2004-2005 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Chan, HLW

Choy, CL

Luo, H

Wan, X

Zhao, X

Zheng, RK

Journal of Applied Physics

2004-2005 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishedVoR allowe

PolyU Institutional Repository

Abnormal phase transitions for tetragonal 1−xPbMg1/3Nb2/3O3-xPbTiO3single crystals at low temperatureXinming Wana)The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, ShanghaiInstitute of Ceramics, Chinese Academy of Sciences, Jiading, Shanghai 201800, Chinaand Department of Applied Physics, The Hong Kong Polytechnic University,Hung Hom, Hong Kong, ChinaR. K. Zheng, H. L. W. Chan, and C. L. ChoyDepartment of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, ChinaXiangyong Zhao and Haosu LuoThe State Key Laboratory of High Performance Ceramics and Superfine Microstructure, ShanghaiInstitute of Ceramics, Chinese Academy of Sciences, Jiading, Shanghai 201800, China(Received 20 July 2004; accepted 30 August 2004)Dielectric spectroscopy and ultrasonic sound velocity were investigated for tetragonal PMN-xPTsingle crystals over a wide range of temperature. In addition to the expected phase transition fromthe tetragonal ferroelectric phase to the cubic paraelectric phase at a high temperature, an abnormalferroelectric phase transition below room temperature is observed. The low-temperature phasetransition shows a relaxor behavior though tetragonal PMN-xPT single crystal is considered to betypical normal ferroelectrics. By comparing the dielectric constants of poled and depoled conditionsand by analyzing fBO6g oxygen-octahedra units of perovskite structure, it is suggested that thelow-temperature phase transition in poled tetragonal PMN-xPT single crystal could be tetragonal torhombohedral ferroelectric phase transition. © 2004 American Institute of Physics.[DOI: 10.1063/1.1808906]I. INTRODUCTIONThe complex perovskite s1−xdPbsMg1/3Nb2/3dO3-xPbTiO3 (PMN-xPT) single crystals are well-known ferro-electrics. The solid solutions have a morphotropic phaseboundary (MPB) near x,0.33 to 0.35,1 dividing the rhom-bohedral phase and the tetragonal one. For compositionswith xø0.35, the compounds have a tetragonal structurewith symmetry of P4mm, for those with xł0.33, the crystalsymmetry is rhombohedral R3m. Normal ferroelectric behav-ior increases as a function of x, whereas relaxor behaviordecreases.2 PMN-xPT single crystals have come into promi-nence in the past few years due to their very high piezoelec-tric coefficients sd33.2500 pC/Nd, extremely large piezo-electric strains s.1.7%d, and extrahigh electromechanicalcoupling factor in the longitudinal bar mode sk33,94%d.3–6Such excellent properties point to a potential revolution in anelectromechanical transduction for a broad range of applica-tions. Recent studies show that the materials with tetragonalstructure exhibit excellent optical properties,7,8 especially theelectro-optic properties.9,10 The result shows that tetragonalPMN-xPT single crystals are also promising candidates foroptical devices.8Recently, an intensive theoretical and experimental re-search has been undertaken in order to understand the originof these enhanced dielectric and piezoelectric properties. Fuand Cohen pointed out that a large piezoelectric responsecould be driven by a polarization rotation induced by anexternal electric field.11 At almost the same time, Nohedaet al. revealed the existence of a monoclinic phase betweenthe previously established tetragonal sFETd and rhombohe-dral sFERd regions in PbZr0.52Ti0.48O3, which further supportsthe concept of polarization rotation.12,13 Subsequently, simi-lar experiments were carried out on PMN-xPT single crys-tals, and it was found that monoclinic sFEMd orthorhombicand sFEOd phases can also be induced in a narrow range ofcompositions with an appropriate poling history.14 Based onthe dielectric constants obtained on a poled k001l-orientedPMN-0.3PT single crystal, Viehland et al. suggested that thetransition