摘要:
Weiss分子场理论（WMFT）对晶体中顺磁-铁磁和顺电-铁电相变特征的定量描述是相当成功的.由于是平均场理论,又可作为初步分析结构无序体系和复杂组分体系相变行为的理论依据.但是迄今为止,并没有对有外场时WMFr的相变特征进行详细研究.而对铁电体系,仅仅对分子取向为两个状态时WMFT的相变特征进行了研究.另外,虽然铁磁与铁电体系的WMFT描述极为相似,但是由于两种体系中微观磁化和极化的单元不同,导致相应的数学描述与结果也有所不同.本文首先对外电场中分子取向包含任意状态的铁电体系的WMFT相变特征,包括自发极化、内能和比热以及静态极化率随温度变化进行严格推导,然后对相变特征随外电场的演变进行了研究.结果表明：1）无外场时,体系发生二级顺电-铁电相变,且随状态数的增加,相变温度减小,这是与铁磁体系不同的地方,同时单分子的平均极化强度减小,而内能、比热和极化率增大;2）外场的存在,使得体系原有的二级相变转化为弥散相变,且外场越强,弥散温区越大.上述结果对深入研究铁电体系的相变,特别是弥散相变无疑是有益的.
Abstract:
The descriptions of the paramagnetic-ferromagnetic and paraelectric-ferroelectric phase transitions（PTs） by Weiss＇s molecular field theory（WMFT） are quite successful,and the WMFT is also a theoretical basis of initial analysis of the PTs of structural disorder and complex compositional systems because of its mean-field characteristic.However,there is not any study on the PT behaviors of the WMFT with external field,and only the case of two orientational states of molecules for ferroelectric systems has been investigated by the WMFT.Although the descriptions of the above two kinds of PTs by the WMFT are quite similar,the exact ones and the corresponding results are different more or less due to the difference in microscopic unit between the magnetization and polarization In this work,the exact descriptions of the ferroelectric systems with arbitrary orientational states of molecules by the WMFT are provided,including the temperature dependences of the spontaneous polarization,the internal energy,the specific heat and the static polarizability,and then the evolutions of the PTs with an external field are studied.The obtained results are as follows.1） Without the external field,the PTs of the systems are of the second order,and the transition temperatures and spontaneous polarizations decrease, which are different from those of the ferromagnetic systems,but the internal energy,the specific heat and the polarizability increase with the increase of the orientational states.2） The external field drives the second order PT to a diffusive one,and diffusive temperature range becomes wider as the field is increases.The results mentioned above would benefit the deep studies of ferroelectric PT,especially the diffusive one.
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