Origin of the Anomalous Color of Egyptian and Han Blue Historical Pigments: Going beyond the Complex Approximation in Ligand Field Theory

详细信息    查看全文

• 作者：Pablo García-Fernández ; Miguel Moreno ; José Antonio Aramburu
• 刊名：Journal of Chemical Education
• 出版年：2016
• 出版时间：January 12, 2016
• 年：2016
• 卷：93
• 期：1
• 页码：111-117
• 全文大小：415K
• ISSN：1938-1328

文摘

The complex approximation is widely used in the framework of the Ligand Field Theory for explaining the optical properties of crystalline coordination compounds. Here, we show that there are essential features of these systems that cannot be understood with the usual approximation that only considers an isolated complex at the correct equilibrium geometry. We also show that a quantitative understanding of such optical transitions cannot, in general, be reached unless the internal electric field, E_R(r), created by the whole crystal on active electrons confined in the complex, is also taken into consideration. Seeking to prove the key role played by this internal field, usually ignored in crystalline transition metal compounds, we focus on the origin of the color displayed by the Egyptian Blue pigment (CaCuSi₄O₁₀), the first ever synthesized by humans. This pigment, together with Han Blue (BaCuSi₄O₁₀), are chosen as model systems because the anisotropic E_R(r) field produces huge shifts, up to ～0.9 eV, in their d–d transitions, which are unusual compared to the majority of compounds containing the same square-planar CuO₄^6– chromophore. The relevance of the internal field for explaining phenomena such as the distinct color of ruby and emerald or the optical spectrum of CuF₆^4– complexes in layered perovskites is also emphasized.