†, Y. Furukawa†

*Materials Research Laboratory and Dept. of Materials Science and Engineering,Pennsylvania State University, University Park, PA 16802

†National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, 305-0044 Japan

Abstract. We report a dramatic influence of lithium nonstoichiometry in LiTaO3 crystals on the180° coercive fields, internal fields, domain wall strain, optical birefringence, and domain shape.Congruent composition of LiTaO3 ( [Li]/([Li]+[Ta])~0.485) show large coercive fields of~21kV/mm for domain motion and built-in internal fields of ~4.5kV/mm, domain wall strains ofεzz~(1-10)x10-6, optical birefringence in the z-plane, and triangular domains with domain wallsparallel to the crystallographic x-axes. On the other hand, near-stoichiometric crystals of LiTaO3([Li]/([Li]+[Ta])~0.499) studied here show low coercive fields of ~1.7kV/mm, internal fields of<0.1kV/mm, negligible domain wall strain or optical birefringence, and six-sided domains withdomain walls parallel to crystallographic y-axes.

INTRODUCTION

Lithium Niobate (LiNbO3) and Lithium tantalate (LiTaO3) ferroelectrics have found awide range of applications in integrated optics [1] and acoustic devices [2]. Many ofthese applications depend on the shaping of antiparallel (180°) ferroelectric domains [3,4]. This has driven the recent interest in the study of domain processes in thesematerials. Recently, a technique called double crucible Czhochralski technique wasdeveloped which enables the growth of single crystals of LiNbO3 and LiTaO3 withvarying lithium content [5, 6]. This has enabled the study of the influence of lithiumnonstoichiometry in LiTaO3 crystals on the physical properties of the material. Thiswork will focus on the domain reversal characteristics and domain wall structure.

DOMAIN REVERSAL AND WALL STRUCTURE

We will compare the domain reversal fields, strains and optical contrast at domainwalls in congruent and stoichiometric crystal compositions of z-cut LiTaO3 crystals.

Domain Reversal

Figure 1 shows the hysteresis loops for domain reversal at room temperature forcongruent and stoichiometric crystals of z-cut LiTaO3.

Figure 1. Hysteresis loops of congruent and stoichiometric LiTaO3 crystals at room temperature. TheEint , indicated by the arrow, depicts the internal field as measured by the offset in the hysteresis loop forthe congruent crystals. The crystals are cycled starting from state I through state II and back to state I.

The congruent LiTaO3 had a lithium concentration of [Li]/([Li]+[Ta])~0.485,whereas, the near-stoichiometric composition used in this study had[Li]/([Li]+[Ta])~0.499. Starting from a single crystal, single domain wafer, thedomains were reversed using liquid electrodes (saturated salt solution of KNO3 inwater) [7]. Starting from state I, the polarization was reversed to state II (forwardpoling) and then back to state I (reverse poling). The coercive field for forward poling(Ef) and reverse poling (Er), were respectively ~21 kV/mm and ~12 kV/mm forcongruent crystal. The difference reflects a large built-in internal field in the form of theoffset of the hysteresis loop along the field axis, given by Eint= (Ef-Er)/2 ~4.5k V/mm.In the stoichiometric crystals [7], the coercive fields were Ef~1.7 kV/mm and Er~1.5kV/mm, resulting in an Eint ~0.1 kV/mm. Clearly, there is a strong correlation betweenthe presence of internal field and the lithium deficiency in the crystals. This is also

accompanied by an increase in the coercive field values in congruent crystals by anorder of magnitude.

Figure 2 shows the change in the coercive fields as a function of temperature for bothcongruent and stoichiometric LiTaO3 crystals. The coercive field in congruent crystalsshows a sharp decrease by a factor of 4 from room temperature (22°C) to 250°C [8].This is accompanied by a broadening of the range of electric fields over which domainreversal takes place, and the disappearance of internal fields at 250°C. The dcconductivity of the crystal also increases exponentially as a function of temperature withactivation energy of 0.66 ±0.14 eV. On the other hand, the stoichiometric crystals showlittle change in the coercive field over the entire range as shown.

