NUCLEAR AND MAGNETIC-STRUCTURES OF THE DISTORTED HEXAGONAL PEROVSKITES ND4FECL3 AND NH4FECL3

The crystal structure of NH4FeCl3 and ND4FeCl3 is related to hexagonal perovskite: chains of face-sharing FeCl6 octahedra lie parallel to th e c-axis, giving quasi-one-dimensional magnetic properties. At high te mperatures the nonspherical ion NH4+ or ND4+ is fully disordered, and the structure may be modelled in space group P6(3) with a primitive ce ll of dimensions a x a x c, Powder neutron diffraction and single crys tal X-ray measurements show that on cooling the ammonium group becomes more ordered and the rest of the crystal structure distorts, with pha se transitions at 181 and 19.5 K. At 3.5 K the magnetic moments freeze to give long-range magnetic order with ferromagnetic correlation with in the chains and an antiferromagnetic array in the frustrated nb plan e. (C) 1998 Published by Elsevier Science B.V. All rights reserved.