  Despite belonging to a well-studied family of transition metal trihalides,
VI_3 has received significant attention only recently. As a hard ferromagnetic
van der Waals compound with a large coercivity, it attracted much attention
because of its potential use in atomically thin spintronic and optoelectronic
devices. However, practical exploration of VI_3 is challenging due to its
instability under ambient conditions. Here, we present a comprehensive set of
optical, x-ray diffraction, magnetization, and ellipsometric measurements of
VI3 and demonstrate that, similarly to the related van der Waals ferromagnet
CrI_3, the degradation process is accelerated by the presence of moisture. The
VI_3 surface was covered by selected media commonly used in physical
measurements to test its stability and lower the degradation rate three times
or higher, providing practical information for experimentalists interested. The
decomposition study at ambient conditions shows that the VI_3 single crystal
can be used for most of the bulk, magnetization, and optical measurements
without any noticeable change of physical properties, as the significant
degradation appears first after ~ 2 hours of exposition as illustrated, e.g.,
by the evolution of the ferromagnetic T_1 and T_2 transitions. The
ellipsometric measurement demonstrates that even the surface remains optically
stable for at least 5 minutes.
