Powder and single-crystal neutron diffraction measurements were performed on RbCoBr3. The magnetic phase transitions of this compound are well explained by the Ising spin model for a distorted triangular lattice. Magnetic peaks of powder diffraction patterns at 34 and 20K closely fit a partial disordered phase and a ferrimagnetic phase, respectively. The temperature dependences of the (h, h, 1) scan profile and magnetic Bragg peak heights at (1, 1, 1), (1/3, 1/3, 1), and (2/3, 2/3, 1) were measured for a single-crystal sample. We conclude that the ferrimagnetic phase only exists below 30.5 K. The partial disordered phase is identified between 30.5 (= TN2) and 37.2K (= TN1). We found a characteristic linear increase in the (1, 1, 1) peak height below TN2 with decreasing temperature. These results show that a lattice distortion couples to the spin system. Numerical simulations are also performed to estimate magnetic interaction parameters.
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