  The interactions between the dielectric polarization and the fluctuations of
the strain (stress) tensor in relaxor ferroelectrics are shown to give rise to
the anisotropy of the anharmonic P4-term in the Landau-type free energy,
however, the harmonic P2-term is still properly described by the rigid
spherical random bond--random field model. These are the essential features of
the compressible spherical dipolar glass model, which is used to calculate the
singularities of the specific heat near field-induced critical points. The
results agree with recent high-resolution calorimetric experiments in PMN
[110].
