  Two-dimensional (2D) layered magnetic materials are generating a great amount
of interest for the next generation of electronic devices thanks to their
remarkable properties associated to spin dynamics. The recently discovered
layered VI$_3$ ferromagnetic phase belongs to this family, although a full
understanding of its properties is limited by an ill-defined crystallographic
structure. This is not any longer true. Here, we investigate the VI$_3$ crystal
structure upon cooling using both synchrotron X-ray and neutron powder
diffraction and provide structural models for the two structural transitions
occurring at 76 K and 32 K. Moreover, we confirm by magnetic measurements that
VI$_3$ becomes ferromagnetic at 50 K and discuss the difficulty of solving its
full magnetic structure by neutrons. We equally determined the magnetic
properties of our recently reported LiVI$_3$ phase, which is alike the
well-known CrI$_3$ ferromagnetic phase in terms of electronic and
crystallographic structures and found to our surprise an antiferromagnetic
behavior with a N\'eel temperature of 12 K. Such a finding provides extra clues
for a better understanding of magnetism in these low dimension compounds.
Finally, the easiness of preparing novel Li-based 2D magnetic materials by
chemical/electrochemical means opens wide the opportunity to design materials
with exotic properties.
