Structural transitions in interacting lattice systems

19 pages, 8 figures ; We consider two-dimensional systems of point particles located on rectangular lattices and interacting via pairwise potentials. The goal of this paper is to investigate the phase transitions (and their nature) at fixed density for the minimal energy of such systems. The 2D rectangle lattices we consider have an elementary cell of sides $a$ and $b$, the aspect ratio is defined as $\Delta=b/a$ and the inverse particle density $A = a b$; therefore, the "symmetric'' state with $\Delta=1$ corresponds to the square lattice and the "non-symmetric'' state to the rectangular lattice with $\Delta\ne 1$. For certain types of the interaction potential, by changing continuously the particle density, such lattice systems undertake at a specific value of the (inverse) particle density $A^*$ a structural transition from the symmetric to the non-symmetric state. The structural transition can be either of first order ($\Delta$ unstick from its symmetric value $\Delta=1$ discontinuously) or of second order ($\Delta$ unstick from $\Delta=1$ continuously); the first and second-order phase transitions are separated by the so-called tricritical point. We develop a general theory on how to determine the exact values of the transition densities and the location of the tricritical point. The general theory is applied to the double Yukawa and Yukawa-Coulomb potentials.