Phase Transitions in Pb(Mg 1/3 Nb 2/3 )O 3 –PbTiO 3 Studied by Low-Frequency Internal Friction Measurement Feng Yan, w,z Peng Bao, y Jingsong Zhu, y Yening Wang, y Helen L. W. Chan, z and Chung Loong Choy z z Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China y National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China The low-frequency internal friction (0.1 Hzofo10 Hz) of (100x%)Pb(Mg 1/3 Nb 2/3 )O 3 x%PbTiO 3 (x 5 0, 13, 23, 33) ceramics has been measured in the temperature range between 90 and 500 K. All of the internal friction peaks show a height that is inversely proportional to the measurement frequency, thus they can be attributed to first-order phase transitions. The phase transitions between rhombohedral, monoclinic, and te- tragonal phases in ceramics with x 5 33 occur over a very broad temperature range from 150 to 400 K, indicating that the giant piezoelectric effect in this material may be attributed to a field- induced phase transition process. I. Introduction R ELAXOR ferroelectric materials, such as (100x%)Pb (Mg 1/3 Nb 2/3 )O 3 x%PbTiO 3 (PMNTx) and (100x%)Pb (Zn 1/3 Nb 2/3 )O 3 x%PbTiO 3 (PZNTx), have attracted extensive interest due to their high piezoelectric effect, which has enabled applications of these materials as actuators, transducers, sen- sors, etc. The underlying mechanism of the above property has been attributed to the electric-field-induced phase transition process. 1,2 PMNT and PZNT have complicated phase diagrams especially for the composition near the morphotropic phase boundary (MPB). 3 For PMNTx (x429), the rhombohedral (R) or monoclinic (M) ferroelectric phase normally transfers to the tetragonal phase at tens of degrees above room temper- ature 3,4 ; thus these phase transitions can be shifted to room temperature by applying a bias electric field. Under a high- enough bias electric field E ! along [001], PMNT can be more stable in the tetragonal phase at room temperature due to the coupling term of the electric field and the polarization P ! in the free-energy expression:  E !  P ! ; thus an electric-field-induced phase transition occurs. As previously reported, PMNT31 has a phase transition sequence of R/Ma-Mc-tetragonal with an increase of the applied electric field along [001]. 4,5 Here Ma and Mc are two types of M phase with polarization in (110) and (100) planes, respectively. The phase diagram of PMNTx has been well studied, 3,4,6,7 but the kinetics of such phase transitions are not clear yet. To better understand the electric-field-induced phase transition process, the kinetics of the phase transition need to be studied. The elastic properties are very sensitive to phase transitions. 7–11 Softening of Young’s modulus often appears near a structural phase transition due to the coupling of the stress and order pa- rameters. On the other hand, the relaxation of order parameters, motion of phase boundaries, and nucleation process during a phase transition normally induce a mechanical loss, i.e., an in- ternal friction (IF). It is necessary to point out here that a phase transition-induced IF peak can be easily discriminated from a normal relaxation peak. 12 A relaxation-type IF peak normally shows a frequency-dependent peak temperature, which can be fitted with the Arrehnius or Voger–Fulcher relationship, while a phase transition-induced IF peak has a fixed peak temperature regardless of the measurement frequency. In principle, the me- chanical loss of a ferroelectric material may be comparable with its dielectric loss in terms of their frequency and temperature dependency. The phase lag in the relaxation of domains and boundaries under a sinusoidal force (mechanical or electrical) is directly reflected in the mechanical loss or dielectric loss. How- ever, for relaxor materials, the dielectric response due to ferro- electric nanodomains is so high that it may cover up the anomaly induced by a phase transition. Thus it is advantageous to study the transitions in relaxors by IF measurements. In this paper, we will report the IF of PMNTx ceramics and to better understand the complex phase transitions in the materials. II. Experimental Procedure The PMNTx ceramic samples with nominal compositions x% 5 0%, 13%, 23%, and 33% were prepared with raw mate- rials of high purity using the improved two-step Columbite pre- cursor method as described by Swart and Shrout. 13 In the first step, the starting oxides of MgO and Nb 2 O 5 (499.9%) were thoroughly ground in ethanol, cold pressed into a pellet, and calcined at 11001C for 10 h to form the pure columbite phase of MgNb 2 O 6 (the stoichiometry was achieved by careful weight compensation of MgO based on our thermal gravimetric anal- ysis, since commercial MgO is well known to adsorb a signifi- cant amount of moisture and CO 2 from the atmosphere). In the second step, the columbite precursor powder was reacted with PbO or PbO and TiO 2 , with 3 wt% excess of PbO added to compensate for the evaporation of PbO during the subsequent calcining and sintering processes. Each composition was thor- oughly ground and calcined at 9501C for 2 h. The calcined powder was then reground with the addition of a few drops of binding agent [polyvinyl alcohol (PVA)] and cold pressed into a required shape, which was first heated up to 5001C in an alu- minum crucible for 5 h in open air to drive off the PVA, and then sintered at 12001C for 2 h in a sealed aluminum crucible with a PbO-enriched atmosphere to form high-density ceramics. The samples have a pure perovskite phase as indicated by X-ray diffraction patterns (y–2y scan, CuKa radiation). Keˆ type inverted torsional pendulum was used for the low-frequency IF measurements (frequency f and temperature T range: 0.1 Hzofo10 Hz, 90 KoTo500 K). The samples for the IF mea- surements were cut to dimensions of 40 mm  5 mm  0.2 mm and were kept in vacuum during the measurement. III. Results and Discussions The low-frequency IF results are shown in Fig. 1. Pure PMN sample shows a hump in the IF at a temperature below 250 K D. Viehland—contributing editor w Author to whom correspondence should be addressed. e-mail: apafyan@polyu.edu.hk Manuscript No. 23056. Received April 14, 2007; approved May 16, 2007. J ournal J. Am. Ceram. Soc., 90 [10] 3167–3170 (2007) DOI: 10.1111/j.1551-2916.2007.01865.x r 2007 The American Ceramic Society 3167