Temperature dependence of soft mode frequency and loss tangent of ammonium iron alum

The pseudo spin-lattice coupled mode model has been modified by adding third- and fourth-orders phonon anharmonic interactions terms, with the help of double-time temperature dependent Green's function method expressions. The values of ferroelectric mode frequency, dielectric constant and loss tangent have been derived. Model values of physical quantities in above expressions have been fitted. Temperature dependence of above quantities has been calculated numerically. Theoretical results have been compared with experimental results of others. A good agreement has been found. (author)