PHASE-TRANSITIONS IN AN ISING-MODEL FOR MONOLAYERS OF COADSORBED ATOMS

A Monte Carlo method is used to study a simple S=1 Ising (lattice-gas) model appropriate for monolayers composed of two kinds of atoms on cubic metal substrates H=2 (where nn denotes nearest-neighbor and nnn next-nearest-neighbor pairs). The phase diagram is determined over a wide range of and T for KnnJnnn=14. For small (or negative) we find an antiferromagnetic 2×1 ordered phase separated from the disordered state by a line of second-order phase transitions. The 2×1 phase is separated by a line of first-order transitions from a c(2×2) phase which appears for larger. The 2×1 and c(2×2) phases become simultaneously critical at a bicritical point and the phase boundary of the c(2×2)disordered transition shows a tricritical point. © 1979 The American Physical Society.