Crystal Structures and Phase Transitions of the van-der-Waals Ferromagnet VI3

TL;DR

This study investigates the temperature-dependent crystal structures and phase transitions of VI3, revealing structural changes associated with magnetic ordering and magnetoelastic interactions through X-ray diffraction and thermodynamic measurements.

Contribution

It provides detailed insights into the structural phase transitions of VI3, including the identification of monoclinic and triclinic phases and their relation to magnetic properties, which was not previously characterized.

Findings

VI3 has a room-temperature trigonal structure R-3.

A structural phase transition occurs at ~79 K to a monoclinic phase.

Further cooling leads to a triclinic phase at 32 K associated with magnetic changes.

Abstract

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.