Cr²⁺ in Square Planar Coordination: Durable and Intense Magenta Pigments Inspired by Lunar Mineralogy

To date, no terrestrial mineral has been reported to contain Cr2+ as one of its major components. However, the occurrence of Cr2+ in lunar phases has stimulated interest in investigating the mineral chemistry of inorganic compounds containing the divalent form of chromium. Furthermore, Cr2+ is similar to Mn3+ (the chromophore responsible for the intense color in YInMn blue (YIn1–xMnxO3)), and both are d4 and exhibit Jahn–Teller distortion, opening up the possibility for Cr2+ being a chromophore for designing pigments. Bulk samples of Ca1–xMgxCrSi4O10 containing Cr2+ were synthesized by high-temperature solid-state reaction methods under reduced oxygen fugacity (under a vacuum and/or N2/H2 gas mixture) in search of intense and durable pigments. The polycrystalline phases crystallize in a tetragonal crystal structure (space group: P4/ncc) and are isostructural to mineral gillespite (BaFeSi4O10) and Egyptian blue (CaCuSi4O10). Magnetic susceptibility measurements confirmed the presence of chromium in the divalent state. The Ca1–xMgxCrSi4O10 (x = 0–0.2) samples exhibit different shades of magenta, varying with Mg content. The optical spectra of the series are consistent with Cr2+ (d4) in square planar coordination with 5B1g as the ground state, corresponding to the configuration d1z2, d2xz, yz, d1xy, dx2–y2. The observed spectra can be attributed to electronic transitions from the 5B1g state to other quintet states of Cr2+: 5B2g, 5Eg, and 5A1g. The origin of the intense magenta color in the solid solutions is ascribed to the synergistic effect of d-d transitions allowed via vibronic coupling through simultaneous excitation of the u vibrational mode of the [CrO4] unit consisting of A2u, B2u, and Eu symmetries. The formation of intense reddish-magenta pigments with Cr2+ in square planar coordination opens the door to exploring novel pigments with divalent chromium in diverse coordination environments.