  Incorporating magnetism into two dimensional (2D) van der Waals (VdW)
heterostrutures is crucial for the development of functional electronic and
magnetic devices. Here we show that Nb3X8 (X = Cl, Br) is a family of 2D
layered trimerized kagom\'e magnets that are paramagnetic at high temperatures
and undergo a first order phase transition on cooling to a singlet magnetic
state. X-ray diffraction shows that a rearrangement of the VdW stacking
accompanies the magnetic transition, with high and low temperature phases
consistent with STEM images of the end members {\alpha}-Nb3Cl8 and
\b{eta}-Nb3Br8. The temperature of this transition is systematically varied
across the solid solution Nb3Cl8-xBrx (x = 0-8), with x = 6 having transitions
near room temperature. The solid solution also varies the optical properties,
which are further modulated by the phase transition. As such, they provide a
platform on which to understand and exploit the interplay between
dimensionality, magnetism, and optoelectronic behavior in VdW materials.

Materials Science