X-Ray Study on the Successive Phase Transitions of K₂CoCl₄

The structural phase transition of K 2 CoCl 4 is investigated by means of X-ray scattering. The transition temperatures are 558 K, 454 K and 142 K. With decreasing temperature, the symmetry of the crystal changes as follows: phase I, normal [ Pmcn ]; phase II, incommensurate [ q =(1/3-δ) c * ]; phase III, commensurate [ P 2 1 c n , q = c * /3]; and phase IV, commensurate [ C 1 c 1, q = a ^*/2+ b * /2+ c * /3]. Here the reciprocal vectors refer to the normal phase. In a narrow temperature range just above 142 K, the superreflection at (5/2 3/2 0) splits into two diffuse peaks. The general feature of the successive transitions is very similar to K 2 ZnCl 4 .