Magnetic and crystal phase transitions in KNiCl3

Neutron scattering measurements revealed that even a seemingly perfect crystal of KNiCl3 at room temperature may segregate into two phases with different lattice distortions in the basal ab plane at temperatures below T≊270 K. In phase A, distortion leads to an increase of the unit cell size by a factor of 3, giving rise to nuclear Bragg peaks of type (h/3,h/3,l). In phase B the distortion gives rise to Bragg peaks at (3h/4,0,l), with integer h and l. Two different magnetic structures have been observed with TN=12.5 and 8.6 K in phases A and B, respectively. Magnetization measurements confirm the values of TN found by neutron scattering for the two magnetic structures.