Critical properties of the
q-state Potts and Potts-lattice-gas models are calculated by means of the Kadanoff variational renormalization group for dimensions 1.58, 2, and 2.32, and the Migdal approximation for arbitrary dimensions. Accurate results for the critical and tricritical exponents are obtained as well as for
qc, the largest value of
q for which the Potts transition is continuous. It is concluded unambiguously that the three-state Potts model in three dimensions undergoes a first-order phase transition.
Based on the connection between a sublattice dilute q-state Potts model (SDQPM) and a correlated percolation model, we define a nonpercolating geometrical factor
gf and a percolating geometrical factor
gp, which depend only on q and geometrical properties of the system. The thermal properties of the SDQPM may be derived simply from
gf and
gp. The theory is confirmed by an exact calculation for one-dimensional SDQPM and may be extended to other interacting systems.