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

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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.