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

To date, no terrestrial mineral has been reported to contain Cr 2+ as one of its major components. However, the occurrence of Cr 2+ in lunar phases has stimulated interest in investigating the mineral chemistry of inorganic compounds containing the divalent form of chromium. Furthermore, Cr 2+ is similar to Mn 3+ (the chromophore responsible for the intense color in YInMn blue (YIn 1– x Mn x O 3 )), and both are d 4 and exhibit Jahn–Teller distortion, opening up the possibility for Cr 2+ being a chromophore for designing pigments. Bulk samples of Ca 1– x Mg x CrSi 4 O 10 containing Cr 2+ were synthesized by high-temperature solid-state reaction methods under reduced oxygen fugacity (under a vacuum and/or N 2 /H 2 gas mixture) in search of intense and durable pigments. The polycrystalline phases crystallize in a tetragonal crystal structure (space group: P 4/ ncc ) and are isostructural to mineral gillespite (BaFeSi 4 O 10 ) and Egyptian blue (CaCuSi 4 O 10 ). Magnetic susceptibility measurements confirmed the presence of chromium in the divalent state. The Ca 1– x Mg x CrSi 4 O 10 ( x = 0–0.2) samples exhibit different shades of magenta, varying with Mg content. The optical spectra of the series are consistent with Cr 2+ ( d 4 ) in square planar coordination with 5 B 1g as the ground state, corresponding to the configuration d 1 z 2, d 2 xz, yz, d 1 xy, d x 2 – y 2 . The observed spectra can be attributed to electronic transitions from the 5 B 1g state to other quintet states of Cr 2+: 5 B 2 g, 5 E g, and 5 A 1 g . 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 thro u gh simultaneous excitation of the u vibrational mode of the [CrO 4 ] unit consisting of A 2 u, B 2 u, and E u symmetries. The formation of intense reddish-magenta pigments with Cr 2+ in square planar coordination opens the door to exploring novel pigments with divalent chromium in diverse coordination environments.