from a normal ferroelectric state (macrodomains)to a relaxor state (microdomains) with an increasing tem-perature is responsible for the secondary transformation near90 °C.15 For a poled k001l-oriented PMN-0.33PT singlecrystal, Lu et al. also found an additional ferroelectric phasetransition (FE-FE) at 80 °C using dielectric constant mea-surements. However, they were uncertain whether this phasechange corresponds to a FEO-FET or a FER-FEO transition.16From the dielectric constant measurements, the secondaryphase transition below Tm (the temperature of the permittiv-ity maximum) has been found for PMN-xPT single crystalsby many groups.17–21 But all the investigations are limited tocompositions near the MPB, and there is no report on tetrag-onal PMN-xPT single crystals.The aim of this work is to investiagate tetragonalPMN-xPT single crystals over a wide range of temperatureusing dielectric spectroscopy. In addition to a phase transi-tion at Tm, an abnormal ferrolectric phase transition belowroom temperature is observed in all the main crystallo-a)Electronic mail: xmwan@citiz.netJOURNAL OF APPLIED PHYSICS VOLUME 96, NUMBER 11 1 DECEMBER 20040021-8979/2004/96(11)/6574/4/$22.00 © 2004 American Institute of Physics6574graphic directions k001l, k110l, and k111l. The phase transi-tion has been supported by an ultrasonic sound velocity mea-surement.II. EXPERIMENTAL PROCEDURELarge-size and high-quality PMN-xPT single crystalswere grown directly from melt by the modified Bridgmantechnique.22,23 The crystals were oriented and cut along threemain crystallographic directions k001l, k110l, and k111l byan x-ray diffractometer. Rectangular-shaped specimens s63631 mm3d were prepared for property measurements.Silver paste was painted and sintered at 580 °C for30 min. Then, the samples were poled with an electric fieldof 10 kV/cm for 15 min at 160 °C in a silicone oil and thenslowly cooled to room temperature with the half of the ap-plied electric field.In this work, dielectric spectroscopy was measured usinga HP4194A impedance analyzer in the temperature rangefrom −180 to 200 °C. The sample was placed inside a liquid-nitrogen bath cryostat (Oxford DN1704-type). The tempera-ture of the sample was stabilized by a digital temperaturecontroller (Oxford ITC 601-type). The longitudinal ultra-sonic sound velocity was measured using the AUW-100 Ad-vanced Ultrasonic Workstation with a 10-MHz LiNbO3transducer.III. RESULTS AND DISCUSSIONFigure 1 shows the dielectric constants versus tempera-ture curve of the k110l-oriented PMN-xPT single crystal.The dielectric constant exhibits a sharp change at 180 °C, sothe PT content in the sample is about 0.38, which is deducedfrom the equation, Tm=5x-10 (where Tm is the temperaturein Celsius scale and x is the PT content).7 PMN-0.38PTsingle crystal is known to possess the tetragonal structure atroom temperature and shows a normal ferroelectric behavior.The sharp change corresponds to the phase transition fromthe tetragonal ferroelectric phase sFETd to the cubic paraelec-tric phase sPECd, and the dielectric constants exhibit verylittle frequency dispersion at this range, as shown in the insetof Fig. 1. All the properties agree well with the previousreports.24,25The most remarkable result of our dielectric measure-ments is the second dielectric peak at about −120 °C, alsoshown in Fig. 1, which has not been observed for the tetrag-onal PMN-xPT single crystal so far. Furthermore, as weknow, PMN−0.38PT single crystal is considered to be typi-cal normal ferroelectrics, while the dielectric constants nearthe second peak are frequency-dependent. It shows that thispeak possesses relaxor behavior and demonstrates a “micro-to macrodomain” transition, similar to that previously re-ported PMN-xPT single crystals in the MPB region.15 Afterthe first heating measurement to 220 °C, the sample waddepoled. We also measured the dielectric spectroscopy forthe deploed sample and found that the peak near −120 °Cdisappeared, which