  A realistic theory of the quantum paraelectric - ferroelectric transition is
presented, involving parameters determined from band calculations and a
renormalization group treatment of critical fluctuations. The effects of
reduced dimensionality and deviations from cubic symmetry are determined.
Expressions for the pressure dependence of the critical temperature, as well as
pressure and temperature dependence of the specific heat are derived, and
evaluated for realistic materials parameters for the systems BaTiO3 and PbTiO3.
In these materials the ferroelectric soft mode dispersion apparently exhibits a
very strong cubic anisotropy, which affects results in an important, albeit
quantitative, manner. A change in order parameter orientation from (100) to
(111) is predicted as quantum criticality is approached.
