This paper presents a quasiharmonic model for the lattice dynamics of the (Formula presented) crystal. The model, constructed as a combination of the valence force field and the polarizable ion model with a majority of potential parameters fixed by auxiliary Hartree-Fock calculations, enables us to calculate vibrational spectra for both the paraelectric and the ferroelectric structure. The calculated eigenvector of the lowest frequency optic mode corresponds to the order parameter of the ferroelectric phase transition and can be identified with the symmetry-breaking soft mode. Interpretation of the obtained results and their comparison with available experimental data is provided.