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Entropy-driven phase transitions in multitype lattice gas models

In multitype lattice gas models with hard-core interaction of Widom--Rowlinson type, there is a competition between the entropy due to the large number of types, and the positional energy and geometry resulting from the exclusion rule and the activity of particles. We investigate this phenomenon in four different models on the square lattice: the multitype Widom-Rowlinson model with diamond-shaped resp. square-shaped exclusion between unlike particles, a Widom-Rowlinson model with additional molecular exclusion, and a continuous-spin Widom-Rowlinson model. In each case we show that this competition leads to a first-order phase transition at some critical value of the activity, but the number and character of phases depend on the geometry of the model. Our technique is based on reflection positivity and the chessboard estimate.