Magnetic structures of the three-dimensional Heisenberg antiferromagnets K2FeCl5.D2O and Rb2FeCl5.D2O

The crystal and magnetic structures of K2FeCl5.D2O and Rb2FeCl5/D2O have been determined by neutron powder diffraction. The crystal structure of K2FeCl5.D2O has been redetermined using single-crystal diffraction. All compounds crystallize with the orthorhombic space group Pnma. The magnetic structure of both deuterated compounds can be described by ferromagnetic planes perpendicular to the b axis coupled antiferromagnetically to each other. The critical behaviour of the staggered magnetization below TN=14.2 K and 10.0 K for K2FeCl5.D2O and Rb2FeCl5.D2O, respectively, can be described by a power law for reduced temperature 0.03critical exponent beta =0.35+or-0.02.