Structural and magnetic phase transitions in the synthetic clinopyroxene LiCrGe₂O₆: a neutron diffraction study between 0.5 and 1473 K

The pyroxene-type compound LiCrGe₂O₆, the Li- and Ge-analogue to the silicate mineral kosmochlor, has been synthesized at 1373 K and investigated by neutron diffraction between 0.5 and 1473 K in order to investigate the variation in magnetic and crystal structure with temperature. A structural phase transition from a low-temperature P2₁/c to a high-temperature C2/c structure was found around 1140 K. The two different structures exhibit different thermal expansion behavior with temperature with a reversal of the largest thermal expansion from the c-axis to the b-axis in the P2₁/c and C2/c phase, respectively. The structural phase transition is accompanied by a large volume increase of 1.9 % and sharp discontinuities in bond lengths, especially for the Li-O and - to a lesser extent - for the Cr-O bonds. At low temperature, some additional nonlinear changes in lattice parameters occur, which are associated with a magnetoelastic couplings of the lattice. Magnetic ordering is observed below 6 K in the neutron diffraction data. Data could be indexed with k = (0 0 0), giving rise to magnetic space group P2₁'/c. This model of the magnetic structure has a pure antiferromagnetic arrangement of spins, both within and between the M1 chains. The spins are oriented within the a-c plane with an almost nil component along [0 1 0]. A shift of the Cr atom out of the center in the equatorial plane of the octahedron is observed below 6 K and is associated with the magnetic phase transition. (orig.)