Effective-field theory identity for studying first order phase transitions of Ising-like spin models

Abstract An effective-field theory identity (EFTI) for studying first order phase transitions of Ising-like spin models is proposed. Via an analytical study, this identity can be used to obtain the phase transitions between two distinct states that presents the same free energy value. In order to show the results obtained by the application of the EFTI, the phase diagram of the spin-1 Blume-Capel model is constructed for the hexagonal, square and cubic lattices. For the cubic lattice, some magnetizations and quadrupoles curves are also presented.