  The rotational instability of the electric polarization P during phase
transformations between ferroelectric phases is of great practical interest,
since it may be accompanied by extremely large values of the piezoelectric
coefficient, and a divergence of the coupled shear compliance contributes to
such enhancements. In the literature, this had been explicitly calculated in
the framework of the Landau theory and discussed with specific numerical
simulations involving tetragonal, orthorhombic and rhombohedral ferroelectric
phases. When monoclinic phases are involved, such an approach is practically
impossible, and an approximated treatment had been proposed, based on the
observation that in those cases there are shear strains almost linearly coupled
to the transverse component of P, implying a divergence of the Curie-Weiss type
in the associated compliances. Here the argument is extended to the general
case of transitions whose major effect is a rotation of the polarization, and
the limits of its validity are discussed. As experimental verification, the
elastic response of BaTiO3 is measured and analyzed, together with those of
other ferroelectric perovskites available in the literature, like KNN.
