  The results of a single-crystal X-ray-diffraction study of the evolution of
crystal structures of VI3 with temperature with 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 (148) has
been confirmed. Upon cooling, VI3 undergoes a structural phase transition to a
monoclinic phase at Ts ~ 79 K. Ts is reduced in magnetic fields applied along
the trigonal c-axis. When VI3 becomes ferromagnetic at TFM1 ~ 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.
