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Abstract
Single crystals of PMN have been studied between 10 and 300 K by means of dielectric, polarization and poling/depoling current measurements, associated with optical domain observations under bias field. Evidence for a first order electric field-induced phase transition, from the mean cubic phase to a macroscopically polar phase, has been disclosed by studying, as a function of temperature, the dielectric anomalies, the establishment of induced polarization, the onset of poling current and the appearance of the birefringence and phase boundary. By means of combined optical and domain switching analysis, the symmetry of the induced phase has been deduced to be trigonal 3m. The induced macropolarization Pind can be switched by a field of opposite polarity. The thermal depoling under a bias field takes place at a temperature depending on the field strength, whereas the zero-field depoling of an induced state occurs always at T do = 213 K, independently of the initial poling field. An electric field/temperature phase diagram, delimiting the pseudo-cubic, the induced reversible and the frozen 3m phase, has been proposed. The dynamics and the character of the induced phase transition can be related to the micropolar regions. The ferroelectric-like behaviour in PMN displayed by dielectric hysteresis can be understood to result in reality from the field-induced polar phase.
