Optical Study on the Structural Phase Transition in Hydrogen-Bonded Ferroelectrics

磷酸一氢铅(LHP)及其同质异构体PbHAsO_4(LHA)是一种准一维铁电体，在310K和315K时为二级结构相变。磷酸盐或砷酸盐四面体通过氢键连接到线性链上，而不是相互交联。与已有研究的KH_2PO_4相反。(1)首次研究了LHA的拉曼光谱在10-320K温度范围内的变化。根据LH_2PO_4的拉曼光谱结果，对LHA的晶格模进行了对称性的指认。-1&gt；，并且与LHP中的等效模相比，表现出较弱的温度依赖性。LHA和LHP的拉曼结果比较表明，软模并不驱动LHA和LHP中的铁电转变。(2)首次测量了氢键铁电体LHP和LHA在紫外区和真空紫外区的反射光谱。在LHP中，极化E//a和E//c的第一激子跃迁分别在5.62 eV和5.77 eV处观察到尖锐的反射峰。激子跃迁被认为是Pb2+&Gt；离子中6s*6p的阳离子激发。(3)详细测量了LHP在10-430K的吸收光谱。在14K时，基波吸收边位于5.08 eV处。基波吸收尾波的对数曲线在30K以上有一条直线，这表明乌尔巴赫定律对吸收波尾是成立的。研究还发现，陡度参数的温度依赖关系与自发极化参数的温度依赖关系有很好的比例关系。激子是研究铁电相变的灵敏探针。

Lead monohydrogen phosphate PbHPO_4 (LHP) and its isomorph PbHAsO_4 (LHA) are one of the quasi-one dimensional ferroelectrics that has been clarified as a second order structural phase transition at 310 K and 315 K.The phosphate or arsenate tetrahedra are linked to linear chains by hydrogen bonds and not cross-linked to one another, in contrast to well studied KH_2PO_4.(1)The Raman spectrum of LHA has been studied for the first time as a function of temperature between 10 and 320 K.Symmetry assignments of the lattice modes in LHA have been made with the help of results from the Raman spectrum of LHP.A soft mode is observed at 72 cm^<-1>, and exhibits a weaker temperature dependence compared with the equivalent mode in LHP.A comparison of Raman results for LHA and LHP shows that the soft mode does not drive the ferroelectric transition in either LHA or LHP.(2)Reflection spectra of hydrogen-bonded ferroelectrics LHP and LHA have been measured for the first time in UV and VUV regions with polarized light. Sharp reflection peaks due to the first exciton transitions are observed at 5.62 eV for polarization E//a and at 5.77 eV for E//c in LHP.The exciton transition has been assigned as the cationic excitation of 6s* 6p in Pb^<2+> ions.(3)Absorption spectra of LHP have been measured at temperatures 10-430 K in detail. The fundamental absorption edge is located at 5.08 eV at 14 K.It is found that a logarithmic plot of the fundamental absorption tail gives a straight line above 30 K, indicating that the Urbach's rule holds for the absorption tail. It is also found that the temperature dependence of the steepness parameter is well scaled to that of the spontaneous polarization Ps. Excitons are found to be a sensitive probe for a study of the ferroelectric phase transition.