At sufficiently low density, the kinetic energy of electrons is overwhelmed by Coulomb repulsion, forming a triangular Wigner crystal lattice.
The Wigner parameter r_s = a/a_B (ratio of inter-particle spacing to Bohr radius) controls the crystal transition. Wigner crystal forms for r_s ≳ 37 (2D). Phonon modes (plasmons at long wavelength, ω ~ √q) become soft at melting.