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[Tomohiro Takayama](https://orcid.org/0000-0002-3492-2025), Kenji Ishii, Sebastian Bette, Jürgen Nuss, Yosuke Matsumoto, Katrin Fürsich, Matteo Minola, Dita Puspita Sari, Isao Watanabe, Aleksandra Krajewska, [Robert Dinnebier](https://orcid.org/0000-0003-2778-2113), Bernhard Keimer, [Hidenori Takagi](https://orcid.org/0000-0001-5700-3761)

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[Non-magnetic ground state in A2WCl6, A = Cs, Rb, K: Face-centered cubic system with spin-orbit-entangled J = 2 states](https://mdr.nims.go.jp/datasets/52c0f03a-9547-45e2-b33e-d3ff58f548ba)

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npj | quantummaterials ArticlePublished in partnership with Nanjing Universityhttps://doi.org/10.1038/s41535-026-00945-wNon-magnetic ground state in A2WCl6, A =Cs, Rb, K: Face-centered cubic systemwith spin-orbit-entangled J = 2 statesCheck for updatesTomohiro Takayama 1,2,3 , Kenji Ishii4, Sebastian Bette1, Jürgen Nuss1, Yosuke Matsumoto1,Katrin Fürsich1, Matteo Minola1, Dita Puspita Sari5,6, Isao Watanabe6, Aleksandra Krajewska1,2,7,Robert Dinnebier 1, Bernhard Keimer1 & Hidenori Takagi 1,2,3,8Heavy 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 ofantifluorite-type tungsten chloride A2WCl6 (A = Cs, Rb, and K), comprising a face-centered cubiclattice of W4+ ions. The 5d2 configuration of W4+ ions yields a spin-orbit-entangled J = 2 state, whichsplits into a T2g triplet and a non-Kramers Eg doublet under a cubic crystal field. The spin-orbitmanifolds have been discussed to give rise to multipolar ordering such as charge quadrupolar ormagnetic octupolar ordering. We found that K2WCl6 undergoes a cubic-to-tetragonal structuraltransition which reconstructs the low-energy J = 2manifold, leading to a non-magnetic singlet groundstate. By contrast, Rb2WCl6 andCs2WCl6 showno signs of phase transition and remain non-magneticdown to the lowest temperature measured. At low temperatures, signatures of weak structuralanomalies were revealed, which may point to the presence of local distortions of the WCl6 octahedra.We argue that the subtle structural distortion arises from the local quadrupolar component, but thepresence of chemical disorder likely prevents the formation of long-range multipolar ordering.The interplay between strong spin-orbit coupling (SOC) and electron cor-relation, particularly that in heavy transition-metal compounds containing4d or 5d elements, has provided a plethora of novel quantum states ofmatter1–3. When 4d/5d elements are octahedrally coordinated by ligandanions, their d-electrons are accommodated in the t2g manifold due to thelarge cubic crystal field. The SOC of 4d/5d elements, which can be com-parable to or even stronger than the noncubic crystal field, even if it ispresent, entangles the spin and orbital degrees of freedom, yielding a spin-orbit-entangled Jeff state. The magnetic interactions between localized Jeffpseudospins can take formsdistinct fromthosebetween spin-onlymomentsbecause of the inherited orbital moments3,4. Unprecedented magneticground states arising from these interactions, such asKitaev spin liquid5 andexcitonic magnetism6, have been theoretically predicted to emerge, andmaterial realization has been intensively explored.When the number of d-electrons in the t2gmanifold is one or two, SOCforms Jeff = 3/2 or 2 state, respectively. These states have been proposed todisplay theorderingof electronicmultipoles suchas chargequadrupoles andmagnetic octupoles7–9. The face-centered cubic (FCC) lattice has been putforward as a platform for such multipolar ordering, and rocksalt-ordereddouble-perovskite oxides with a general formula of A2B’B”O6 (A: alkali-earth or lanthanoid ions, B’(B”): transition-metal ion or other cationscoordinated octahedrally by oxygen atoms) have been a particular focus inthis respect.When one of the B cations, B”, is non-magnetic, theirmagneticand electronic properties are governed by the FCC sublattice of B’ ions. In5d1 double-perovskite oxides hosting Jeff = 3/2 state like Ba2NaOsO6 andBa2MgReO6, a cubic-to-tetragonal structural transition has been observed,followed by long-range magnetic dipolar ordering10–12. The precedingstructural transition, which releases sizeable electronic entropy, has beendiscussed toderive from the formation of charge quadrupole ordering out ofthe Jeff = 3/2 manifold13,14.The FCC lattice of d2 ions was originally proposed to undergo analo-gous charge quadrupolar ordering from the Jeff = 2 manifold8,9. However,1Max Planck Institute for Solid State Research, Stuttgart, Germany. 2Institute for Functional Matter and Quantum Technologies, University of Stuttgart,Stuttgart, Germany. 3National Institute for Materials Science, Tsukuba, Japan. 4Synchrotron Radiation Research Center, National Institutes for Quantum Scienceand Technology, Sayo, Hyogo, Japan. 5Graduate School of Engineering and Science, Shibaura Institute of Technology, Saitama, Japan. 6Nuclear StructureResearch Group, RIKEN Nishina Center for Accelerator-Based Science, Wako, Saitama, Japan. 7ISIS Neutron and Muon Source, STFC Rutherford AppletonLaboratory, Chilton, Didcot, Oxon, UK. 8Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo, Japan. e-mail: TAKAYAMA.Tomohiro@nims.go.jpnpj Quantum Materials |           (2026) 11:83 11234567890():,;1234567890():,;https://doi.org/10.1038/s41535-026-00945-whttp://crossmark.crossref.org/dialog/?doi=10.1038/s41535-026-00945-w&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41535-026-00945-w&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41535-026-00945-w&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41535-026-00945-w&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41535-026-00945-w&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41535-026-00945-w&domain=pdfhttp://orcid.org/0000-0002-3492-2025http://orcid.org/0000-0002-3492-2025http://orcid.org/0000-0002-3492-2025http://orcid.org/0000-0002-3492-2025http://orcid.org/0000-0002-3492-2025http://orcid.org/0000-0003-2778-2113http://orcid.org/0000-0003-2778-2113http://orcid.org/0000-0003-2778-2113http://orcid.org/0000-0003-2778-2113http://orcid.org/0000-0003-2778-2113http://orcid.org/0000-0001-5700-3761http://orcid.org/0000-0001-5700-3761http://orcid.org/0000-0001-5700-3761http://orcid.org/0000-0001-5700-3761http://orcid.org/0000-0001-5700-3761mailto:TAKAYAMA.Tomohiro@nims.go.jpwww.nature.com/npjquantmatsmost of thed2 double-perovskites have been reported to display only a singletransition of long-range magnetic order or spin-glass freezing, and nocharge quadrupole ordering has been identified15,16. The