  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.
