The composition dependence of soft mode in Pb_xSr_1-xTiO₃ (PST) single crystals has been theoretical investigated in para-electric phase for various compositions. Using model Hamiltonian [ For pure anharmonic crystal in presence of defect (impurity)] and Green's functions technique. Soft mode is estimated for various values of x. As x increases value of soft mode frequency decreases at constant temperature and as T increases the value of soft mode frequency increases at constant x.

Keywords: Model Hamiltonian, Green's Functions Technique, Soft Mode Frequency, Anharmonic Crystal, Single Crystal.