Three mixed crystals, κ-(ET)2Ag2x Cu2(1–x)(CN)3 [ET is bis­(ethylenedithio)­tetrathiafulvalene; 0.24 < x < 0.71] with a κ-type packing motif of face-to-face ET dimers, were obtained by electrocrystallization. Regardless of the composition, each ET dimer fits into a hexagonal anionic opening (i.e., key-on-hole packing) similar to its parent spin liquid candidate, κ-(ET)2Cu2(CN)3. X-ray diffraction and energy dispersive spectroscopy analyses revealed that Cu and Ag atoms are statistically disordered with a fairly homogeneous distribution in a crystal. A structural variation depending on x is responsible for the change in the calculated band parameters related to intermolecular interactions, electron correlations, and frustrations. A salt with nearly equimolar amounts of Ag and Cu (x = 0.49) is semiconductive at ambient pressure and undergoes a Mott transition upon application of hydrostatic pressure. Along with the positive pressure dependence of the transition temperature, the temperature-independent amplitude of magnetic torque at low temperatures suggests that the insulating phase is a quantum spin liquid. Further application of pressure results in the appearance of a superconducting phase. Contrary to those of the parent salts, κ-(ET)2Cu2(CN)3 and κ-(ET)2Ag2(CN)3, the transition temperature increases as the pressure increases and eventually reaches 4.5 K at 1.65 GPa.

We report the synthesis, crystal and band structures, and transport properties of organic conductor κ-(ET) 2 Cu[Au(CN) 2 ]Cl [ET = bis(ethylenedithio)tetrathiafulvalene], which has a triangular spin−lattice (S = 1 / 2 ) composed of (ET) 2•+ dimers and polyanions with no disorder. The anisotropy of triangular lattice t′/t = 1.19 and physical properties indicate that this material is the first ET-based quantum-spin-liquid candidate having a nearly regular triangular lattice with a disorder-free anion.

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