Temperature dependence of dielectric constant and loss tangent in methyl ammonium aluminium alum

A model of pseudospin-lattice coupled mode for methyl ammonium aluminium alum (MASD) has been modified to include cubic and quartic phonon anharmonic interactions. With the help of double-time temperature dependent Green's function method and modified Hamiltonian, expressions for soft mode frequency, dielectric constant and loss tangent have been evaluated. By fitting model values of physical quantities in the theoretical expressions, temperature dependence of soft mode frequency, dielectric constant and loss tangent have been obtained and compared with experimental results of Pepinsky et al.(15), which show a good agreement.