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

Abstract The clinopyroxene compounds LiFeSi 2 O 6 and LiFeGe 2 O 6 have been investigated by constant wavelength neutron diffraction at low temperatures and by bulk magnetic measurements. Both compounds are monoclinic, space group P 2 1 / 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. LiFeSi 2 O 6 shows a simple magnetic structure with no indication of an incommensurate modulation. The magnetic space group is P 2 1 / c ′ and the structure is described by a ferromagnetic coupling of spins within the infinite M 1 chains of edge-sharing octahedra, while the coupling between these M 1 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 LiFeGe 2 O 6 is different to the silicate: the space group is P 2 1 ′ / c and the magnetic unit cell doubled along the a -direction. Within the M 1 chains spins are coupled antiferromagnetically, while the chain to chain coupling is antiferromagnetic when coupling goes via the Ge B tetrahedron and ferromagnetic when it goes via the Ge A tetrahedron.

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