five-foldJeff = 2 state is expected to split into a doublet Eg and a triplet T2g in thecubic environment because of the residual crystal field, i.e., by the admixtureof excited t2g1eg1 configuration induced by spin-orbit coupling3,17,18 (wedenote the d-orbitalmanifolds split by cubic crystalfield as t2g and eg in italiclower case, while the doublet and triplet out of the Jeff = 2 state are indicatedas Eg and T2g in upper case). The lower-energy Eg state is a non-Kramersdoublet carrying no magnetic dipole moment, but it does host a chargequadrupole andmagnetic octupolemoment, whichmay exhibit long-rangeordering at low temperatures. Although the Eg-T2g splitting was formerlyassumed to be smaller than themagnitudeof exchange interactions and thusnegligible8, the presence of this gap with a size of 10–20meV was experi-mentally confirmed by inelastic neutron scattering measurements ondouble-perovskite oxides with 5d2 Os6+ ions17. Notably, the osmates havebeen proposed to developmagnetic octupolar ordering from the Eg doublet.Ba2MOsO6 (M = Mg, Ca, Zn) displays a phase transition at ~50 K with asubtle anomaly in the magnetic susceptibility, accompanied by an entropychange close toRln2.Althoughmuon-spin precessionswere observed in thezero-field condition below the transition temperature, no magnetic Braggpeak was found in the neutron diffraction. Furthermore, powder X-raydiffraction and Raman spectroscopy measurements did not reveal anystructural distortions expected for a quadrupolar-ordered state17,19. Theformation of magnetic octupolar ordering has been advocated to fullyaccount for these observations17–21.Nevertheless, the ground state of d2 double perovskites remains underdebate. The appearance of magnetic octupole ordering was initially attrib-uted to the predominant ferro-octupolar coupling arising from interactionsinvolving the excited T2g manifold18, which was supported by the micro-scopic study using exact diagonalization and classical Monte Carlosimulation21. Pourovskii et al. performed many-body first-principles cal-culations and showed that octupolar ordering is formedby the predominantferro-octupolar interactionbetween theEg doublets22. In contrast, Khaliullinet al. argued from themicroscopic theory that the quadrupole interaction isstronger than octupolar coupling when the dominant d-electron hoppingprocesses are considered, and the formation of charge-quadrupole orderingshould bemore plausible23. Churchill andKee extended the investigation byincluding all the allowed exchange processes and showed that the Osdouble-perovskites lie at the verge of magnetic octupole and charge quad-rupole ordered states24. In contrast to the osmates, the cubic double-perovskite rhenate Ba2YReO6 with Re5+ (5 d2) ions undergoes long-rangemagnetic dipolar ordering, as revealed by polarized neutron scatteringmeasurements25. This is argued to arise from the lower-energy triplet withthe reversed energy hierarchy of Eg-T2g states20,26.A large body of previous discussions on the ground state of d2 double-perovskites suggests the presence of rich electronic phases in a subtle balanceof competing interactions. That is, fine-tuning of relevant electronic para-meters such as spin-orbit coupling, Hund’s coupling, and degree of d-phybridization may result in the formation of distinct multipolar phases. Wetherefore visited the antifluoriteK2PtCl6-type tungsten chlorideA2WCl6 (A=Cs,Rb,K),which comprises anFCC lattice ofW4+ (5d2) ions27,28. TheW4+ ionis octahedrally coordinated by Cl- ions in a perfect cubic environment. Thereduced hybridization between 5dW4+ andCl- ions, compared to those in 5doxides,may change the landscape ofmultipolar interactions. Previous studieson A2WCl6 [A = Cs, Rb] found no signs of magnetic ordering, although thenature of the magnetic ground state or electronic structure has not yet beenclarified29. Our resonant inelastic X-ray scattering (RIXS) measurementsindicate that the low-energy statesof the5d2 electronsofW4+ ionsderive fromthe spin-orbit-entangled J = 2 manifold, which provides a platform formultipole physics. Nevertheless, Cs2WCl6 and Rb2WCl6 do not show a clearsign of phase transition down to the lowest accessible temperature, whereasK2WCl6undergoes a structural transition to a tetragonalphase, likely inducedby steric instability. Nomagnetic orderingwas identified inA2WCl6 [A =Cs,Rb] in both thermodynamic andmuon-spin relaxation (μSR)measurements.Although A2WCl6 [A = Cs, Rb] appears to retain cubic symmetry at lowtemperatures, subtle signatures of structural distortion were discerned in ourlow-temperature Raman scattering and powder X-ray diffraction (PXRD)measurements. We speculate that the individualWCl6 octahedra are slightlydistorted at low temperatures and the degeneracy of the Eg doublet is lifted bylocal quadrupolar interaction, but the presence of chemical disorder mayhinder the formation of long-range multipolar ordering.ResultsElectronic structure and magnetic properties of A2WCl6(A =Cs, Rb, K)Theobtained sampleswere found tobenearly a singlephaseofA2WCl6witha small amount (1-2 wt%) of ACl impurity (see Fig. S1 for the PXRDpatterns). A2WCl6 crystallizes in a K2PtCl6-type cubic structure (spacegroup: Fm-3m, No. 227) at room temperature, and the WCl62- octahedraand A+ cations form an antifluorite-type arrangement27,28, where WCl6octahedra are isolated fromeachother (Fig. 1a).TheW4+ ions are placed in aperfect cubic environment and compose the FCC sublattice. This structurecanbe viewed as a rock-salt-ordereddouble-perovskite structurewithoneofthe B-site cations vacant. The W4+ ions have two 5d electrons that areaccommodated in the t2g manifold. The S = 1 and Leff = 1 state of the (t2g)2configuration in the presence of spin-orbit coupling is expected to form aJeff = 2 state in the LS-coupling scheme, which splits into the Eg doublet andthe T2g triplet under a cubic crystal field.To clarify the local electronic state, we performed W L3-edge RIXSmeasurements on single crystals of Rb2WCl6 and K2WCl6 and a poly-crystalline pellet of Cs2WCl6. Figure 1c shows the obtained RIXS spectrameasured with the incident X-ray energy Ei of 10.203 keV. The spectra ofCs2WCl6 and Rb2WCl6 were collected at T = 8K, and the data of K2WCl6were taken at room temperature, which is well above the structural transi-tion temperature discussed below. The energy resolution of the measure-mentswas about 120meV. In addition to the elastic signal, several electronicexcitations were clearly resolved around 0.4, 1.0, 1.9, and 3.2 eV for all thesamples. The observed peak did not show a visible momentum Q-depen-dence, indicative of the localized nature