The magnetic structure of clinopyroxene-type LiFeGe2O6 and revised data on multiferroic LiFeSi2O6

The clinopyroxene compounds LiFeSi₂O₆ and LiFeGe₂O₆ have been investigated by constant wavelength neutron diffraction at low temperatures and by bulk magnetic measurements. Both compounds are monoclinic, space group P2₁/c and do not exhibit a change in nuclear symmetry down to 1.4 and 5 K respective. However, they transform to a magnetically ordered state below 20 K. LiFeSi₂O₆ shows a simple magnetic structure with no indication of an incommensurate modulation. The magnetic space group is P2₁/c′ and the structure is described by a ferromagnetic coupling of spins within the infinite M1 chains of edge-sharing octahedra, while the coupling between these M1 chains is antiferromagnetic. The magnetic phase transition is accompanied by magnetostriction of the lattice when passing through the magnetic phase transition. The magnetic structure of LiFeGe₂O₆ is different to the silicate: the space group is and the magnetic unit cell doubled along the a-direction. Within the M1 chains spins are coupled antiferromagnetically, while the chain to chain coupling is antiferromagnetic when coupling goes via the GeB tetrahedron and ferromagnetic when it goes via the GeA tetrahedron.