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

Abstract

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 $\Delta S\approx 5\mathrm{J}\cdot {\mathrm{K}}^{-1}\cdot \mathrm{mol}{f.u.}^{-1}$, 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.

23 pages, 9 figures, 4 tables in Inorg. Chem. Front., 2017

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