  Solidification as a first order phase transition is described in the Landau
theory by the same equation as tricritical phenomena. Here, the solidification
or melting temperature against pressure curve is modelled to end at a
tricritical point. The model gives the phase transition temperature's
dependence on pressure up to the quadratic term with a definite expression for
the coefficients. This formula is expected to be generally valid for pure
materials having melting curves with dT/dP approaching zero at very high P.
Excellent experimental agreement is obtained for iron, the material having the
most high pressure data which rather accurately determines the value of the
coefficient defining the curvature. The geophysically interesting iron
solidification temperatures at the Earth's core pressures are obtained. In
addition, the general formulae for entropy change, latent heat and volume
contraction in solidification are found and calculated for iron as functions of
pressure and temperature.
