PHASE-DIAGRAM OF THE 5-VERTEX MODEL

Within the ice-type models, the solution of the five-vertex model is o btained with the use of the Bethe ansatz. Since the allowed number of vertex types is odd, the arrow-reversal symmetry of the system is brok en by construction. Due to this, the exact solution obtained and the p hase diagram are very different from those of the symmetric six-vertex model. A connection to the asymmetric six-vertex model (of which the five-vertex model is an extreme case) is made. The different regions o f the phase diagram are described and the transitions between them are analyzed. Several aspects of the phase diagram are unusual, i.e., the ordered phases (both ferroelectric and antiferroelectric) are frozen- in phases and the disordered phase is replaced by a ferrielectric phas e. In the free-fermion case, the known results of the modified potassi um dihydrogen phosphate model are recovered.