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 P21'/c 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. - Graphical abstract: Section of the nuclear and magnetic structure of the synthetic clinopyroxene-type compound LiFeGe2O6 displaying the antiferromagnetic coupling of spins within the chains of Fe3+O6 octahedra and the antiferromagnetic (via GeB sites) and ferromagnetic (via GeA sites) coupling between these chains.