The magnetic structure of clinopyroxene-type LiFeGe 2O 6 and revised data on multiferroic LiFeSi 2O 6

Abstract

The clinopyroxene compounds LiFeSi 2O 6 and LiFeGe 2O 6 have been investigated by constant wavelength neutron diffraction at low temperatures and by bulk magnetic measurements. Both compounds are monoclinic, space group P2 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 2O 6 shows a simple magnetic structure with no indication of an incommensurate modulation. The magnetic space group is P2 1/ 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 LiFeGe 2O 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 M1 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.