Low temperature structures of the layered ferromagnet VI3

VI3 is of much topical interest as a 2-dimensional magnet but the evolution of the low temperature crystal structure is unclear. VI3 has R3 ̅ symmetry at 300 K and is reported to undergo a structural distortion at Ts = 79 K, ferromagnetic order at Tc1 = 50 K, and a further magnetic/structural transition at Tc2 = 32 K. We propose a high-resolution powder x-ray diffraction study to determine the crystal structure changes through these transitions. A thermally annealed VI3 powder will be used to minimise sample microstrain. Accurate knowledge of the structure is needed to help understand neutron spectroscopy and other measurements.