of the intra-atomic excitations. Thepeak intensities above 3 eV were enhanced when the energy of the incidentX-rays was increased to 10.206 keV, which means that they represent theexcitations between the t2g and eg manifolds of the W 5 d orbitals, namelythose across 10Dq (see Fig. S2 for the Ei- andQ-dependences and Fig. S3 forthe fitting of the RIXS spectra). The t2g-eg excitation shows a shoulder on thehigh-energy side, and its origin is unclear at the moment. We suspect thatthismaybe ascribed to the spin-conservingand spin-flip processes of the t2g-eg excitation or to the Jeff = 3/2–1/2 split of the final (t2g)1(eg)1 state, namely,(Jeff = 3/2)1(eg)1 and (Jeff = 1/2)1(eg)1 states.The peaks below 2.5 eV represent excitations within the t2g manifold.We consider a single-ion Hamiltonian including SOC in the cubic limit toclarify the relevant intra-atomic excitations. Figure1b illustrates the sche-matic energy diagram of the spin-orbital multiplet for the (t2g)2 configura-tion in theLS coupling scheme. The (t2g)2 electronic configuration splits into3P (S = 1, Leff = 1), 1D2(S = 0, Leff = 2), and 1S0 (S = 0, Leff = 0) states byHund’s coupling JH. By spin-orbit coupling ζ, the3P state further splits into3P2 (Jeff = 2), 3P1(Jeff = 1), and 3P0 (Jeff = 0) states. Figure 1d shows the energylevels of these spin-orbit multiples as a function of ζ, where JH is fixed at0.3 eV, a reasonable value for 5 d compounds30. The LS-coupling picture isvalid for JH » ζ, while the large ζ limit corresponds to the jj-coupling regimewhere two d-electrons are accommodated in the jeff = 3/2 manifold (Fig. S4shows the energy levels in the large ζ limit. Here, we denote the spin-orbitalmultiplets of LS-coupling scheme as Jeff with uppercase letters, whereasthose of the jj-coupling picture are expressed as jeff in lowercase). Thevertical axis represents the energy difference from the Jeff = 2 state. Here, forsimplicity, we did not consider the small splitting of the Jeff = 2 state by thecubic crystal field, which should be smaller than the experimental energyresolution. The peak positions in the RIXS spectrumof Rb2WCl6 are shownas red horizontal lines in the figure. The peaks at 0.95 and 1.85 eV can beattributed to the excitations from the Jeff = 2 state to the 1D2 and1S0 states,https://doi.org/10.1038/s41535-026-00945-w Articlenpj Quantum Materials |           (2026) 11:83 2www.nature.com/npjquantmatsrespectively. The one at 0.36 eV represents the excitation to the states cor-responding to the Jeff = 1 and0 states in the small ζ limit. By optimizing ζ andJH numerically to the peak positions, we obtained ζ ~ 305meV andJH ~ 310meV, indicating that A2WCl6 lies in the intermediate regionbetween the LS- and jj-coupling schemes, as in other 5 d compounds30–32.Despite being in the intermediate regime, the low-energy states of the 5d2configuration in A2WCl6 comprise five spin-orbit-entangled states, and wedenote them as the J = 2 state hereafter.As discussed in the Introduction, the J = 2 state is expected to split intothe T2g triplet and Eg doublet by the cubic crystal field, but this was notresolved in the RIXS spectra. The energy gap Δc between Eg and T2g, whichwas found to be 10-20meV for the Os double-perovskites17, is proportionalto ζ2/10Dq18. The ζ of W is close to that of Os, while the 10Dq of A2WCl6(~3.2 eV) is smaller than that of Os double-perovskite oxides (4-5 eV). Thepossible Δc of A2WCl6 should be similar in magnitude to that ofBa2MgOsO6, that is, a few tens of meV. This energy scale is well below theenergy resolution of current RIXS experiments, and the excitation, if any,could be masked by elastic scattering. Moreover, the Eg-T2g excitation wasnot identified in the powder inelastic neutron scattering (INS) spectra ofRb2WCl6 either (Fig. S5). The INS map is dominated by strong phononexcitations below 50meV, and no clear signs of magnetic excitations couldbe resolved. Nevertheless, we cannot exclude the cases where the Eg-T2gexcitation is present at a higher energy above 30meV but decays quicklywithQ or is strongly coupled with the phonon excitations and substantiallybroadened, making it difficult to resolve by powder INS.Magnetic and thermodynamic properties of A2WCl6The J = 2 state of A2WCl6 is expected to display transitions associated withthe ordering of charge or magnetic multipoles. Figures 2a and b show themagnetic susceptibility χ(T) and heat capacity C(T) measured for thepolycrystalline samples. χ(T) shows a Curie-Weiss-like increase uponcooling from room temperature, but no clear anomalies are seen down to2 K for all the A2WCl6 samples. The effective moment μeff and Curie-Weisstemperature θCWare 1.41μB/Wand -62K forK2WCl6, 1.46μB/Wand -51Kfor Rb2WCl6, and 1.39 μB/W and -46 K for Cs2WCl6, respectively, as esti-mated from the Curie-Weiss fit shown in the inset. The magnitude of μeff isclose to the value expected for the Jeff = 2momentwitha g-factor of 1/23, thatis, 1.22 μB. The deviation of μeff from that of an ideal Jeff = 2 is smaller inA2WCl6 than in the 5d2 double-perovskite oxides (μeff = 1.5–2.3 μB), mostlikely reflecting the reduction of d-p hybridization that otherwise suppressesthe orbital moment. Effective magnetic moments of nearly isolated para-magnetic ions have been well described by the Kotani model34, while somedeviations from the model have been found for A2WCl629. Our data forRb2WCl6 deviates from the Kotani model as well, especially at low tem-peratures (Fig. S6a), which may be attributed to the presence of inter-siteinteractions35 or to the non-Kramers Eg doublet state36,37.The heat capacity C(T) of Cs2WCl6 and Rb2WCl6 did not show anysignature of a phase transition down to 2 K as shown in Fig. 2b. No phasetransition was observed down to 50mK either in the measurements of theRb2WCl6 single crystals (Fig. 3a). Below 1 K, a broad hump of C(T)/T,centered at ~0.2 K in zero field, was observed, and the hump shifted tohigher temperatures when a magnetic field was applied. This suggests thatthe hump originates from defect spins. The entropy change associated withthe defect spins was estimated to be approximately 0.9 J/mol·K up to 2 K,which is about 15% of spin-1/2. Similar amounts of defects were also foundin the magnetization curve at low temperatures for the Rb2WCl6 andK2WCl6 polycrystalline samples, where the Brillouin function-like con-tributions, superposed on the paramagnetic background, were attributed toFig. 