Vibrational Spectra, Hydrogen Bonding, and Ferroelectricity in the PbHPO₄ Family

The infrared and Raman spectra of polycrystalline PbHPO 4 , PbHAsO 4 , CaHPO 4 , and BaHPO 4 were measured at room temperature in the range between 4000 and 30 cm −1 , and the temperature dependence of the dielectric constant of these materials was studied. The OHO hydrogen bonds in PbHPO 4 and PbHAsO 4 are found to be very similar to those in KH 2 PO 4 and KH 2 AsO 4 and the XO 4 groups (X = P, As) are – in contrast to CaHPO 4 and BaHPO 4 – as well close to being tetrahedral. The symmetry of the soft mode in PbHPO 4 is deduced and it is found that the ferroelectric transition could be of second order as far as group theory is concerned. A microscopic model of the ferroelectric orderdisorder phase transition based on the interaction between the chains of OHO bonds linking the PO 4 groups is proposed.