A NEW 1D-ANTIFERROMAGNET - CRYSTAL AND MAGNETIC-STRUCTURES OF TLMNF4.H2O

The synthesis, crystal structure, and magnetic properties of a new fluoromanganate (III) TlMnF4. H2O are investigated. Single crystals are obtained from 40% hydrofluoric acid at low temperature (5-degrees-C). The crystal structure is refined down to R/wR = 0.034/0.030 in the monoclinic C 2/c space group with the following unit cell constants: a = 1.3784(1) nm, b = 0.6631(1) nm, c = 1.0537 nm, beta = 103.66(1)-degrees (Z = 8). The hydrogen positions are determined from powder neutron diffraction data at 20 K. As isostructural RbMnF4.H2O, the compound is characterized by infinite zigzag chains of alternating [MnF6] and [Mn(H2O)2F4] octahedra sharing trans corners. The Jahn-Teller elongation is directed along the chain direction in the first type of octahedra, whereas it corresponds to the perpendicular Mn-O equatorial bond in [Mn(H2O)2F4] species. The magnetic properties can be fitted using the isotropic Heisenberg model for chains of finite S = 2 spins and are compared to those of RbMnF4.H2O. The corresponding exchange constants: J/k = -3.9[-4K] for Tl[Rb]MnF4. H2O are weaker than those generally observed in chain-type fluoromanganates (III) because of a low Mn-F-Mn bridging angle of about 138-degrees. A 3D magnetic ordering occurs below T(N) = 8.1(3)K [8.3(3)K] for Tl[Rb]MnF4. H2O. Along the chain an antiferromagnetic arrangement is observed with components on a- and c-axes. The total magnetic moments are M = 3.39(6)-mu-B for TlMnF4. H2O at 1.3 K and M = 3.46(6) mu-B for RbMnF4. H2O at 4 K.