1 | Crystal structure and electronic propertiesof A2WCl6. a Crystal structure of A2WCl6 (A = Cs,Rb, and K) forming a cubic lattice with the spacegroup Fm-3m (No. 225). The crystal structure isvisualized using the VESTA software33. b Spin-orbital multiplet structure of d2 configuration in theLS-coupling scheme. The inset depicts the split ofJeff = 2 (3P2) state into the non-Kramers doublet Egand theT2g triplet with a gap ofΔc. cWL3-edgeRIXSspectra for A2WCl6. The energy of the incidentX-ray is 10.203 keV. The measurements on K2WCl6and Rb2WCl6 were performed on single crystals,while the polycrystalline pellet was used forCs2WCl6. The spectra for the former were collectedat the L point of the Brillouin zone. d Calculatedenergy levels for spin-orbit multiplets of (t2g)2 con-figuration as a function of spin-orbit coupling ζ. Thevertical axis shows the energy difference from theJeff = 2 (3P2) state. The Hund’s coupling JH is fixed at0.3 eV. The horizontal red dashed lines show theRIXS peak positions for Rb2WCl6, and the pinkshaded areas correspond to the width of eachexcitation peak.https://doi.org/10.1038/s41535-026-00945-w Articlenpj Quantum Materials |           (2026) 11:83 3www.nature.com/npjquantmatsthe defect spins. (Fig. S6). Such a sizeable number of defects may beattributed to the presence of quenched disorder such as A-site deficiencyand oxygen contamination29 (Fig. S7). In contrast, the C(T) of K2WCl6clearly indicates the appearance of a phase transition at 180 K (Fig. 2b). Asdiscussed below, this transition represents a cubic-to-tetragonal structuraltransition. The entropy change estimated to be ~1.3 J/mol·K through thistransition suggests the involvement of electronic degrees of freedom.To obtain further insights into the ground state, μSRmeasurements inthe zero-field condition (ZF-μSR) were performed on powder samples ofA2WCl6 (A =K, Rb, Cs). Figures 4a and b show the time dependence of theasymmetry parameter of the muon-spin polarization, A(t), at the selectedtemperatures forRb2WCl6 andK2WCl6, respectively. The data forCs2WCl6are presented in Fig. S8, and the data in the presence of the longitudinalfieldof 100 Oe are shown in Fig. S9. For all the samples, A(t) showed no oscil-latory signals or the recovery of the so-called “1/3-tail” down to the basetemperature of 2 K, indicating the absence of staticmagnetism such as long-range magnetic order or a spin-glass state. The absence of muon-spinprecession contrasts with theOsdouble-perovskites inwhich the oscillatoryresponse of ZF-μSR has been interpreted as the appearance of magneticoctupole ordering17,38. A(t) was fitted using a Lorentzian function A(t) =A0exp(-λ/t) + Abg, where A0 denotes the initial asymmetry at t = 0, and λrepresents themuon-spin relaxation rate. Abg is the background componentassociated with muons stopping at a silver sample holder and is assumedtemperature-independent. The spectra exhibited only weak relaxationwithin the accessible experimental time window, and a simple exponentialfunctionwas therefore adopted to characterize the temperature dependenceof the relaxation rate. The temperature dependence of λ is shown in Fig. 4c,d. λ(T) increases monotonically upon cooling from room temperature inboth samples. For K2WCl6, a small drop in λ(T) is seen around 180 Kwhereaphase transitionwas found inC(T).Althoughnoclear anomalywas seen inλ(T) of Rb2WCl6 (Fig. 4c) and Cs2WCl6 (Fig. S8), λ(T) appears to level offaround 100 K and then increases again upon further cooling. This behaviormay be related to structural instability discussed in the next section.Structural instability of Rb2WCl6The thermodynamic data presented above did not reveal any signature ofphase transitions in Rb2WCl6 down to 50mK. The ZF-μSR measurementexcludes the presence of magnetic dipolar or octupolar ordering in contrastto the double perovskite oxides like Ba2YReO6 and Ba2MgOsO616,17,25.Likewise, a spin-glass freezing of J = 2 moments is ruled out. The groundstate of Rb2WCl6 thus appears to be non-magnetic.One of the possible mechanisms leading to a non-magnetic groundstate is the appearance of charge quadrupole ordering out of the Egdoublet18,23,24 or from the J = 2manifold by distorting the lattice8. To explorethis point,we investigated the low-temperature crystal structure ofRb2WCl6using Raman spectroscopy and X-ray diffraction measurements. Figure 5ashows the Raman scattering spectra measured on the (111) surface of aRb2WCl6 single crystal. The upper figure shows the spectra obtained for theparallel geometry, where the incident and scattered light have the samepolarization, while the lower one shows the data from the crossed geometrywith orthogonal polarizations. The polarization direction of the incidentlight was arbitrary within the (111) surface. In the crystal structure ofFig. 3 | Specific heat of Rb2WCl6 single crystals. a Specific heat divided by tem-perature, C(T)/T, of Rb2WCl6 single crystals at low temperatures below 2 K, mea-sured at several magnetic fields. The magnetic field is applied parallel to the [111]direction. b Entropy change S estimated from the C(T)/T data. S is as large as 0.9 J/W-mol·K at 2 K at zero magnetic field.Fig. 2 | Magnetic and thermodynamic of A2WCl6 (A=Cs, Rb, K). a Temperaturedependence of magnetic susceptibility χ(T) and b specific heat C(T) of the poly-crystalline samples of A2WCl6. The inset of (a) shows the inverse of χ(T), and theblack dotted line delineates the Curie-Weiss fitting for the data of Rb2WCl6.https://doi.org/10.1038/s41535-026-00945-w Articlenpj Quantum Materials |           (2026) 11:83 4www.nature.com/npjquantmatsRb2WCl6 (space groupFm-3m), there are fourRaman-activemodes:A1g,Eg,and two T2g modes. Note that phonon modes are expressed in italicuppercase to distinguish from the spin-orbital manifold. Three phononpeaks were clearly observed in the spectra. Considering the Raman spectraof other antifluorite-type A2MCl6 compounds (M: transition-metal), thethree peaks at ~170, ~230, and~350 cm−1 can be assigned asT2g,Eg, andA1gmodes, respectively, which originate from the bending, elongation, andbreathing distortions of WCl6 octahedra, respectively29,39,40. A weak andasymmetric feature around 80 cm-1 likely originates from the other T2gmode involving the motion of Rb atoms39,40. There is another subtle peak at~270 cm−1 whose origin is not clear, possibly due to a disorder-activatedforbidden mode. The phonon modes associated with the WCl6 octahedradid not show any clear changes between 300 and 15 K in both the paralleland crossed geometries, suggesting that Rb2WCl6 retains cubic symmetry.Nevertheless, a weak signature of a structural anomaly was found in thetemperature dependence of the Raman spectra. Figure 5b shows a contourplot of the parallel-geometry data. Although no splitting of phonon peaks orappearance of new peaks is seen, the T2g phonon mode near 