FIGURE 2. The coercive field for domain reversal for congruent (CLT) and stoichiometric (SLT)LiTaO3 crystals. The symbol notation is as follows: Filled squares: Ef-start (CLT); Open squares: Ef-end(CLT); Filled circles: Er-start (CLT); Open circles: Er-end (CLT); Open rhombus: Ef (SLT); Star: Er(SLT); filled rhombus: dc conductivity (CLT). The solid line fit to dc-conductivity is an exponential fitwith activation energy of 0.66±0.14 eV.

The current understanding of the defect chemistry in nonstoichiometric LiTaO3crystals is consistent with a charge balance between tantalum antisites ([TaLi]4+) andfour lithium vacancies (4[Vli]-) [9, 10]. Evidence from diffuse X-ray scattering, [11, 12]neutron scattering, [13] and Nuclear Magnetic Resonance (NMR) measurements [14,15] point to the likelihood of these point defects forming dipolar clusters ordered along

the Lithium and tantalum chains in the pseudo-cubic directions of the crystal. Based onthese studies, we present a qualitative model to visualize the changes in coercive andinternal fields seen above in terms of point defect complexes. Figure 3 shows aschematic of the stable states for the possible defect dipole for an “up” and a “down”domain, the notation referring to the polarization direction, Ps.

(a) (b)

TaLi

VLi

+y

+y +z +z +y

+y

TaLi

Ps

Ps

FIGURE 3. Schematic describing a plausible model for the stable states of a 4[VLi]- -[TaLi]4+ dipolardefect in an “up” domain (a) and in a “down” domain (b). In this model, upon domain reversal at roomtemperature from +Ps in (a), to -Ps in (b) , the tantalum antisite ion (TaLi) moves from its position in (a) tothat in (b). However, the lithium vacancies (VLi) remain in their original positions in (a), resulting in a“frustrate state” of the defect dipole. To achieve the stable state of defect dipole in (b), thermal activationis required by heating over 150°C.

The position of the lithium vacancy with respect to tantalum antisite has beenarbitrarily chosen here assuming that defect dipole formed by [Vli]-- [TaLi]4+ along the z-axis orients parallel to the matrix polarization (Ps). When the Ps is reversed usingelectric field at room temperature from Fig. 3 (a) to Fig. 3(b), the lithium atoms, and thetantalum atom at the antisite is forced through the close packed oxygen triangle to aposition just below the oxygen plane. The field-induced motion of the tantalum antisiteacross the oxygen plane may be responsible for the increase in the coercive field incongruent crystals. However, it appears that the lithium vacancies surrounding theantisite do not move under the electric field at room temperature. In this state, the

defect dipole at room temperature is in a “frustrated state” after domain reversal atroom temperature. After heating the crystal to 150°C and above, the lithium ions areknown to become increasingly mobile resulting in increase in the ionic conductivityseen in Fig. 2. This results in a randomization of the lithium vacancies in the lattice,and the disappearance of the defect dipole and hence the internal fields at highertemperatures. On cooling the crystal back, the lithium ions reorder to form a defectdipole in the opposite direction as shown in Fig. 3(b), resulting in the re-establishmentof large internal fields and increased coercive fields at room temperature [16].

Domain Wall Strain and Optical Contrast

We present experimental evidence that suggest that the “frustrated defect dipoles”arising from lithium deficiency as proposed above also give rise to large regions ofstrain and optical birefringence adjacent to domain walls in congruent LiTaO3 crystals.These effects disappear in upon heating and cooling of the congruent crystals as well asin stoichiometric crystals. Figure 4 shows the optical image of domains created in congruent LiTaO3 at roomtemperature using electric field. The image is taken looking down the z-axis of thecrystal,