indicates that the abnormal dielectricpeak only exists in the poled sample. From the theory ofpolarization rotation,11 the dipoles will rotate along differentpaths under different poling directions. When the poling wasdone, the polar direction is expected to incline close to theelectric field in each domain, possibly resulting in an in-creased tetragonal lattice distortion. However, the distortiondoesn’t result in the structure transformation at room tem-perature because there is no additional dielectric peak fromroom temperature to Tm. Decreasing the temperature mayresult in the collapse of the distortion and induce anotherphase transition at a low temperature.The low-temperature phase transition has been supportedby ultrasonic measurement. Figure 2 shows the temperaturedependence of the longitudinal ultrasonic sound velocitypropagated along the k110l direction for the PMN-0.38PTsingle crystal. With a decreasing temperature from 27 °C,the sound velocity softens smoothly and reaches a localminimum at about −60 °C, and then follows a slight stiffen-ing with further cooling of the temperature. The broad soft-ening of the sound velocity reflects that the acoustic phonondamps over a wide temperature range. As is known, the com-positional fluctuations-induced random electric fields cansuppress the development of a long-range ferroelectric orderand give rise to order-parameter fluctuations in a wide tem-perature range.26 The coupling between the elastic strain andthe fluctuating order parameters, hence, leads to the smoothsoftening of the sound velocity in a wide temperature rangefrom 27 to −30 °C. Similar acoustic anomalies have alsobeen observed in PbMg1/3Nb2/3O3 (PMN)27,28 andPbMg1/3Ta2/3O3 (PMT)29 by the measurements of micro-Brillouin scattering. In contrast to the smooth softening ofthe sound velocity at high temperatures, the sound velocityFIG. 1. Temperature dependence of dielectric constants of k110l-orientedPMN-0.38PT single crystal (solid line: 100 Hz; dashed line: 1 kHz; dottedline: 10 kHz).FIG. 2. Temperature dependence of longitudinal ultrasonic sound velocityof k110l-oriented PMN-0.38PT single crystal.J. Appl. Phys., Vol. 96, No. 11, 1 December 2004 Wan et al. 6575shows a drastic softening with a decreasing temperature from−90 to −120 °C, which has not yet been observed in similarPMN and PMT single crystals. Due to the increasing largeattenuation with the decreasing temperature, the ultrasonicecho signal disappears completely below −120 °C (see Fig.3), which possibly resulted from the scattering of anotherphase below −120 °C. We note that with an increasing tem-perature from a low temperature s77 Kd to room tempera-ture, the ultrasonic echo signal appears again at about−120 °C, which may correspond to a reverse structuralphase transition. The large ultrasonic attenuation below−120 °C makes it impossible to measure the sound velocitybelow −120 °C. We want to stress that such a large attenu-ation, together with a large softening of the sound velocitybelow −90 °C, strongly implies a structural phase transitionassociated with the dielectric constants anomaly. Similarlarge softening of the sound velocity has been observed nearthe structural phase-transition points in Ba1-xSrxTiO3.30From the dielectric measurements, we observed an ab-normal relaxor peak at about −120 °C for normal ferroelec-tric PMN-0.38PT single crystal. The peak was confirmed tobe corresponding with a phase transition by ultrasonic soundvelocity measurement. We know that the lattice distortionsand the angles in different phases for the PMN-xPT singlecrystal differ only slightly, so they are very different to bedetected,31 especially at such a low temperature. However, itis estimated that the low-temperature phase couldn’t be theorthorhombic phase, which has been observed in PMN-xPTsingle crystals with rhombohedral structure or in the MPBregion.16–20 It is generally accepted that for PMN-xPT single,crystals, the dielectric constant shows the minimum