  An exactly solvable lattice model with infinite-range potential is applied to
uniaxial ferroelectrics. Asymptotically exact expression for free energy as a
function of an order parameter at any temperatures is obtained. Effect of
thermal expansion of lattice unit cell is taken into account. The free energy
expansion in powers of order parameter in the vicinity of critical point is
presented. Corrections to Landau expansion are obtained. In particular, it is
shown that summand with external field contains a contribution of higher powers
over order parameter.
  Keywords: Lattice model, Free energy, Phase transition, Long-range
interatomic potentials, Curie temperature, polarization
  PACS: 05.20.-y, 05.70.-a, 82.65.+r
