Millimetre-wave investigation of the soft mode in ferroelectric PbHPO4



The complex dielectric function equation image along the a axis of PbHPO4 single crystals is determined from reflection measurements at frequencies between 70 and 225 GHz in the paraelectric and the ferroelectric phase near the transition temperature Tc = 310 K. The data are complemented by a remeasurement of the properties of the lowest far-infrared mode. The mm-wave data can be described by a simple Debye formula. The relaxation time exhibits critical slowing down, and the corresponding relaxation frequency varies between about 2 and 60 GHz near Tc. Proton tunneling appears to play no essential role in this dielectric mechanism related to proton ordering.