Phase Diagram of the Lattice Restricted Primitive Model

We present a comprehensive study of the lattice restricted primitive model, i.e., a lattice gas consisting of an equal number of positively and negatively charged particles interacting via on-site exclusion and a 1/r potential. On the cubic lattice, Monte Carlo simulations show a line of Neel points separating a disordered, high-temperature phase from a phase with global antiferromagnetic order. At low temperatures the (high-density) ordered phase coexists with the (low-density) disordered phase. The Neel line meets the coexistence curve at a tricritical point, T_t \simeq 0.14, rho_t \simeq 0.4. A simple mean-field analysis is in qualitative agreement with simulations.