Non-magnetic ground state in A2WCl6, A = Cs, Rb, K: Face-centered cubic system with spin-orbit-entangled J = 2 states

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Heavy transition metal compounds with strong spin-orbit coupling have appeared as a platform for d -electron multipolar physics. We report the electronic, magnetic, and structural properties of antifluorite-type tungsten chloride A 2 WCl 6 (A = Cs, Rb, and K), comprising a face-centered cubic lattice of W 4+ ions. The 5 d 2 configuration of W 4+ ions yields a spin-orbit-entangled J = 2 state, which splits into a T 2g triplet and a non-Kramers E g doublet under a cubic crystal field. The spin-orbit manifolds have been discussed to give rise to multipolar ordering such as charge quadrupolar or magnetic octupolar ordering. We found that K 2 WCl 6 undergoes a cubic-to-tetragonal structural transition which reconstructs the low-energy J = 2 manifold, leading to a non-magnetic singlet ground state. By contrast, Rb 2 WCl 6 and Cs 2 WCl 6 show no signs of phase transition and remain non-magnetic down to the lowest temperature measured. At low temperatures, signatures of weak structural anomalies were revealed, which may point to the presence of local distortions of the WCl 6 octahedra. We argue that the subtle structural distortion arises from the local quadrupolar component, but the presence of chemical disorder likely prevents the formation of long-range multipolar ordering.