  The mean field theory is revisited in the classical and quantum mechanical
limits. Taking into account the boundary conditions at the phase transition and
the third law of the thermodynamics the physical properties of the ordered and
disordered phases were reported. The equation for the order parameter predicts
the occurrence of a saturation of $\Psi^2$ = 1 near $\Theta_S$, the temperature
below the quantum mechanical ground state is reached. The theoretical
predictions are also compared with high resolution thermal expansion data of
SrTiO$_{\text{3}}$ monocrystalline samples and other some previous results. An
excellent agreement has been found suggesting a universal behavior of the
theoretical model to describe continuous structural phase transitions.
