Order-Disorder Transformation in RuBr3 and MoBr3: A Two-Dimensional Ising Model

Several trihalides (Cl, Br, I) of elements of groups 4, 5, 6, and 8 of the periodic table, such as TiI3, show a crystal structure made by parallel infinite columns, running along c, of face-sharing octahedra [MX6]. The metals placed within the columns may interact with one of the nearest neighbors, so that long and short MM distances regularly alternate along the columns; two distinct conformations are possible for the columns, one with M-M pairs centered at z = 1/4, the other with M-M pairs centered at z = 3/4. Below a certain temperature the distribution of the two types of columns in the a,b plane is ordered, giving rise to an orthorhombic form, with space group Pnmm. Over this temperature a transition to a hexagonal form, with space group P63/mcm, is observed. We, through X-ray diffraction studies carried out at different temperatures, estimate this transition temperature for RuBr3 and MoBr3 crystals and show that the hexagonal form too consists of ordered columns, the higher symmetry of that phase being due to a random distribution of the two types of columns in the a,b plane. We suggest that an equal spanning of the metals in the columns, as proposed by previous authors for the hexagonal phase, is never realized in any compound with the TiI3 structure type and present a possible mechanism of the order-disorder phase transition in this class of compounds and discuss it within the frame of a two-dimensional Ising model.