ND4FEBR3, A NEW ONE-DIMENSIONAL S = 1 ANTIFERROMAGNET

Compounds of chemical formula AFeX3 (A = Rb,Cs,Tl, and X = Cl, Br) provide an isomorphous series of one-dimensional induced-moment magnets with an effective spin S = 1. They display a variety of ordered magnetic structures at low temperatures, depending on the sign and relative magnitudes of the intra- and interchain exchange and any magnetic dipole-dipole interactions. We report the first preparation of the salt NH4FeBr3. Powder neutron diffraction measurements on a perdeuterated sample showed that at room temperature the unit cell is hexagonal with cell parameters a = 7.398(3) angstrom and c = 6.331(2) angstrom. On cooling, structural phase transitions to cells of successively lower symmetry were observed. These are probably caused by cooperative reorientations of the ND4+ tetrahedra. At 4.8(2) K and 2.8(2) K additional Bragg peaks were observed. These additional reflections were attributed to magnetic long-range order with antiferromagnetic exchange both in the basal plane and along the c axis and could be indexed on an orthorhombic unit cell of dimension 2a X square-root 3a X c. Magnetic susceptibility measurements taken with a SQUID magnetometer on a polycrystalline sample at 2-100 K with an applied magnetic field of 0.1 T showed very similar behavior to that for RbFeBr3, with a broad cusp at approximately 8 K and theta congruent-to -43 K.