Structural, Electronic, and Magnetic Curiosities of\nan Unprecedented Chromate (II)

The ternary sulfido\nchromate (II), K<sub>2</sub>[Cr<sub>3</sub>S<sub>4</sub>], was synthesized\nthrough a straightforward solid-state\nmethod as the first alkali metal chalcogenido chromate with the formal\noxidation state +2, which was verified by X-ray absorption spectroscopy.\nSingle-crystal diffraction analysis reveals the chromium ions to be\ncoordinated by sulfur in two geometric arrangements: square planar\nand square pyramidal. Both environments are unusual for transition\nmetal complexes with a <i>d</i><sup>4</sup> electron configuration.\nStructural distortions from the ideal arrangement are present in both\ncoordination environments. Measurement of the magnetic moment indicates\na value of 3.60 μB per chromium ion, which appears at first\nglance to contradict the standard ligand field theory. Quantum chemical\ncalculations suggest high-spin states for both coordination geometries\nwith a spin delocalization due to Cr–Cr interactions, leading\nto an intermediate-spin state with magnetic moment values very close\nto the experimental results, and attributing the structural distortions\nas the first example of the Jahn–Teller active <i>d</i><sup><i>4</i></sup> system with nonoctahedral coordination\ngeometries. The optical, dielectric, and impedance measurement results\nindicate the potential as a synergic insulator, capacitor, and high-dielectric-constant\nmaterial.