School of Materials Science and Technology, Institute of Technology, Banaras Hindu University^* Laboratory for Neutron scattering, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland^** Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea^*** Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 790-784, Korea^****
○Akhilesh Kumar Singh^* Dhananjai Pandey^* Oksana Zaharko^** Songhak Yoon^*** Namsoo Shin^**** Sunggi Baik^***

Solid solution system (1-x)Pb(Mg_1/3Nb_2/3)-xPbTiO₃ (PMN-xPT) is technologically important material for piezoelectric transducer and actuator applications due to its very high electromechanical response about the morphotropic phase boundary (MPB) composition. Recently, we have discovered two new monoclinic phases with space groups Cm and Pm in the MPB region, the structure of which was believed to be mixture of rhombohedral and tetragonal phases for decades [A.K. Singh and D. Pandey, Phys. Rev.B 67,064102(2003)]. In the present work, we have carried out temperature dependent dielectric, piezoelectric resonance frequency and powder diffraction studies on various compositions of PMN-xPT near MPB, which reveals several new phase transitions. Rietveld analysis of the high-resolution neutron and synchrotron x-ray powder diffraction data has been done to characterize different crystallographic phases evolving with composition and temperature near MPB. Our results suggest the presence of a succession of three phase transitions, not reported earlier, corresponding to structural changes form the monoclinic Cm to the monoclinic Pm to the tetragonal to the cubic phases for 0.26<x<0.31 on heating above room temperature. For the compositions with 0.30<x<0.35, phase transitions from monoclinic Pm to tetragonal to cubic phases have been observed on heating above room temperature. The tetragonal compositions close to the MPB transform to monoclinic Pm phase on cooling below room temperature. The implications of these discoveries in terms of the polarization rotation model [H. Fu and R. E. Cohen, Nature 403, 281(2000), D. Vanderbilt and M. H. Cohen, Phys. Rev. B 63, 094108 (2001)] for high piezoelectricity of PMN-xPT ceramics are also discussed.