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 T_N=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.