Neutron diffraction study of the crystal and magnetic structure of Rb2CrCl4: a two-dimensional ionic ferromagnet

The crystal and magnetic structure of Rb 2 CrCl 4 , a compound that exhibits two-dimensional ferromagnetism with nearly easy-plane behaviour, has been investigated using single-crystal neutron diffraction at 5.5, 77 and 300K. Selected Bragg reflections have also been examined at temperatures up to 573K. At all temperatures the structure is described by the space-group Cmca (D 2h 18 ), a subgroup of the K 2 NiF 4 structure I4/mmm. There is an antiferrodistortive displacement of the Cl - ions in the basal plane such that the CrCl 6 units are tetragonally elongated with principal axes alternately along (011) and (011) of the Cmca unit cell. At 5.5K the nearest-neighbour Cr-Cl distances are 2.700(7) and 2.387(6) AA within the basal plane and 2.371(22) AA parallel to the a axis. The ordered moments are canted alternately to one side and the other of the (010) or (001) axes of the Cmca cell by +or-(5.2+or-1.1 degrees ).