Phase Diagram of Lattice-Spin System RbCoBr₃

We study the lattice-spin model of RbCoBr$_3$ which is proposed by Shirahata and Nakamura, by mean field approximation. This model is an Ising spin system on a distorted triangular lattice. There are two kinds of frustrated variables, that is, the lattice and spin. We obtain a phase diagram of which phase boundary is drawn continuously in a whole region. Intermediate phases that include a partial disordered state appear. The model has the first-order phase transitions in addition to the second-order phase transitions. We find a three-sublattice ferrimagnetic state in the phase diagram. The three-sublattice ferrimagnetic state does not appear when the lattice is not distorted.