170 cm−1appears to broaden slightly upon cooling. The upper inset of Fig. 5a depictsthe full width at half maximum (FWHM) for the T2g phonon mode in theparallel geometry, obtainedbyfitting thepeakwith apseudo-Voigt function.The FWHM decreases on cooling from 300 K, but starts to increase below150 K. A similar broadening was observed for the crossed geometry (lowerinset of Fig. 5a). In contrast, the FWHM of the other two phonon modesdecreases monotonically upon cooling (Fig. S10). Such broadening ofphonon peaks may suggest the presence of local distortion despite theabsence of global structural transitions down to the base temperature.In agreement with the findings from Raman spectra, X-ray diffractionmeasurements displayed signatures of local distortion. Single-crystal X-raydiffraction (SCXRD)performed at 100K indicates a cubic structurewith thespace group Fm-3m, which is unchanged from room temperature(Table S2). The PXRD diffraction patterns also showed no clear signs ofstructural transition down to 12 K as shown in Fig. 6a. The temperaturedependence of the cubic lattice parameter a shown in the inset of Fig. 6adisplayed a monotonic decrease with conventional thermal contractionbehavior. Nevertheless, we found that some Bragg peaks, such as (400),broadened at low temperatures, as depicted in Fig. 6b. The FWHM of thispeak increased gradually upon cooling (Fig. 6c). For other Bragg reflections,the (220) peak showed a weaker broadening at very low temperatures(Fig. S11), while the (111) reflection did not show broadening upon cooling.This implies that specific local distortions grow in the lattice at low tem-peratures, although they are not coherent and do not induce a structuraltransition. Similar behaviorswere found inCs2WCl6, although the degree ofbroadening was weaker (Fig. S12).Singlet ground state of K2WCl6 induced by structural transitionIn contrast to Cs2WCl6 and Rb2WCl6, K2WCl6 displayed a clear sign of aphase transition around 180 K in the specific heatC(T). SCXRDandRamanscattering indicate that this transition involves a structural change from thecubic to a tetragonal phase. Figure 7a shows the crystal structure of K2WCl6at 100 K from the SCXRD. The refined structural parameters are presentedin Table 1. The WCl6 octahedra are elongated along the c-axis and rotatedabout the c-axis. The bond length between the W and apical Cl atoms (W-Clap) along the c-axis is 2.409(4)Å,whereas that in theab-planes (W-Cleq) is2.365(5) Å. The direction of rotation could not be refined satisfactorily,likely because of the presence of disorder associated with defects. TheRaman spectra showaclear splittingof theT2gphononmodenear 170 cm−1,and some new features are seen at 15 K, pointing to a symmetry lowering(Fig. 7b). The splitting of the T2g phonon mode occurred at the transitiontemperature of 180 K and grows gradually upon cooling (Fig. 7c).This structural transition is accompanied by a change in the electronicground state. TheW L3-edge RIXS spectrum at 8 K shows a sizeable spectralFig. 4 | Muon-spin relaxation measurements on A2WCl6 (A = Rb, K). a, b Temperature dependence of μSR time spectrum measured on the powder of Rb2WCl6 andK2WCl6. The solid lines show the fit of experimental data by the Lorentzian function. c, dTemperature dependence of muon-spin relaxation rate λ of Rb2WCl6 and K2WCl6.https://doi.org/10.1038/s41535-026-00945-w Articlenpj Quantum Materials |           (2026) 11:83 5www.nature.com/npjquantmatsweight in the low-energy region adjacent to the elastic scattering (Fig. 8a andits inset). This contribution can be explained by the presence of low-energyexcitation at ~80meV superposed on the elastic scattering (the solid red anddottedblack lines inFig. 8a, respectively), pointing to the reconstructionof thelow-energy states by the tetragonal distortion of the WCl6 octahedra. Fig-ure 8b shows the calculated energy levels of spin-orbital multiplets as afunction of the tetragonal crystal field. We employed SOC ζ = 0.3 eV andHund’s coupling JH = 0.3 eV based on the results for Rb2WCl6. By increasingthe tetragonal elongation distortion, the J= 2 manifold splits into the uppertwo doublets (Jz = ±2 and ±1) and the lower singlet (Jz = 0). The excitation ataround 80meV can be accounted for by the excitation from the ground-statesinglet to the Jz = ±2 doublet when the tetragonal crystal field is about130meV.We note that this tetragonal splitting of the J= 2manifold is muchlarger than the expected T2g-Eg splitting that is supposed to be around tens ofmeV. This indicates that the ground state of K2WCl6 is a non-magneticsinglet split from the J = 2manifold. An analogous low-energy excitation hasbeen observed in the distorted double-perovskite Sr2YReO6,where aRe5+ ionhas a 5d2 configuration, and theReO6octahedra are distorted already at roomtemperature in the way that they are elongated in the ab-plane (see Fig. S13).Sr2YReO6 shows no magnetic transition down to low temperatures41, con-sistent with the singlet ground state. We speculate that the finite magneticsusceptibility of K2WCl6 at low temperatures (Fig. S6c), as well as that ofSr2YReO641, could be ascribed to van Vleck paramagnetism associated withthe excitation from the Jz = 0 to Jz = ±1 and ±2.The structural transition thus results in a non-magnetic ground statein K2WCl6 by splitting the low-energy J = 2 manifold. We suspect thatFig. 5 | Raman scattering measurement on aRb2WCl6 single crystal. a Raman scattering spectrain the parallel (upper) and the crossed geometries(lower) of light polarizations. The measurementswere performed on the (111) surface of the singlecrystal, and the polarization of incident light was inan arbitrary direction within the (111) surface. Theupper inset displays the temperature dependence ofthe full width at half maximum (FWHM) of the T2gphonon mode at ~170 cm-1 in the parallel geometry.The lower inset shows the normalized intensities ofthe T2g phonon mode at 15 and 300 K in the crossedgeometry. A slight broadening is seen at 15 K.b Temperature dependence of Raman scatteringspectra in the parallel geometry displayed as a con-tour plot.https://doi.org/10.1038/s41535-026-00945-w Articlenpj Quantum Materials |           (2026) 11:83 6www.nature.com/npjquantmatsthis structural transition does not represent electronically inducedquadrupolar ordering. In fact, a similar structural transition has beenobserved in a number of A2MX6 compounds (X: halogen atom) when theA-site cation has a small ionic radius like K+, regardless of whether Mcations are magnetic or non-magnetic39,40,42,43. For instance, K2TaCl6displays a structural transition yielding elongated TaCl6 octahedra, incontrast to Rb2TaCl6 and Cs2TaCl6 where the tetragonal compression ofTaCl6 octahedra observed at low temperatures is attributed to the chargequadrupole ordering44. The structural instability of K2TaCl6 is associatedwith the size mismatch between the small K+ ions and TaCl6 octahedra.In line with the Ta chlorides, we speculate that the phase transitionobserved in K2WCl6 is not of electronic origin but is induced by stericinstability with small K+ ions.DiscussionsFor a 5d2 transition-metal ion in an octahedral coordination, spin-orbitcoupling entangles the spin andorbital degreesof freedomand formsa spin-orbit-entangled J = 2 manifold. Under a cubic crystal field, the J = 2 state issplit into the T2g triplet and the non-Kramers Eg doublet, where ordering ofelectronic multipoles such as charge quadrupole ordering or magneticoctupole ordering is expected to emerge17–24. 