  Insulating Nb$_3$Cl$_8$ is a layered chloride consisting of two-dimensional
triangular layers of $S_{eff}$ = 1/2 Nb$_3$Cl$_{13}$ clusters at room
temperature. Magnetic susceptibility measurement show a sharp, hysteretic drop
to a temperature independent value below $T = 90$ K. Specific heat measurements
show that the transition is first order, with $\Delta S \approx 5\ \mathrm{J
\cdot K^{-1} \cdot mol\ \mathit{f.u.}^{-1}}$, and a low temperature $T$-linear
contribution originating from defect spins. Neutron and X-ray diffraction show
a lowering of symmetry from trigonal $P\bar{3}m1$ to monoclinic $C2/m$
symmetry, with a change in layer stacking from -AB-AB- to
-AB$^\prime$-BC$^\prime$-CA$^\prime$- and no observed magnetic order. This
lowering of symmetry and rearrangement of successive layers evades geometric
magnetic frustration to form a singlet ground state. It is the lowest
temperature at which a change in stacking sequence is known to occur in a
Van-der-Waals solid, occurs in the absence of orbital degeneracies, and
suggests that designer 2-D heterostructures may be able to undergo similar
phase transitions.
