Powder X-ray diffraction study of Cs2CoI4 from room temperature down to 40 K

Crystal structures of Cs2CoI4 at room temperature, 250 K, 200 K, 150 K, 100 K, 60 K in Phase I and at 40 K in Phase II are investigated by powder X-ray diffraction method. The beta-K2SO4-type structure goes on from room temperature down to the I-II phase transition point (51 K on cooling). The temperature dependence of lattice constants shows a difference between the a-axis direction and the other two axes ones. On the basis of the lattice constants at room temperature, the rates of contraction are about 0.5% in the a-axis direction and about 1% in the b- and c-axes ones, respectively. This is due to the interatomic distances of Cs2(1)-Co(1) and Cs2(2)-Co(2) almost parallel to the a-axis direction. The temperature dependence of these distances are in a reverse order each other. In Phase II, the most possible crystal-system is triclinic.