![]() ![]() The temperature variations of the unit cell parameters and atomic shifts are also presented to throw light on the nature of the various phase transitions in this mixed system. In these notes we give a short review, at an introductory level, of some topics concerning the study of crystal structures by means of neutron powder diffraction. Rietveld analyses of the powder neutron diffraction data at various temperatures on a pseudorhombohedral composition with x = 0.25 suggest that the short range M B type monoclinic order present at room temperature grows to long range monoclinic order on lowering the temperature. Neutron powder diffraction (NPD) is the most advantageous technique for using the RM due to the simple peak shape produced by the relatively coarse resolution of neutron diffractometers. All these transitions are found to be accompanied with anomalies either in the temperature dependence of dielectric constant or the piezoelectric resonance frequency or both. ![]() The double monochromator system, consisting of two parallel copper crystals in (2 2 0) mode was aligned to give a wavelength of 1.470(1) Å. In addition, our studies confirm the earlier findings on transitions from the monoclinic M C to the tetragonal to the cubic phases for 0.31 ⩽ x ⩽ 0.34 on heating above room temperature and tetragonal to monoclinic M C phase on cooling below room temperature for x = 0.36. Neutron diffraction measurementsNPD patterns were obtained at the neutron powder diffractometer at the 50 MW R2 Research Reactor at Studsvik, Sweden. Our results suggest the presence of a succession of three phase transitions, not reported earlier, corresponding to structural changes from the monoclinic M B to the monoclinic M C to the tetragonal to the cubic phases for 0.27 ⩽ x ⩽ 0.30 on heating above room temperature. A phase diagram of the PMN − x PT system showing the stability fields of ergodic relaxor, monoclinic M B, monoclinic M C, tetragonal and cubic phases is presented. The dielectric peak temperature and the Vogel-Fulcher freezing temperature are found to increase linearly with “ x” while their difference, after decreasing linearly with x, vanishes at x = 0.35 suggesting a crossover form relaxor ferroelectric to normal ferroelectric behavior at this composition. The frequency dependent dielectric studies reveal relaxor ferroelectric behavior for x < 0.35 and a normal ferroelectric behavior for x ⩾ 0.35. Anomalies in the temperature dependence of piezoelectric resonance frequency and dielectric constant are correlated with structural changes using Rietveld analysis of powder neutron diffraction data. Dielectric, piezoelectric resonance frequency, and powder neutron diffraction studies as a function of temperature have been performed on several compositions of ( 1 − x ) − x Pb Ti O 3 ( P M N − x PT ) ceramics in and outside the morphotropic phase boundary (MPB) region to investigate the phase transitions and phase stabilities in this mixed system. ![]()
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