SPIN-ONE-ISING MODEL FOR (CO)(1-X) (N-2)(X) MIXTURES - A FINITE-SIZE-SCALING STUDY OF RANDOM-FIELD-TYPE CRITICAL PHENOMENA

A qualitative model for solid mixtures of diatomic molecules, where on e species (called CO, to be specific) carries both a dipole moment and a quadrupole moment, while the other species (called N-2) has only a quadrupole moment, is studied by Monte Carlo methods. We use spins S-i = +/- 1 to represent the orientations of the CO electric dipole momen t, if the lattice site i is taken by a CO molecule, while S-i = 0 if t he site is taken by an N-2 molecule. Assuming nearest-neighbor antifer roelectric interactions between CO molecules, and a bilinear dipole-qu adrupole coupling between CO and N-2, the randomly quenched N-2 molecu les act like random fields do in the random field Ising model. In prev ious work it was already shown that this crude model is in very good a greement with experimental data in two dimensions (adsorbed layers), w here the random fields induces a rounding of the transition. Here Mont e Carlo simulations of the three-dimensional version of this model are presented and analyzed with finite size scaling concepts. As expected from the theory, a behaviour qualitatively different from the two-dim ensional case is detected. The Monte Carlo data provide qualitative ev idence that the random field induces crossover to an universality clas s with critical exponents distinct from the pure Ising model, but it i s not feasible to us to study large enough systems that would allow a reliable estimation of these exponents. But the results show that dilu tion without dipole-quadrupole coupling has much less drastic effects on the critical behavior, and that in the presence of this coupling ve ry small impurity concentrations do indeed change the critical behavio r.