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 (YIn_1–xMn_xO₃)), and both are d⁴ and exhibit Jahn–Teller distortion, opening up the possibility for Cr²⁺ being a chromophore for designing pigments. Bulk samples of Ca_1–xMg_xCrSi₄O₁₀ 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 Ca_1–xMg_xCrSi₄O₁₀ (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 ⁵B_1g as the ground state, corresponding to the configuration d_¹z², d²_xz, yz, d_¹xy, d_x²–y². The observed spectra can be attributed to electronic transitions from the ⁵B_1g state to other quintet states of Cr²⁺: ⁵B_2g, ⁵E_g, and ⁵A_1g. 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 A_2u, B_2u, and E_u 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.

中文翻译：

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方形平面配位中的 Cr2+：受月球矿物学启发的耐用且浓郁的洋红色颜料

迄今为止，还没有报道称陆地矿物含有 Cr ²⁺作为其主要成分之一。然而，月相中 Cr ²⁺的出现激发了人们对研究含有二价铬的无机化合物的矿物化学的兴趣。此外，Cr ^{2+与 Mn 3+}相似（导致 YInMn 蓝色 (YIn _{1– x} Mn _x O ₃ ) 中强烈颜色的发色团），并且两者都是d ⁴并表现出 Jahn-Teller 畸变，这开启了可能性Cr ²⁺是用于设计颜料的发色团。通过高温固相反应方法在降低氧逸度（真空和/或 N ₂ /H ₂气体混合物下）下合成含有 Cr ²⁺的 Ca _{1– x} Mg _x CrSi ₄ O ₁₀块状样品，以寻找浓烈且持久的颜料。多晶相以四方晶体结构（空间群：P 4/ ncc）结晶，并且与矿物锌锌矿（BaFeSi ₄ O ₁₀）和埃及蓝（CaCuSi ₄ O ₁₀）同构。磁化率测量证实了二价态铬的存在。 Ca _{1– x} Mg _x CrSi ₄ O ₁₀ ( x = 0–0.2) 样品呈现出不同深浅的洋红色，随镁含量的变化而变化。该系列的光谱与以⁵ B _1g为基态的方平面配位的Cr ²⁺ ( d ^{4 ) 一致，对应构型}d _{¹ z ²} , d ² _{xz, yz} , d _{¹ xy} , d _{x ² – y ²}。观察到的光谱可归因于从⁵ B _1g^{态到 Cr 2+}的其他五重态的电子跃迁：⁵ B _{2 g}、⁵ E _g和⁵ A _{1 g}。固溶体中强烈的洋红色的起源归因于dd的协同效应通过同时激发由A _{2 u}、B _{2 u}和Eu_对称性组成的[CrO _{4 ] 单元的}u振动模式，通过电子振动耦合实现跃迁。在方形平面配位中与 Cr ²⁺形成强烈的红洋红色颜料为在不同的配位环境中探索具有二价铬的新型颜料打开了大门。