Magnetic structures of the M2TbF6 (M=Li, K, Rb) fluorides: A complex behavior resulting from frustration

Neutron powder diffraction has been performed on Li2TbF6, K2TbF6 and Rb2TbF6 fluoroterbates. Incommensurate long-range magnetic order is observed below TN=2.02, 1.60 and 2.07 K. The square-modulating of the magnetic structures can be correlated with the geometric frustration induced by the pseudo-hexagonal packing of the [TbF6]2− chains in these hexafluorides. This frustration and the magnetic interactions are discussed on the basis of experimental data and topological considerations. The magnetic structures encountered in this series, and the particular thermal evolution of the Li2TbF6 magnetic structure may result from the competition between the magnetic interactions taking place in the chains and the magnetic interactions coupling the chains.