5d2 double-perovskite oxideshave been intensively studied in this regard, and the formation of magneticoctupole ordering was proposed in the double perovskite osmates17,although their ground state has not yet been firmly established.The tungsten chlorides A2WCl6 (A = Cs, Rb, and K) possess an elec-tronic configuration and FCC sublattice of W ions similar to those ofdouble-perovskite osmates. The formation of a low-energy J = 2 multipletFig. 6 | Powder X-ray diffraction (PXRD) mea-surement on Rb2WCl6 at low temperatures.a PXRD patterns at 12 and 295 K collected with Cu-Kα1 radiation. The data at 12 K is shifted verticallyfor brevity. There is no clear structural change downto 12 K. The inset depicts the temperature depen-dence of the cubic lattice parameter a. The dashedline is a guide for the eyes. b (400) Bragg reflection ofthe PXRD patterns at selected temperatures. Uponcooling, some broadening of the peak is seen.c Temperature dependence of full-width at halfmaximum (FWHM) for the (400) (upper) and (111)(lower) Bragg reflections. The increase in FWHM isseen for the (400) reflection on cooling from roomtemperature, while no broadening is found for the(111) peak.https://doi.org/10.1038/s41535-026-00945-w Articlenpj Quantum Materials |           (2026) 11:83 7www.nature.com/npjquantmatsstate was confirmed in A2WCl6 by RIXS, providing another platform formultipolar physics, although the Eg-T2g splitting has not been confirmedexperimentally yet.K2WCl6 displays a structural transition at~180 K,whichreconstructs the J = 2 manifold and yields a singlet ground state. We arguethat this transition is caused by structural instability owing to the sizemismatch between the K+ ion and WCl6 octahedra, rather than by chargequadrupolar interactions.By contrast, Rb2WCl6 and Cs2WCl6 did not show any signs of phasetransitions down to low temperatures. Although the W-Cl bonds shouldhave smaller degrees of hybridization and themagnitude of super-exchangeinteractions may be reduced compared to those of 5 d oxides, the appear-ance of octupolar ordering has been predicted at around 5.5 K for Cs2WCl6from first-principle-calculations36, and the multiple interactions should notbe negligibly small. Other factors like frustration, chemical disorder or bothmay prevent the formation of long-range multipolar ordering.The structural investigation at low temperatures on Rb2WCl6 revealedsignatures of local structural distortion despite the absence of a globalstructural transition. The (400) Bragg reflection in the PXRDmeasurementwas broadened upon cooling, while the (111) peak did not show visiblebroadening. This suggests a local tetragonal distortion, possibly associatedwith the distortion of the WCl6 octahedra. This distortion is unlikely to becaused by steric instability as in K2WCl6, because such distortions have notbeen observed in other 5 d FCC chlorides with larger Rb+ ions such asRb2TaCl6 and Rb2ReCl644,45.One of the possible origins of the weak structural anomaly is inci-pient quadrupole ordering. The quadrupolar interaction between Egdoublet has been shown to be predominantly antiferro-type, in contrastto the ferromagnetic octupolar one. The antiferro quadrupole interactionis frustrated on the FCC lattice23, and the presence of disorder, such asoxygen incorporation29 (Fig. S7) or A-site deficiency inferred from theACl impurities (Fig. S1), may impede the formation of long-rangequadrupolar ordering, leading to a quadrupolar glassy state. Consideringthe broadening of the phonon peak in the Raman scattering (Fig. 5a) andthat of PXRD Bragg reflections (Fig. 6b), local distortions may set inaround 100 K and gradually develop on further cooling. By scrutinizingthe heat capacity data (Fig. 2b), the C(T) of A2WCl6 [A = Rb, Cs] isslightly larger than that of K2WCl6 with quenched electronic degrees offreedom below ~100 K, which might reflect the gradual release of elec-tronic entropy, although quantitative evaluation is difficult because of thedifferent crystal structures.On the other hand, the ab initio calculation on Cs2WCl6 suggeststhat ferromagnetic octupolar interaction is predominant, as in the Osdouble perovskites36. Although ferromagnetic octupolar interaction is notfrustrated, the presence of disorder has been proposed to reduce thetransition temperature for the octupolar ordering46, which may smear outthe transition. In such a case, the signature of weak structural distortionmay be related to the local quadrupolar ordering at the W4+ ions in thevicinity of the defects, where quadrupolar coupling prevails over theoctupolar one. That is, only parts of W4+ ions near the defects areinvolved in the local distortion via quadrupolar ordering, unlike thequadrupolar glassy state discussed above.The balance between quadrupolar and octupolar interactions islikely very subtle in A2WCl6. The 10Dq of A2WCl6 [A = Rb, Cs] issmaller than that of Os double-perovskite oxides, and the Eg-T2g gap,which is in proportion to ~ζ2/10Dq, would be enhanced. This may reducethe ferro-octupolar coupling via the excited T2g state18, making theoctupolar coupling less pronounced compared to the osmate double-perovskites. In addition, coupling with phonons may strongly renor-malize the magnitude of multipolar interactions23,47,48. The coupling ofthe spin-orbital state with local lattice deformation, i.e., dynamic Jahn-Fig. 7 | Low-temperature crystal structure of K2WCl6. a Crystal structure ofK2WCl6 at 100 K analyzed by single-crystal X-ray diffraction measurement. TheWCl6 octahedra are elongated along the c-axis and rotated within the ab-planes. Theperiodic pattern of the WCl6 rotations was not determined precisely by the refine-ments, probably due to the presence of multiple domains associated with defects.The inset