  A density dependent relativistic mean-field model is determined to reproduce
the components of the nucleon self-energy at low densities. This model is used
to investigate spinodal instabilities in isospin asymmetric nuclear matter at
finite temperatures. The inhomogeneous density distributions in the spinodal
region are investigated through calculations in a cubic Wigner-Seitz cell.
Compared to results obtained in phenomenological calculations the spinodal
region is large, i.e. the spinodal region at zero temperature can reach
densities above 0.12 fm$^{-3}$. The predicted spinodal region is concentrated
around isospin symmetric nuclear matter and the critical temperature is
considerably lower than in the previous microscopic based investigation within
a non-relativistic Brueckner-Hartree-Fock approach.
