  We analyze the rhombic to square vortex lattice phase transition in
anisotropic superconductors using a variant of Ginzburg-Landau (GL) theory. The
mean-field phase diagram is determined to second order in the anisotropy
parameter, and shows a reorientation transition of the square vortex lattice
with respect to the crystal lattice. We then derive the long-wavelength elastic
moduli of the lattices, and use them to show that thermal fluctuations produce
a reentrant rhombic to square lattice transition line, similar to recent
studies which used a nonlocal London model.
