The ternary sulfido chromate (II), K2[Cr3S4], was synthesized through a straightforward solid-state method as the first alkali metal chalcogenido chromate with the formal oxidation state +2, which was verified by X-ray absorption spectroscopy. Single-crystal diffraction analysis reveals the chromium ions to be coordinated by sulfur in two geometric arrangements: square planar and square pyramidal. Both environments are unusual for transition metal complexes with a d4 electron configuration. Structural distortions from the ideal arrangement are present in both coordination environments. Measurement of the magnetic moment indicates a value of 3.60 μB per chromium ion, which appears at first glance to contradict the standard ligand field theory. Quantum chemical calculations suggest high-spin states for both coordination geometries with a spin delocalization due to Cr–Cr interactions, leading to an intermediate-spin state with magnetic moment values very close to the experimental results, and attributing the structural distortions as the first example of the Jahn–Teller active d4 system with nonoctahedral coordination geometries. The optical, dielectric, and impedance measurement results indicate the potential as a synergic insulator, capacitor, and high-dielectric-constant material.
