Crystal structures and phase transitions of the van der Waals ferromagnet VI3

Abstract

The results of a single-crystal x-ray-diffraction study of the evolution of the crystal structures of $\mathrm{V}{\mathrm{I}}_{3}$ with temperature with an emphasis on phase transitions are presented. Some related specific-heat and magnetization data are included. The existence of the room-temperature trigonal crystal structure $R\overline{3}$ (148) has been confirmed. Upon cooling, $\mathrm{V}{\mathrm{I}}_{3}$ undergoes a structural phase transition to a monoclinic phase at ${T}_{\mathrm{s}}\ensuremath{\sim}79\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. ${T}_{\mathrm{s}}$ is reduced in magnetic fields applied along the trigonal $c$ axis. When $\mathrm{V}{\mathrm{I}}_{3}$ becomes ferromagnetic at ${T}_{\mathrm{FM}1}\ensuremath{\sim}50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, 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.