Crystal structure evolution in the van der Waals transition metal trihalides

TL;DR

This study compares the structural phase transitions of vanadium-based and chromium-based transition-metal trihalides, revealing contrasting symmetry changes and hysteresis behaviors in their temperature-dependent crystal structures.

Contribution

It provides new insights into the differing structural transition mechanisms in V and Cr trihalides through experimental measurements and analysis.

Findings

VCl3, VBr3, VI3 have high-temperature phases with higher symmetry.

CrCl3, CrBr3, CrI3 exhibit low-temperature phases with lower symmetry.

V-based trihalides show no measurable hysteresis in phase transitions.

Abstract

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.