Dielectric and photovoltaic phenomena in tungsten-doped Pb ( Mg 1 ∕ 3 Nb 2 ∕ 3 ) 1 − x Ti x O 3 crystal Chi-Shun Tu, F.-T. Wang, R. R. Chien, V. Hugo Schmidt, C.-M. Hung, and C.-T. Tseng Citation: Applied Physics Letters 88, 032902 (2006); doi: 10.1063/1.2165278 View online: http://dx.doi.org/10.1063/1.2165278 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/88/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Dielectric∕piezoelectric resonance in high-strain Pb ( Mg 1 ∕ 3 Nb 2 ∕ 3 ) 1 − x Ti x O 3 crystals J. Appl. Phys. 97, 126105 (2005); 10.1063/1.1948523 Effect of anisotropic in-plane strains on phase states and dielectric properties of epitaxial ferroelectric thin films Appl. Phys. Lett. 86, 052903 (2005); 10.1063/1.1855389 Growth and characterization of Fe-doped Pb ( Zn 1/3 Nb 2/3 ) O 3 − PbTiO 3 single crystals J. Appl. Phys. 93, 9257 (2003); 10.1063/1.1571966 Low-temperature transverse dielectric and pyroelectric anomalies of uniaxial tungsten bronze crystals J. Appl. Phys. 92, 1536 (2002); 10.1063/1.1491995 Pseudolinearity in the inverse dielectric susceptibility of poled Pb ( Mg 1/3 Nb 2/3 ) O 3 – PbTiO 3 crystals J. Appl. Phys. 92, 638 (2002); 10.1063/1.1483111 Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 153.90.170.56 On: Fri, 29 Apr 2016 17:20:36 Dielectric and photovoltaic phenomena in tungsten-doped Pb„Mg1/3Nb2/3…1−xTixO3 crystal Chi-Shun Tua and F.-T. Wang Graduate Institute of Applied Science and Engineering, Fu Jen University, Taipei 242, Taiwan R. R. Chien and V. Hugo Schmidt Department of Physics, Montana State University, Bozeman, Montana 59717 C.-M. Hung and C.-T. Tseng Department of Physics, Fu Jen University, Taipei 242, Taiwan Received 16 September 2005; accepted 13 December 2005; published online 17 January 2006 This work investigates dielectric and photovoltaic behaviors in PbMg1/3Nb2/30.64Ti0.36O3 single crystal doped with 0.5 mol % WO3. Dielectric permittivities measured as functions of temperature and frequency reveal two first-order-type phase transitions upon heating and cooling. The photovoltaic response strongly depends on illumination wavelength, sample thickness, and prior electric-field poling. The relation of photovoltage and light intensity under near-ultraviolet  =406 nm illumination for the poled samples can be expressed by an exponential equation. Optical transmission reveals that the cutoff wavelength is near 400 nm and indicates a minimum electronic energy gap of 3.0 eV. © 2006 American Institute of Physics. DOI: 10.1063/1.2165278 High-strain ferroelectric PbMg1/3Nb2/31−xTixO3 PMNTx single crystals have demonstrated very large pi- ezoelectric coefficients compared with PbZr1−xTixO3 ceramics.1 Physical properties of PMNT strongly depend on Ti content, external electric E-field, and crystallographic orientation.1–4 The ultrahigh piezoelectricity has been theo- retically attributed to polarization rotations between tetrago- nal T and rhombohedral R phases through intermediate monoclinic M or orthorhombic O symmetries.5 Photostrictive effect, which directly converts photonic energy to mechanical motion and is originated from the cou- pling of photovoltaic field and converse piezoelectric re- sponse, has the great potential of applications in wireless photoactuators. The tungsten-doped Pb1−xLaxZryTiz1−x/4O3 ceramics, i.e., PLZTx /y /z, exhibits a large photovoltaic ef- fect and photoinduced strain under near-ultraviolet illumination.6–10 The photovoltage exhibits obvious depen- dence on polarization direction of illumination light in PLZT3/52/48 ceramics doped with WO3 and Ta2O5.7 The optimal illumination wavelength for obtaining maximum photovoltage is 366 nm for PLZT3/52/48 ceramics. It was found that the photoinduced electric response is sensitive to sample thickness and surface roughness in 0.5 mol % WO3-doped PLZT3/52/48 ceramic and the optimum thickness is 33 m.8–10 The cutoff of optical transmission for PLZT ceramics was observed near 400 nm.11 A recent study showed that the photovoltage of PLZT ceramics depends on grain size and can reach 6 kV/cm with grain size 0.42 m.12 The photovoltage across a single grain in ceramics is theo- retically proportional to the electric polarization along the photovoltaic direction, suggesting that a higher poling field is needed to induce stronger photovoltage.13 Impurity doping, which can enhance structural asymmetry and electric con- ductivity, is believed to play a critical role in the photoin- duced electric and elastic phenomena. Photovoltaic studies to date in ferroelectric