valuewhen the crystals are poled along the spontaneous polariza-tion direction. For example, the dielectric constants at roomtemperature for tetragonal PMN-0.38PT single crystal areabout 780, 6972, and 9784, respectively, for the samplespoled along k001l , k110l, and k111l directions.25 The dielec-tric constant of the spontaneous polarization direction k001lshows the minimum value. As shown in Fig. 1, the dielectricconstants for the k110l-poled sample around the low-temperature phase-transition region are much larger thanthose of the depoled condition. Such evidence may be be-lieved that the k110l direction is not the polarization direc-tion. That is to say, the phase that appeared near −120 °Ccannot be an orthorhombic phase.The structure of PMN-xPT single crystals can be re-garded as the one made of fBO6g (fTiO6g, fNbO6g, andfMgO6g) coordination polyhedra linked by their vertexes.Figure 4 shows the schematic diagram of the perovskitestructure composed of fBO6g oxygen octahedra. FerroelectricPMN crystals have rhombohedral phase and its Tm is about−10 °C, ferroelectic PT crystals have tetragonal phase andits Tm is about 490 °C. It is in favor of the rhombohedralphase by adding PMN in the PMN-xPT solid solutions ordecreasing temperature. Therefore, it is reasonable to thinkthat the polarization in fTiO6g octahedra appears dominantalong the k001l direction, it’s dominant along k111l infNbO6g or fMgO6g octahedra. In tetragonal single crystals,fNbO6g and fMgO6g octahedra are forced to distort along thek001l direction in the pressing of fTiO6g octahedral. Whendecreasing temperature or adding PMN, fNbO6g and fMgO6goctahedra slip the leash of fTiO6g octahedra, and simulta-neously make fTiO6g octahedra distort along k111l, resultingin the FET-FER ferroelectric phase transition. The FET-FERphase transition has been found in the other compositions.For example, the FET-FER phase-transition temperature TRTis about 89, 78, and 66 °C, respectively, for PMN-0.256PT,PMN-0.296PT, and PMN-0.324PT.17 One can draw the con-clusion that the TRT decreases with an increasing PT concen-tration. The TRT of our sample, PMN-0.38PT, is about−120 °C, which is consistent with the trend. Combined withits dielectric properties and ultrasonic sound velocity results,it is assume that the phase below −120 °C in the poledFIG. 3. Pulse-echo patterns at (a) −58 °C, (b) −83 °C, and (c) −143 °C.FIG. 4. Schematic diagram of ABO3 perovskite structure composed offBO6g oxygen octahedra by their vertexes ( A:Pb2+ ;B:Mg2+ ,Nb5+, or Ti4+).6576 J. Appl. Phys., Vol. 96, No. 11, 1 December 2004 Wan et al.PMN-0.38PT single crystal could be the ferroelectric rhom-bohedral phase and the peak could be the FET-FER phasetransition.We note that the similar peaks also appear in the k001land k111l direction of poled tetragonal PMN-xPT singlecrystals. Figure 5 shows the temperature dependence of di-electric constants of poled k001l and k111l-orientedPMN-xPT single crystals. They both have a sharp changeand Tm is about 196 °C for the k001l direction and 199 °Cfor k111l one, indicating that the PT content for them isabout 0.412 and 0.418, respectively. So they are both in te-tragonal phase. As shown in the inset of Fig. 5, they alsohave an additional frequency-dependent peak at about−145 °C, as observed for the k110l direction. But they aremuch “evener” compared with the peak of the k110l direc-tion. We should also mention here that accurate structuremeasurements for the low-temperature phases, such as high-resolution powder x-ray diffraction measurements, must becarried out to find out the nature of the phase. The authenticexperimental evidences are still under investigation and moredetails will be discussed in later work.IV. CONCLUSIONDielectric spectroscopy and ultrasonic sound velocitywere investigated for tetragonal PMN-xPT single crystalsover a wide range of temperature. In addition to the expectedphase transition from the tetragonal ferroelectric phase to thecubic paraelectric phase