STUDY ON THE DIELECTRIC PROPERTY AND PHASE TRANSITION OF PEROVSKITE STRUCTURE TYPE RELAXOR MATERIALS

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This study was carried out in order to investigate the relationship between electrical properties and crystal structures of lead based perovskite materials and make the mechanism of diffused phase transition of them clear.At first, effect of Mn addition to lead zirconate titanate (PZT) on electromechanical properties was studied from viewpoints of crystal and electronic structures. Then, it was confirmed that Mn addition made PZT a piezoelectrics with both high coupling factor and low mechanical loss. It was clarified that the high coupling factor of Mn-doped PZT was originated from the decrease in the axis ratio c/a caused by Mn addition and that the low mechanical loss was caused by the existence of oxygen defects introduced with Mn3+ and/or Mn2+ for charge compensation.Secondary, we studied the mechanism of the diffused phase transition of lead magnesium niobium oxide (PMN). The following facts were confirmed. 1)PMN shows ferroelectricity under -40C and the degree of the ferroelectricity increases with the decrease in temperature. 2)The crystal structure of PMN keeps cubic one down to -100C. 3)PMN shows large dielectric dispersion around 0C and there are ion hoppings near 0C. From these results, we supposed the following model related to the diffuse phase transition of PMN. PMN has ferroelectric micro domain which size is smaller than coherent length of X-ray and the region of it become large as temperature decreasing. It thermally fluctuates near the room temperature and the fluctuation relates to the ion hoppings and the relaxation frequency of them decreases with the decrease in temperature. Around Tc, PMN shows a large dielectric dispersion under 1000kHz because the relaxation frequency is found near there. As a result, it was clarified that the diffuse phase transition of PMN was a phenomenon originated from the increase in the region of the ferroelectric micro domain and the freezing of the micro domain fluctuation with the decrease in temperature.