MAGNETIC-STRUCTURES OF THE 3-DIMENSIONAL HEISENBERG ANTIFERROMAGNETS K2FECL5-CENTER-DOT-D2O AND RB2FECL5-CENTER-DOT-D2O

The crystal and magnetic structures of K2FeCl5 . D2O and Rb2FeC15 . D2 O have been determined by neutron powder diffraction. The crystal stru cture of K2FeCl5 . H2O has been redetermined using single-crystal diff raction. All compounds crystallize with the orthorhombic space group P nma. The magnetic structure of both deuterated Compounds can be descri bed by ferromagnetic planes perpendicular to the b axis coupled antife rromagnetically to each other. The critical behaviour of the staggered magnetization below T-N = 14.2 K and 10.0 K for K2FeCl5 . D2O and Rb2 FeCl5 . D2O, respectively, can be described by a power law for reduced temperature 0.03 < t < 0.2 by a critical exponent beta = 0.35 +/- 0.0 2.