Crystal structure evolution in the van der Waals vanadium trihalides

Most transition-metal trihalides are dimorphic. The representative chromium-based triad, CrCl₃, CrBr₃, CrI₃, is characterized by the low-temperature (LT) phase adopting the trigonal BiI₃-type while the structure of the high-temperature (HT) phase is monoclinic of AlCl₃type (C2/m). The structural transition between the two crystallographic phases is of the first-order type with large thermal hysteresis in CrCl₃and CrI₃. We studied crystal structures and structural phase transitions of vanadium-based counterparts VCl₃, VBr₃, and VI₃by measuring specific heat, magnetization, and x-ray diffraction as functions of temperature and observed an inverse situation. In these cases, the HT phase has a higher symmetry while the LT 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 ratioc/aof the unit cell parameters, types of crystal structures, and nature of the structural transitions in V and Cr trihalides are discussed.