Non-magnetic ground state in A₂WCl₆ (A = Cs, Rb, K): A face-centered cubic system of spin-orbit-entangled J = 2 states

Heavy transition metal compounds with strong spin-orbit coupling 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 in a cubic environment yields a spin-orbit-entangled $J$ = 2 state, which has 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 lifts the degeneracy of the $J$ = 2 state, 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 of the $J$ = 2 state but the frustrated quadrupolar interaction, together with chemical disorder, inhibits the formation of long-range quadrupolar ordering.