A discussion of critical behaviour in a mixed-spin Ising model

The critical behaviour of a mixed and spin-1 Ising system is investigated by the use of the two-site cluster effective-field theory. The general expressions for evaluating the transition temperature and the tricritical point are obtained. Particular attention is drawn to the tricritical behaviour of the square lattice and it is shown that it strongly depends on the accuracy of calculations with a tendency to disappear when the accuracy is improved.