materials have mainly focused on the PLZT ceramics. However, the physical origin of photoinduced voltaic response still re- mains unclear. In this work, we have investigated dielectric and photovoltaic properties in the high-strain PMNT 36% single crystal doped with 0.5 mol % WO3, i.e., PMNT/W36/0.5. The crystal was grown using a modified Bridgman method by H.C. Materials Corp. The samples were cut perpendicular to the 001 direction and the edges’ orientations are 010 and 001. The Ti concentration was estimated by using the dielectric maximum temperature Tm.14 For dielectric measurements, a Wayne–Kerr Precision Ana- lyzer PMA3260A with four-lead connections was used to obtain real  and imaginary  parts of dielectric permit- tivity. A Janis CCS-450 cold head was used with a Lakeshore 340 temperature controller. The sample dimensions are 5 51 mm3 and gold electrodes were deposited on sample surfaces by dc sputtering. Two processes were used in the dielectric measurements, called “zero-field heated” ZFH and “zero-field cooled” ZFC, in which the data were taken upon heating and cooling, respectively, without any E-field poling. The hysteresis loop of polarization versus E field was taken at room temperature RT by using a Sawyer–Tower circuit at f =46 Hz. In photovoltaic measurements, argon and krypton ion la- sers with wavelengths 488 and 406 nm were used for photo- nic energies, respectively. The experimental setup is given in Fig. 1. The basal dimensions of samples are 5.02.5 mm2. The laser polarization is perpendicular to the direction of the prior E-field poling for the poled samples, which were poled at RT. No external dc E field was applied on samples during measurements. Transparent conductive films of indium tin oxide ITO were deposited on sample surfaces by sputter- ing. Note that both ITO film and glass have an optical cutoff near 300 nm. A Thermo Electron EV500 ultraviolet UV- Visible Spectrophotometer was used for optical transmission study. aAuthor to whom all correspondence should be addressed; electronic mail: cstu@physics.montana.edu APPLIED PHYSICS LETTERS 88, 032902 2006 0003-6951/2006/883/032902/3/$23.00 © 2006 American Institute of Physics88, 032902-1 Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 153.90.170.56 On: Fri, 29 Apr 2016 17:20:36 Figure 2 shows temperature-dependent  and  at sev- eral frequencies obtained from ZFH and ZFC, respectively. Obvious thermal hystereses are observed in the temperature regions of 250–310 K and 430–450 K respectively, in- dicating two first-order-type transitions. As shown in the re- ciprocal of  Fig. 2a, two steep dips are seen at 316 291 K and 444 439 K, respectively. This also confirms two first-order-type transitions upon heating cooling. The maximum temperatures Tm of  ZFH and  ZFC oc- cur at 444 and 439 K, respectively, where tetragonal T-cubic C transitions take place. Note that the tungsten trioxide WO3 has a monoclinic M ferroelectric phase and exhibits high electric conductivity at RT.15 The phase dia- gram of PMNT revealed by synchrotron diffraction, predicts a M→T transition near RT for PMNT 36%.4 Thus, the PMNT/W36/0.5 crystal likely undergoes a M→T→C transition sequence near 316 291 K and 444 439 K upon heating cooling. In addition, dielectric absorptions  ZFH and  ZFC exhibit frequency-dependent maxima near 300 and 280 K respectively, but their maximum tem- peratures are frequency independent. These dielectric behav- iors are different from the WO3-doped PMNT 33% ceramics.16 Hysteresis loop of polarization versus E-field taken at RT is given in Fig. 3. The spontaneous polarization and co- ercive field are about 14 C/cm2 and 3.5 kV/cm, respec- tively. The remnant polarization 8 C/cm2 is obviously smaller than 17 C/cm2 in PMNT 35% single crystal,17 likely due to enhancement of electric conductivity by WO3 dopant. Figure 4 shows relations of photovoltage and light inten- sity under blue =488 nm illumination for thicknesses d =0.2 and 0.5 mm. The photovoltaic response strongly de- pends on sample thickness and the thinner sample has higher photovoltage. Thickness-dependent photovoltaic behavior was also observed in the 0.5 mol % WO3-doped PLZT ceramics.8–10 Relations of photovoltage and light intensity under illuminations of =488 and 406 nm for d=0.2 mm are given in Fig. 5 for unpoled and poled samples. The poled samples with prior polings at E=5 and 10 kV/cm exhibit similar photovoltaic responses and have much larger photo- voltage than the unpoled sample. A prior E-field poling can also enhance the