Crystal structure evolution in the van der Waals vanadium trihalides.

Most transition-metal trihalides are dimorphic. The representative chromium-based triad, CrCl 3 , CrBr 3 , CrI 3 , is characterized by the low-temperature (LT) phase adopting the trigonal BiI 3 -type while the structure of the high-temperature (HT) phase is monoclinic of AlCl 3 type ( C 2/ m ). The structural transition between the two crystallographic phases is of the first-order type with large thermal hysteresis in CrCl 3 and CrI 3 . We studied crystal structures and structural phase transitions of vanadium-based counterparts VCl 3 , VBr 3 , and VI 3 by 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 CrX 3 . 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.