钨青铜型结构铁电体的低温扩散相转变

PHASE TRANSITION IN SOME FERROELECTRIC NIOBATE CRYSTALS WITH TUNGSTEN BRONZE STRUCTURE AT LOW TEMPERATURES

对六种钨青铜结构的铌酸盐铁电晶体进行了从15K到室温范围的介电特性及热电特性的研究。分析其介电特性和热电特性与极化电场的关系,用X射线粉末衍射进行佐证。证实在50—70K的范围内,SBN,PBN,KNSBN三类铁电钨青铜型铌酸盐晶体均存在着一个新的相变。相变是由点群4mm铁电相到点群mm2铁电相的转变。铁电自发极化方向由四方晶胞的e轴方向转变到正交晶胞的b轴方向。介电特性的高频及低频测量表明该相变具有扩散(或称瀰散)型特征。比热的实验结果证明相变是属于高于一阶相变的高阶相变。对相变前后的晶胞结构提出了一个模型解释。

Dielectric and pyroelectric properties have been measured in six ferroelectric niabate single crystals with tungsten bronze structure from 15K to room temperature. The dielectric and pyroelectric results showed that, in all the three kinds of ferroelectric niobate single crystals SBN, PBN, KNSBN with TTB structure, a new phase transition occured between 50K and 70K, which were further supported by the X-ray experiment. The crystal symmetry changes from point group 4mm to point group mm^2, and the direction of ferroelectric polar-axis changes from the c-axis of tetragonal lattice cell to the b-axis of orthogonal lattice cell as the temperature drops. The difference between high frequency dielectric constants and low frequency dielectric constants indicates that the phase transition possesses diffuse character. According to the specific heat measurement, it is confirmed that this phase transition is a higher order transition. A model of structural change has been proposed to explain this phase transition.