Green's function theory of phase transition in PbHPO₄ and PbDPO₄ ferroelectrics

Abstract

In this paper, we investigated the newly found order-disorder ferroelectrics PbHPO₄ along with its deuterated analogue PbDPO₄. Because the major contribution of the spontaneous polarization is the dipoles forming by the proton (or deuteron) ordering, not the displacements of heavy atoms as in the case of KDP-type crystals. Therefore in the Hamiltonian, the influence of the lattice vibration may not be considered, but the 4th order interaction of pseudo-spin is included. By introducing the undetermined-constant into the Green's function method, we carried out the spontaneous polarization of PbHPO₄ and PbDPO₄ crystals, the results are good agreement with the experimental data.