at Tm, an abnormal ferroelectricphase transition below room temperature is observed. Fur-thermore, the low-temperature phase transition shows relaxorbehavior though tetragonal PMN-xPT single crystal is re-garded as typical normal ferroelectrics. By comparing thedielectric constants of the poled and depoled conditions andby analyzing fBO6g oxygen-octahedra units of perovskitestructure, it is suggested that the low-temperature phase tran-sition in poled tetragonal PMN-xPT single crystal could betetragonal to rhombohedral ferroelectric phase transition.ACKNOWLEDGMENTSOne of the authors (R.K.Z.) thanks Professor XiaoguangLi for his support in the ultrasonic measurements. This workis supported by the National Natural Science Foundation ofChina (Grant No. 50272075), the High Technology and De-velopment Project of the People’s Republic of China (GrantNo. 2002AA325130), Hong Kong Research Grants Council(Grant No. PolyU 5193/00P), and Center for Smart Materialsof the Hong Kong Polytechnic University.1S. W. Choi, J. M. Jung, and A. S. Bhalla, Ferroelectr., Lett. Sect. 21, 27(1996).2E. V. Colla, N. K. Yushin, and D. Viehland, J. Appl. Phys. 83, 3298(1998).3R. F. Service, Science 275, 1878 (1997).4S.-E. Park and T. R. Shrout, J. Appl. Phys. 82, 1804 (1997).5K. Harada, S. Shimanuki, T. Kobayashi, S. Saitoh, and Y. Yamsshita, KeyEng. Mater. 157, 95 (1999).6G. Xu, H. Luo, P. Wang, H. Xu, and Z. Yin, Chin. Sci. Bull. 45, 491(2000).7X. Wan, H. Luo, J. Wang, H. L. W. Chan, and C. L. Choy, Solid StateCommun. 129, 401 (2004).8X. Wan, J. Wang, H. L. W. Chan, C. L. Choy, H. Luo, and Z. Yin, J. Cryst.Growth 263, 251 (2004).9D.-Y. Jeong, Y. Lu, V. Sharma, Q. Zhang, and H. Luo, Jpn. J. Appl. Phys.,Part 1 42, 4387 (2003).10X. Wan, H. Xu, T. He, D. Lin, and H. Luo, J. Appl. Phys. 93, 4766 (2003).11H. Fu and R. E. Cohen, Nature (London) 281, 403 (2000).12B. Noheda, D. E. Cox, G. Shirane, J. A. Gonzalo, L. E. Cross, and S. E.Park, Appl. Phys. Lett. 74, 2059 (1999).13B. Noheda, D. E. Cox, G. Shirane, R. Guo, B. Jones, and L. E. Cross,Phys. Rev. B 63, 014103 (1999).14Z.-G. Ye, B. Noheda, M. Dong, D. E. Cox, and G. Shirane, Phys. Rev. B64, 184114 (2001).15D. Viehland, J. Powers, L. E. Cross, and J. F. Li, Appl. Phys. Lett. 78,3508 (2001).16Y. Lu, D.-Y. Jeong, Z.-Y. Cheng, Q. Zhang, H. Luo, Z. Yin, and D. Vie-hland, Appl. Phys. Lett. 78, 3109 (2001).17M. C. Shin, S. J. Chung, S.-G. Lee, and R. S. Feigelson, J. Cryst. Growth263, 412 (2004).18J. P. Han and W. W. Cao, Phys. Rev. B 68, 134102 (2003).19X. Zhao, J. Wang, H. L. W. Chan, C. L. Choy, and H. Luo, J. Phys.:Condens. Matter 15, 6899 (2003).20Y. Guo, H. Luo, K. Chen, H. Xu, X. Zhang, and Z. Yin, J. Appl. Phys. 92,6134 (2002).21X. Zhao, B. Fang, H. Cao, and H. Luo, Mater. Sci. Eng., B 96, 254(2002).22Z. Yin, H. Luo, P. Wang, and G. Xu, Ferroelectrics 299, 207 (1999).23H. Luo, G. Xu, P. Wang, and Z. Yin, Ferroelectrics 231, 97 (1999).24H. Cao, B. Fang, H. Luo, Y. Sun, and J. Guo, Ceram. Int. 29, 145 (2003).25H. Cao, B. Fang, H. Xu, and H. Luo, Mater. Res. Bull. 37, 2135 (2002).26V. Westphal, W. Kleemann, and M. D. Glinchuk, Phys. Rev. Lett. 68, 847(1992).27S. G. Lushnikov, J. H. Ko, and S. Kojima, Appl. Phys. Lett. 84, 4798(2004).28C. S. Tu, V. H. Schmidt, and I. G. Siny, J. Appl. Phys. 78, 5665 (1995).29J. H. Ko, S. Kojima, and S. G. Lushnikov, Appl. Phys. Lett. 82, 4128(2003).30V. V. Lemanov, E. P. Smirnova, P. P. Syrnikov, and E. A. Tarakanov, Phys.Rev. B 54, 3151 (1996).31B. Noheda, D. E. Cox, G. Shirane, J. Gao, and Z.-G. Ye, Phys. Rev. B 66,054104 (2002).FIG. 5. Temperature dependence of dielectric constants of poled (a) k001land (b) k111l-oriented PMN-xPT single crystals (solid line: 100 Hz; dashedline: 1 kHz; dotted line: 10 kHz).J. Appl. Phys., Vol. 96, No. 11, 1 December 2004 Wan et al. 6577

Abnormal phase transitions for tetragonal (1-x)Pb(Mg[sub ⅓]Nb[sub ⅔])O₃-xPbTiO₃ single crystals at low temperature

Abstract

Similar works

Full text

Available Versions

PolyU Institutional Repository