Co-operative Jahn–Teller ordering in the crystal structure of Rb 2 CrCl 4 : a two-dimensional easy-plane ionic ferromagnet

Single crystal neutron diffraction at 77 and 5·5 K shows that the Cl atoms in the basal plane in Rb2CrCl4 are moved 0·16 Å from the midpoint of the line joining adjacent Cr atoms giving tetragonally elongated CrCl6 with principal axes alternately along [011] and [01] of the D2h18(Cmca) unit cell; this is the microscopic origin of the ferromagnetic exchange.