1 mm

FIGURE 4. An optical image of the z-plane 180° domains in congruent LiTaO3 taken with samplebetween cross-polarizers aligned along the horizontal and vertical axes of the above image. Thepolarization directions, Ps is normal to the image plane, and crystallographic +y-axis in the matrix domainpoints upwards along the vertical axis of the above image. Domain walls are parallel to thecrystallographic x-axes. The triangular domains are formed by domain walls aligned along the crystallographicx-axes which are perpendicular to the three c-glide planes. In this crystal state, the

dipolar defects in Fig. 3 would be in a “frustrated state” inside the triangular domains,and in stable state outside the triangles [17]. The transition region across a domain wallshows optical contrast. Since the image is taken with the sample between crosspolarizers, and the crystal is optically isotropic in the z-plane, the image is mostly dark.However, light appears to leak through regions adjacent to the domain walls, which areat an angle to the polarizer and the analyzer, suggesting that these regions exhibit opticalbirefringence. The contrast disappears for walls which are parallel or perpendicular tothe cross polarizers (for example the horizontal wall of the triangles in Fig. 4), thusrevealing that the optical birefringence is between the refractive indices parallel (nx)and perpendicular (ny) to the domain walls. This birefringence has been carefullymeasured by Yang and Mohideen [18, 19] using Near-field Scanning OpticalMicroscope (NSOM) as nx-ny~ (2-10)x10-5 over widths ranging from 0.5-3 µms.Since this birefringence has contributions from strains directly through the elasto-opticeffect and indirectly through the combination of piezoelectric and electro-optic effects,one can deduce the strains εy-εx~ 1.4x10-4 and εyz ~2x10-4 in the region of opticalbirefringence. This also corresponds to an electric field Ey~±10 kV/mm locally in thebirefringence region. Yang and Mohideen [18] have also measured a surface step at thewall, corresponding to an approximate strain εzz ~ 1x10-6 and an electric field Ez ~0.15kV/mm. If this electric field is multiplied by the relative dielectric constant,(k~45) itwould correspond to the equivalent external electric field of kEz~6.75 kV/mm which issimilar in magnitude to the internal field measured from the hysteresis loop. As aconfirmation of these deductions, currently ongoing work on X-ray synchrotron imagingof domain walls in congruent LiTaO3 reveal strains in the [0006] lattice planes of theorder of ∆d[0006]/d[0006] ~(1-10)x10-5 over regions of 5-15µm across the domain walls[20].

The optical birefringence disappears when the crystal shown in Fig. 4 is annealed attemperatures above 200°C for a few seconds and cooled back to room temperature [17].In this state, the dipolar defects inside and outside the triangular domains would be instable states as shown in Figs. 3 (a) and (b) respectively. On the other hand, in near-stoichiometric crystals, the optical birefringence is observed to be negligible (estimatedto be < 10-6) at the domain walls. Figure 5 shows two images of 180° domain walls innear-stoichiometric crystals taken with an optical microscope. While the domain wall isbarely visible in Figure 5(a), application of a small but uniform electric field of ~1.6kV/mm across the z-axis (normal to image plane) of the crystal results in an indexcontrast at the wall through the electro-optic effect, which reveals the presence of thedomain wall. In addition, the domain shape observed in stoichiometric crystals consistsof six sided polygons with domain walls parallel to the crystallographic y-axes. One cansee wall facets enclosing 120° angles along the domain wall visible in Fig. 5(b).

FIGURE 5. The optical contrast at a domain wall in stoichiometric LiTaO3 (a) with no field, and (b)under an electric field of 1.6 kV/mm along the z-axis. The polarization direction is normal to the imageplane.

CONCLUSIONS

The results presented here clearly indicate that the presence of a few mole percent oflithium nonstoichiometry results in dramatic changes in the domain wall structure anddomain reversal phenomena in ferroelectric LiTaO3. In particular, the opticalbirefringence, strains and electric fields near the 180° domain walls, as well as the wallorientation itself are directly dependent on lithium nonstoichiometry related defectcomplexes. These results are also qualitatively the same in LiNbO3 with the soleexception that domain wall orientation remains oriented along crystallographic y-axes incongruent as well as stoichiometric LiNbO3 crystals.

In the light of these observations, one needs to take a closer look at the internalstructure of domain walls in ferroelectrics. The observation of large strains over manymicron widths at 180° walls calls into question our usual assumption of an ideal 180°wall. In particular, one has to be keenly aware of the extrinsic contributions to domainwall strains such as those arising from changes in crystal chemistry.

ACKNOWLEDGEMENTS

We would like to thank Dr. U. Mohideen at University of California, Riverside, CAand Dr. B. Steiner, at National Institute for Standards and technology, Gaithersburg,MD, for the valuable discussions on domain wall structure.

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