photovoltage in PLZT ceramics due to the increase of remnant polarization.12 Compared with  =488 nm, near-UV =406 nm illumination induces much stronger photovoltaic responses. It indicates that the photonic energy of =406 nm can be absorbed more effectively in the poled sample, because the photovoltage is excited by the photonic absorption. Wavelength-dependent transmission as seen in Fig. 6 confirms that the cutoff occurs at 400 nm and indicates a minimum electronic energy gap of 3.0 eV FIG. 1. Color online Experimental configuration of photovoltage V/cm. FIG. 2. Color online ZFC and ZFH dielectric permittivities of a  and b . The inset in a is the reciprocal of  taken from f =10 kHz. FIG. 3. Hysteresis loop of polarization vs E field obtained at RT. FIG. 4. Color online Photovoltage vs light intensity =488 nm for the unpoled samples. 032902-2 Tu et al. Appl. Phys. Lett. 88, 032902 2006 Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 153.90.170.56 On: Fri, 29 Apr 2016 17:20:36 Eghc /. The optical transmission of pure PMNT crystals also goes down to zero at 400 nm.17 A TM and T multidomain phases were observed at RT, respectively, in PMNT 35% and 38% single crystals after a prior poling of E=6 kV/cm along 001.3 “TM” represents that dominant tetragonal domains coexist with a smaller frac- tion of monoclinic domains. It was found that tetragonal do- main increases with Ti content while the sample was poled along 001.3 Thus, the 001-poled PMNT/W36/0.5 crys- tal likely has dominant tetragonal macrodomains at RT. It was found that the relation of photovoltage and light intensity can be described by an exponential equation. As indicated by the solid line in Fig. 5, the light intensity- dependent photovoltages under near-UV =406 nm illumi- nation for the poled samples can be approximately described by Eph=5.710−7Iop2.5. EphV/cm and IopmW/cm2 are photovoltage and light intensity, respectively. In poled PLZT3/52/48 ceramic doped with 0.5 mol % WO3, the intensity-dependent photovoltage under illumination of  =366 nm can also be described by an exponential equation, Eph=0.66Iop0.5.9 In conclusion, dielectric and photovoltaic phenomena have been investigated in PMNT/W36/0.5 single crystal. Two first-order-type phase transitions were observed upon heating and cooling. The photovoltaic response exhibits a strong dependence on illumination wavelength, sample thickness, and E-field poling. The relation of light intensity versus photovoltage under illumination of =406 nm can be expressed by an exponential equation in the poled samples. The optical cutoff at 400 nm suggests that a shorter wave- length may excite stronger photovoltage. This work was supported by DoE EPSCoR Grant No. DE-FG02-01ER45869 and NSC Grant No. 94-2112-M-030- 004. 1T. R. Shrout, Z. P. Chang, N. Kim, and S. Markgraf, Ferroelectr., Lett. Sect. 12, 63 1990. 2Z.-G. Ye, B. Noheda, M. Dong, D. Cox, and G. Shirane, Phys. Rev. B 64, 184114 2001. 3C.-S. Tu, R. R. Chien, F.-T. Wang, and V. H. Schmidt, Phys. Rev. B 70, 220103R 2004. 4B. Noheda, D. E. Cox, G. Shirane, J. Gao, and Z.-G. Ye, Phys. Rev. B 66, 054104 2002. 5H. Fu and R. E. Cohen, Nature London 403, 281 2000. 6K. Uchino, IEEE Ultrasonics Symposium IEEE, New York, 1990, p. 721. 7S.-Y. Chu, Z. Ye, and K. Uchino, Smart Mater. Struct. 3, 114 1994. 8P. Poosanaas, A. Dogan, S. Thakoor, and K. Uchino, J. Appl. Phys. 84, 1508 1998. 9P. Poosanaas, K. Tonooka, and K. Uchino, Mechatronics 10, 467 2000. 10K. Uchino, P. Poosanaas, and K. Tonooka, Ferroelectrics 264, 303 2001. 11M. Ichiki, J. Akedo, Y. Morikawa, K. Ozaki, M. Tanaka, and Y. Ishikawa, Ferroelectrics 232, 259 1999. 12K. Takagi, S. Kikuchi, and J.-F. Li, J. Am. Ceram. Soc. 87, 1477 2004. 13P. S. Brody and F. Crowne, J. Electron. Mater. 4, 955 1975. 14Z. Feng, H. Luo, Y. Guo, T. He, and H. Xu, Solid State Commun. 126, 347 2003. 15F. Jona and G. Shirane, Ferroelectric Crystals Pergamon, New York, 1962, pp. 254, 255. 16N. Zhong, X. L. Dong, D. Z. Sun, P. H. Xiang, and H. Du, MRS Bull. 39, 175 2004. 17C.-S. Tu, F.-T. Wang, R. R. Chien, V. H. Schmidt, and G. F. Tuthill, J. Appl. Phys. 97, 064112 2005. FIG. 5. Color online Photovoltage vs light intensity =406 and 488 nm for various prior poling E fields. The solid line is the fitting of equation, Eph=5.710−7Iop2.5, for the poled samples under =406 nm illumination. FIG. 6. Optical transmission vs wavelength taken at RT. 032902-3 Tu et al. Appl. Phys. Lett. 88, 032902 2006 Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 153.90.170.56 On: Fri, 29 Apr 2016 17:20:36