Rearrangement of van der Waals stacking and formation of a singlet state at T = 90 K in a cluster magnet

Abstract

Insulating Nb₃Cl₈ is a layered chloride consisting of two-dimensional triangular layers of S_eff = 1/2 Nb₃Cl₁₃ 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 ΔS ≈ 5 J K⁻¹ mol⁻¹ 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 Pm1 to monoclinic C2/m symmetry, with a change in layer stacking from –AB–AB– to –AB′–BC′–CA′– 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.