shows a close-up view ofWCl6 octahedra, showing theW-Cl bond lengthswith the apical (Clap) and with the equatorial (Cleq) chloride ions. b Raman scat-tering measurements on the (111) surface of a K2WCl6 crystal in the parallel andcrossed configurations. The splitting of phonon modes, together with the appear-ance of new peaks, indicates the symmetry lowering at low temperatures. c. Tem-perature dependence of Raman scattering spectra for K2WCl6 in the parallelgeometry measurement. The splitting of T2g phonon mode near 170 cm-1 is clearlyobserved.https://doi.org/10.1038/s41535-026-00945-w Articlenpj Quantum Materials |           (2026) 11:83 8www.nature.com/npjquantmatsTeller (JT) effect, has been discussed to augment the strength of quad-rupolar interactions23,47. The antifluorite-type structure of A2WCl6 has nointermediate cations between the WCl6 octahedra; the WCl6 is fullyisolated, unlike in Os double-perovskites Ba2MOsO6, where non-magnetic cations, such as Mg2+, Ca2+, or Zn2+, are bonded to the oxy-gen atoms of the OsO6 octahedra. This structural feature may promotecoupling with phonons more effectively than in double-perovskites,boosting quadrupolar interaction. The dynamic Jahn-Teller effect mayalso play a role in broadening the Eg-T2g excitation, making it difficult toobserve the excitation by INS. The estimate of multipolar exchangeinteractions incorporating the dynamic JT effect may be needed to unveilthe predominant interaction in A2WCl6.To summarize, the electronic, magnetic, and structural properties ofantifluorite tungsten chloride A2WCl6 (A = Cs, Rb, and K) were investi-gated. Rb2WCl6 and Cs2WCl6 showed no clear signs of phase transitiondown to the lowest accessible temperature. A close inspection of the low-temperature structure points to the presence of subtle structural distortions.It is tempting to infer that such a structural instability is associated with thequadrupolar component of the J = 2manifold.We suspect that the presenceof chemical disorder impedes the occurrence of long-range multipolarordering, leading to a non-magnetic ground state. To elucidate the nature ofthe ground state, local probes such as extended X-ray fine structure mea-surements, pair-distribution function analysis, or nuclear quadrupoleresonancemayprovide decisive information on the low-temperature crystalstructure. Alternatively, substantial improvement in sample quality maybring about the formation of long-range multipolar ordering. Finally, che-mical modifications, such as 4d counterparts A2MoCl6 or enhanced cova-lency like A2WBr6 and A2WI6, may give rise to distinct electronic groundstates, which should beworthy of future investigations to further explore therich multipolar physics of spin-orbit-coupled d-electrons.MethodsSample preparationPolycrystalline samples of A2WCl6 (A =Cs, Rb, and K) were synthesized via aconventional solid-state reaction. Stoichiometric amounts of ACl, W, andWCl6powderswereweighted andmixed in anAr-filled glovebox.Themixturewas pelletized and subsequently sealed in an evacuated quartz tube. The tubewasheatedat650 °Cfor72 hours. Single crystalsofRb2WCl6andK2WCl6weregrown via a chemical vapor transport reaction using polycrystalline powder.Single crystals of Cs2WCl6, which are large enough for thermodynamic andspectroscopicmeasurements,were not obtainedusing a similar procedure, andthus only the powder samples were used for the measurements.Structural analysisThe powder and single-crystal samples were characterized using X-ray dif-fraction. The powder X-ray diffraction measurements at room temperaturewere performed with a Bruker D2 Phaser (Cu Kα radiation), and the mea-surements at low temperatures were conducted with a Bruker D-8 Advancesystem using Cu Kα1 radiation from a primary Ge (111) Johannson mono-chromator. Oxford Cryosystem Phenix was used to change the temperatureof the sample. Single-crystal X-ray diffraction data were collected at 298 and100K using a SMART APEX-I CCD X-ray diffractometer (Bruker AXS,Karlsruhe, Germany) equipped with a Cryostream 700 Plus cooling device(Oxford Cryo-systems, Oxford, U.K.). Data collection and reduction werecarried out using theBruker Suite software package. The crystal structures arevisualized using the VESTA software33.Magnetic and thermodynamic measurementsThemagnetic and thermodynamicpropertieswere investigateddown to2 Kusing commercial equipment (Quantum Design MPMS3 and PPMS). TheTable 1 | Structural parameters of K2WCl6 at 100 K refined bysingle-crystal X-ray diffractionAtom Site g x y z Uiso (Å2)W 2a 1.0 0 0 0 0.0210(3)K 4d 1.0 0.5 0 0.25 0.0261(6)Cl1 4e 1.0 0 0 0.2356(3) 0.0244(6)Cl1 16l 0.5 -0.2054(6) -0.2806(6) 0 0.0260(7)The space group is I4/mmm (No. 139) and Z = 2. The lattice parameters are a = 6.8001(16) Å andc = 10.225(3) Å. The refinement indices are R = 5.42%, and GOF = 1.149.Fig. 8 | Change of electronic structure in K2WCl6 associated with the structuraltransition. aW L3-edge RIXS spectra of K2WCl6 at 8 K. The energy of the incidentX-ray was 10.203 keV, and the spectra were collected at the L-point [Q = (6.5, 6.5,6.5)] of the original cubic lattice. The black dotted line shows the contribution ofelastic scattering. The inset shows a magnified view near the elastic scattering,together with the spectrum collected at 300 K. For comparison, the two spectra werenormalized by the elastic line. Sizeable intensity is seen at low energies around 0.1 eVat 8 K, superimposed on the elastic scattering. The contribution can be explained by apeak centered around 80 meV shown as the solid red line in the main panel.bCalculated energy levels of the spin-orbitmultiplet as a function of the elongationaltetragonal crystal field Δtetra for a (t2g)2 ion. In the calculation, both spin-orbitcoupling ζ and Hund’s coupling JH are fixed at 0.3 eV based on the result forRb2WCl6. The J = 2 manifold splits into Jz = 0, ±1, and ±2 states by the tetragonalcrystal field. The Jz = 0 singlet is the ground state, and the low-energy peak around80 meV can be explained by the excitation between Jz = 0 and ±2 states withΔtetra ~ 130 meV.https://doi.org/10.1038/s41535-026-00945-w Articlenpj Quantum Materials |           (2026) 11:83 9www.nature.com/npjquantmatscontributions from the core diamagnetic susceptibilities49 have already beensubtracted in the data shown in Fig. 2a. The heat capacitymeasurements ontheRb2WCl6 single crystals at ultra-low temperatureswere performedusinga custom-built apparatus equipped with a dilution fridge50.Resonant inelastic X-ray scatteringTheW L3-edge RIXSmeasurements were carried out at BL11XUof SPring-8. Single crystals of Rb2WCl6 andK2WCl6 were used for