Stabilisation of Paraelectric Phase by Strong Cubic and Quartic Phonon Anharmonicities in KDP Crystal

Abstract

Considering cubic and quartic phonon anharmonic interactions into the pseudospinphonon coupled mode model of Kobayashi6 expressions for shift, width, soft mode frequency and hence Curie temperature, dielectric susceptibility, Curie-Weiss constant and dielectric (tangent) loss have been evaluated for ferroelectric KDP. The method of retarded double-time thermal Green’s function and the treatment of Dyson’s equation have been used in the development. It is shown that cubic and quartic phonon anharmonic interactions render real value to the soft mode frequency in the para-phase. Results are in agreement with the experimental results of others.

Phonon energy renormalization in KDP‐type ferroelectrics

“…The pseudospin-phonon interaction causes a renormalization of the spin-spin interaction constant JO -+ Jen with which is now temperature dependent. From the latter equation it is evident that the anharmonicity parameter Ak will decrease the effective exchange coupling Jeff while the pseudospin-lattice coupling F k , Rk, and T k will increase its value in agreement with Upadhyay et al [19]. These observations for Ak and F k are in agreement with those of Ganguli et al [16].…”

“…Wesselinowa [18] used a Green's function technique to study the influence of the spin-phonon interactions on the transverse dynamical structure factor, which exhibits three peaks due to the coupling of the transverse soft mode to the acoustic phonon mode. Upadhyay et al [19] have considered the stabilization of the paraelectric phase by strong cubic and quartic phonon anharmonicities.…”

Phase Transition Dielectric Properties in Order-disorder Antiferroelectric NH4(H2PO4) (ADP) Crystal

“…Many authors 3,4 have experimentally investigated the dielectric properties of these crystals. The theory was developed to explain the ferroelectric transitions in displacive 41 and order-disorder 42 type crystals. In this paper, we have considered a four sublattice pseudospin lattice coupledmode model Hamiltonian by adding anharmonic phonon interactions, extra spin-lattice interactions, and direct spin-spin interaction has been used to obtain the theoretical expressions for the shift, width, soft mode frequency, dielectric constant and loss tangent.…”