Abstract
The Raman spectra of PbHPO4 and PbHAsO in the para- and ferroelectric phases and also PbDAsO4 (room temperature only) have been measured. Mode assignment of most predicted modes has been made. Weak underdamped soft modes have been observed below 100 cm−1 in the ferroelectric phase of both hydrogen compounds which confirms the role of the proton tunnelling in the double minimum potential. PbDAsO4 seems not to be isomorphous to PbHAsO4 at room temperature.