themeasurements,whereas a polycrystalline sample was used for Cs2WCl6. The energy of theincidentX-raywas tuned to 10.203 keVwhich corresponds to the 2p3/2 to 5dt2g excitation energy of the W4+ ion. The incident X-rays were mono-chromatized by a Si(444) channel-cut monochromator, and the scatteredX-rays from the sample were analyzed by a Si(555) diced and sphericallybent monochromator. The total energy resolution determined by themeasurement of Kapton tape was about 120meV.The Re L3-edge RIXS measurements shown in Supplementary Infor-mation were performed at the same beamline. A polycrystalline pellet ofSr2YReO6 was used in this experiment. The incident X-rays were mono-chromatized by a 4-bounce Si(400) asymmetric monochromator, and thescattered X-rays were analyzed by a Si(119) diced and spherically bentanalyzer. The energy of the incident X-ray was tuned to 10.536 keV. Thetotal energy resolution was ~115meV.Raman spectroscopyRaman scattering measurements were performed on single crystals ofRb2WCl6 and K2WCl6 in the full back-scattering geometry. The incidentpolarization was in an arbitrary direction within the (111) surface of singlecrystals. The measurements were performed using a 632.8 nm He-Ne lasersource and a LabRAM HR800 spectrometer (HORIBA). The spectrum ateach temperature and geometry was collected with a counting time of120 seconds and 7 repetitions.Muon-spin relaxationMuon-spin relaxationmeasurementswere conductedonpowder samplesofA2WCl6 on ARGUS at ISIS Neutron and Muon Source51. The measure-ments were performed at zero field and with a longitudinal field of 100Oe.All the collected data were analyzed using the WIMDA software52.Inelastic neutron scatteringTime-of-flight inelastic neutron scatteringwas carried out using the thermalspectrometer MAPS at the ISIS Neutron and Muon Source53. A 6.7 gpolycrystalline sample was used for the experiment. Measurements wereconducted with the incident energy Ei = 100meV, 70meV, 50meV and20meV and chopper frequencies of 400Hz, 350Hz, 350Hz and 200Hz,respectively.Data availabilityThe datasets generated and/or analyzed during the current study are notpublicly available due to practical constraints associatedwith data archiving,but are available from the corresponding author upon reasonable request.Received: 4 June 2026; Accepted: 15 September 2026;References1. Witczak-Krempa, W., Chen, G., Kim, Y. B. & Balents, L. Correlatedquantum phenomena in the strong spin-orbit regime. Annu. Rev.Condens. Matter Phys. 5, 57 (2014).2. Rau, J. G., Lee, E. K. H. & Kee, H. Y. Spin-orbit physics giving rise tonovel phases in correlated systems: Iridates and related materials.Annu. Rev. Condens. Matter Phys. 7, 195 (2016).3. 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Spin-orbital excitation in d2 FCCmagnetRb2WCl6.STFC ISIS Neutron andMuon Source https://doi.org/10.5286/ISIS.E.RB2190103 (2021).AcknowledgementsWe are grateful to G. Khaliullin, N. Iwahara, G. Jackeli, and K. M. Kojima forinvaluable discussions. The synchrotron radiation experiment wasperformed at the BL11XU of SPring-8 with the approval of the Japan Syn-chrotronRadiationResearch Institute (JASRI) (ProposalNo. 2019B3555and2020A3555). We thank the RIKEN-RAL muon facilities for the allocation ofbeamtime on ARGUS at ISIS Neuron and Muon Source (Proposal No.RB2070012). We acknowledge the provision of beamtime onMAPS at ISISNeutronandMuonSource (ProposalNo.RB2190103). T.T., J.N.,Y.M.,A.K.and H. T. were supported by the Alexander von Humboldt Foundation fortheir research at the Max Planck Institute for Solid State Research, and T.T.was supported by the Izumi Science and Technology Foundation for theresearch at the National Institute for Materials Science.Author contributionsT.T. conceived the project, synthesized the samples, and conducted themagnetic and thermodynamic measurements. T.T. and K.I. collected theRIXS spectra and analyzed the data. S.B. and R.D. measured the low-temperature PXRD patterns and analyzed the data. J.N. conducted theSCXRD measurement and the analysis. Y.M. carried out the ultralow-temperature heat capacitymeasurements and analyzed the data. K.F.,M.M.and B.K. performed the Raman scattering measurements and analyzed thedata. D.P.S. and I.W. performed the μSR measurements and analyzed thedata. A.K. conducted the INS measurement and the analysis. H.T. super-vised the project. T.T. wrote the manuscript with input from all the authors.FundingOpen Access funding enabled and organized by Projekt DEAL.Competing interestsThe authors declare no competing interests.Additional informationSupplementary information The online version containssupplementary material available athttps://doi.org/10.1038/s41535-026-00945-w.Correspondence and requests for materials should be addressed toTomohiro Takayama.Reprints and permissions information is available athttp://www.nature.com/reprintsPublisher’s note Springer Nature remains neutral with regard tojurisdictional claims in published maps and institutional affiliations.Open Access This article is licensed under a Creative CommonsAttribution 4.0 International License, which permits use, sharing,adaptation, distribution and reproduction in anymedium or format, as longas you give appropriate credit to the original author(s) and the source,provide a link to the Creative Commons licence, and indicate if changeswere made. 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To view a copy of thislicence, visit http://creativecommons.org/licenses/by/4.0/.© The Author(s) 2026https://doi.org/10.1038/s41535-026-00945-w Articlenpj Quantum Materials |           (2026) 11:83 11https://doi.org/10.5286/ISIS.E.RB2070012https://doi.org/10.5286/ISIS.E.RB2070012https://doi.org/10.5286/ISIS.E.RB2070012https://doi.org/10.5286/ISIS.E.RB2190103https://doi.org/10.5286/ISIS.E.RB2190103https://doi.org/10.5286/ISIS.E.RB2190103https://doi.org/10.1038/s41535-026-00945-whttp://www.nature.com/reprintshttp://creativecommons.org/licenses/by/4.0/www.nature.com/npjquantmats Non-magnetic ground state in A2WCl6, A = Cs, Rb, K: Face-centered cubic system with spin-orbit-entangled J = 2 states Results Electronic structure and magnetic properties of A2WCl6 (A = Cs, Rb, K) Magnetic and thermodynamic properties of A2WCl6 Structural instability of Rb2WCl6 Singlet ground state of K2WCl6 induced by structural transition Discussions Methods Sample preparation Structural analysis Magnetic and thermodynamic measurements Resonant inelastic X-ray scattering Raman spectroscopy Muon-spin relaxation Inelastic neutron scattering Data availability References Acknowledgements Author contributions Funding Competing interests Additional information