  Most transition-metal trihalides are dimorphic. The representative
chromium-based triad, CrCl3, CrBr3, CrI3, is characterized by the
low-temperature phase adopting the trigonal BiI3 type while the structure of
the high-temperature phase is monoclinic of AlCl3 type (C2/m). The structural
transition between the two crystallographic phases is of the first-order type
with large thermal hysteresis in CrCl3 and CrI3. We studied crystal structures
and structural phase transitions of vanadium-based counterparts VCl3, VBr3, and
VI3 by measuring specific heat, magnetization, and X-ray diffraction as
functions of temperature and observed an inverse situation. In these cases, the
high-temperature phase has a higher symmetry while the low-temperature
structure reveals a lower symmetry. The structural phase transition between
them shows no measurable hysteresis in contrast to CrX3. Possible relations of
the evolution of the ratio c/a of the unit cell parameters, types of crystal
structures, and nature of the structural transitions in V and Cr trihalides are
discussed.
