Crystal structures and phase transitions of the van der Waals ferromagnet VI3

The results of a single-crystal x-ray-diffraction study of the evolution of the crystal structures of VI3 with temperature with an emphasis on phase transitions are presented. Some related specific-heat and magnetization data are included. The existence of the room-temperature trigonal crystal structure R (3) over bar (148) has been confirmed. Upon cooling, VI3 undergoes a structural phase transition to a monoclinic phase at T-s similar to 79 K. T-s is reduced in magnetic fields applied along the trigonal c axis. When VI3 becomes ferromagnetic at T-FM1 similar to 50 K, magnetostriction-induced changes of the monoclinic-structure parameters are observed. Upon further cooling, the monoclinic structure transforms into a triclinic variant at 32 K which is most likely occurring in conjunction with the previously reported transformation of the ferromagnetic structure. The observed phenomena are preliminarily attributed to strong magnetoelastic interactions.