With the use of the modified pseudospin lattice coupled-mode (PLCM) model by adding third-order and fourth-order phonon anharmonic interactions terms and the statistical Green's function technique a phenomenological explanation of phase transition of the lead monohydrogen arsenate (PbHAsO4) crystal has been given. Expressions for shift, width, dielectric constant, loss tangent and modified ferroelectric soft mode frequency have been derived theoretically. By fitting model values of various

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... ities obtained from literature in these derived expressions, their temperature dependence has been evaluated numerically. Present theoretically results agree well with the experimental results of others.