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

Abstract

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.

Identifying the Unknown Content of an Ancient Egyptian Sealed Alabaster Vase from Kha and Merit’s Tomb Using Multiple Techniques and Multicomponent Sample Analysis in an Interdisciplinary Applied Chemistry Course

“…The replica could be prepared by mixing a vegetable oil with a pigment (ochre) and previously aging it in a climate chamber. The approach could be considered similar to the ones used in forensic chemistry. − However, its use has been extended in the field of Cultural Heritage give to students in Archaeology, Conservation Science, or Chemistry the possibility to know how a scientist could operate, even comparing with other approaches. − In fact, in this field, there is a different problematic need to choose the specific and proper approach for the problem solving.…”

Preparation, properties and applications of near-infrared fluorescent silicate nanosheets

“…49,101 For example, the works by García-Fernández et al have explained that, in order to fully understand the hue of EB and HB, the presence of a normally neglected internal electric field and the contribution of sand in the production process are of paramount importance. 102,103 Furthermore, a bright and broadband photon up-conversion of EB and HB has been demonstrated by Chen et al, showing that emission in the Vis region is possible via pumping with a NIR laser diode. 63,104 Binet et al have also very recently further investigated the origins of the NIR luminescence of EB and shown that it can be more efficiently excited in the UV range; this is possible given the existence of four UV-excited emissions that are most likely determined by the presence of trapped excitons of different origins.…”

The Modernity of Ancient Pigments: A Historical Approach

“…For both Egyptian blue and Han blue, the chromophore is CuO 4 6− in a square planar configuration. Embedding the chromophore in the tetrasilicate environment has a considerable effect on the d-d splitting, facilitating virtually complete absorption of the red, yellow and green regions of the visible spectrum, as shown in Figure 11 [51].…”

“…For both Egyptian blue and Han blue, the chromophore is CuO4 6-in a square planar configuration. Embedding the chromophore in the tetrasilicate environment has a considerable effect on the d-d splitting, facilitating virtually complete absorption of the red, yellow and green regions of the visible spectrum, as shown in Figure 11 [51]. Excavations in Pompeii and Cumae demonstrate that the ancient craftsmen recognized the need for excess amounts of silica to produce the best Egyptian blue, although they could have no foreknowledge that a 21st century molecular orbital diagram would provide the theoretical basis for their protocol.…”

“…Qualitative diagram showing the splitting of d levels for Cu 2+ in four different environments. Note the high energy elevation of the a 1g level from the isolated CuO 4 6− to that in CaCuSi 4 O 10on the right (reprinted with permission from Ref [51]…”