Rearrangement of Van-der-Waals Stacking and Formation of a Singlet State at T = 90 K in a Cluster Magnet

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.