Contribution of soft zone-center transverse optical phonons to the shift and width of sharp line electronic transitions in ferroelectrics

The contribution of soft zone-center transverse optical phonons to the shift and width of sharp line electronic transitions in ferroelectrics is assessed. It is pointed out that the soft modes may be important for transitions involving ions at sites lacking inversion symmetry, where a direct coupling to the displacement field is allowed, but are probably not important for ions at centro-symmetric sites where the coupling to the long-wavelength modes is through the strain field.