| Summary: | 碩士 === 輔仁大學 === 物理學系 === 89 === Dielectric permittivities and polarization-electric field (P-E) hysteresis loops have been measured as a function of temperature in relaxor-based ferroelectric single crystals (PMN-x%PT) for x=31, 32, 33 with orientations along {111}, {110}, {211} and {001}, respectively. A sharp ferroelectric with an abrupt step-like discontinuity in polarization was observed near 425 K for PMN-33%PT{001}, 410 K for PMN-32%PT{110}, 415 K for PMN-32%PT{211} and 415 K for PMN-31%PT{111}, respectively. Correspondly, the dielectric permittivity shows a sharp peak in the same temperature region. In the lower temperature range (350-380 K), a diffuse phase transition associated with a broad frequency-dependent dielectric spectrum is observed. During the phase transition, the crystal orientation would be rearranged . This means that the angle between the spontaneous polarization and the orientation (which is the same with the applied electric field) would be changed. On heating process, these different angles reveal gradual or a step-like transformations . The transitions of PMN-32%PT{211} and PMN-31%PT{111} exhibit sharper transitions than those observed in PMN-32%PT{110} and PMN-33%PT{001} .
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