Dielectric properties of ferroelectric methylammonium aluminium sulphate alum (MASD) crystal

By considering two sublattice pseudospin lattice coupled mode model Hamiltonian has been modified to include the extra spin-lattice interactions, the direct spin-spin coupling interaction, an applied electric field and the phonon anharmonic interactions for MASD alum. By using double-time thermal-dependent Green function method and modified Hamiltonian formulation, the expressions for soft mode frequency, dielectric constant and dielectric tangent loss properties have been derived and evaluated. Fitting the model values of various physical parameters in theoretical obtained expressions, the thermal variations of the above quantities have been calculated. Theoretical results compare well with experimental data of others on MASD alum.