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Origin of the Exotic Blue Color of Copper-Containing Historical Article pubs.acs.org/IC Origin of the Exotic Blue Color of Copper-Containing Historical Pigments Pablo García-Fernandez,́ * Miguel Moreno, and JoséAntonio Aramburu Departamento de Ciencias de la Tierra y Física de la Materia Condensada, Universidad de Cantabria, Avenida de los Castros s/n, 39005 Santander, Spain *S Supporting Information ABSTRACT: The study of chemical factors that influence pigment coloring is a field of fundamental interest that is still dominated by many uncertainties. In this Article, we investigate, by means of ab initio calculations, the origin of the unusual bright blue color displayed by historical Egyptian Blue (CaCuSi4O10) and Han Blue (BaCuSi4O10) pigments that is surprisingly not found in other 6− compounds like BaCuSi2O6 or CaCuO2 containing the same CuO4 chromophore. We show that the differences in hue between these systems are controlled by a large red-shift (up to 7100 cm−1) fi 6− produced by an electrostatic eld created by a lattice over the CuO4 chromophore from the energy of the 3z2-r2 → x2-y2 transition, a nonlocal phenomenon widely ignored in the realm of transition metal chemistry and strongly dependent upon the crystal structure. Along 4− this line, we demonstrate that, although SiO4 units are not involved in the chromophore itself, the introduction of sand to create CaCuSi4O10 plays a key role in obtaining the characteristic hue of the Egyptian Blue pigment. The results presented here demonstrate the opportunity for tuning the properties of a given chromophore by modifying the structure of the insulating lattice where it is located. ■ INTRODUCTION even then they remained rare. This was the case for lapis lazuli, Dyes and pigments have a variety of applications and are in a blue-colored stone that was found only in one mine for the 1 whole ancient world, which is located in modern-day huge demand in industry. However, obtaining stable pigments 2−4 ffi Afghanistan. that display particular tonalities has often proven di cult as fi many nonquantified factors are involved in the chemistry of The rst traces of man-made blue pigments date back to 1−7 early Egyptian culture (predynastic era, ∼3600 BC) where the color. Thus, understanding how to modify the hue of a fi chromophore while retaining strong chemical stability is crucial, so-called Egyptian Blue pigment was prepared. This rst as this knowledge would allow a bottom-up approach to synthetically produced pigment in human history involves a pigment design. In this Article, we study the origin of historical mixture containing several phases, including cuprorivaite (CaCuSi4O10), unreacted quartz, and variable amounts of pigment colors used in ancient Egyptian and Chinese 2−4,12 6− glass. The exotic bright blue color of the pigment is due to civilizations based on square-planar CuO4 complexes, with 6− 2−4,8,12 extra attention paid to those displaying an unusual bright blue the square-planar CuO4 chromophore present in the − 2 4 fi insulating CaCuSi4O10 compound displaying the gillespite tonality. Our primary nding is that, in all of these systems, 13 the color is unexpectedly controlled by an electrostatic field that (BaFeSi4O10) structure (tetragonal space group P4/ncc, see 6− Figure 1). The Egyptian Blue pigment is very stable, was used is created by the lattice over the CuO4 chromophore. This key factor, however, is usually ignored in the large body of work extensively for decorative purposes in the early dynasties in in the field of transition metal chemistry, where properties are Egypt (until the end of the Roman period), Greece, and around usually explained considering only the complex at equilibrium the Roman Empire, and can be found, for example, in Amarna, − 14 geometry.5 11 Luxor, the Parthenon, and Pompeii. As a remarkable example, Historically, the technology of pigments was amongst the pure Egyptian Blue was used in the crown of a famous bust of − 2−4 earliest that mankind developed.2 4 By using different colored Queen Nefertiti. Significantly later, and most likely developed independ- earth, or grinding soft rocks to a powder, prehistoric humans 15 could make pictures of different colors, such as the wonderful ently, two pigments involving a colored compound with a drawings in Altamira (Spain) and Lascaux (France) caves. composition somewhat similar to that of cuprorivaite were Interestingly, the color blue is absent in prehistoric paintings because of its scarcity in surface soils. Blue pigments only began Received: August 29, 2014 to appear in human history when mining was developed, and Published: December 17, 2014 © 2014 American Chemical Society 192 dx.doi.org/10.1021/ic502420j | Inorg. Chem. 2015, 54, 192−199 Inorganic Chemistry Article Figure 1. Unit cells corresponding to Egyptian Blue (CaCuSi4O10) and Han Blue (BaCuSi4O10) pigments (left), the Han Purple (BaCuSiO2O6) 2+ 6− pigment (right), and the CaCuO2 compound (white box). Cu ions involved in square-planar CuO4 complexes are depicted in dark blue, whereas 4− SiO4 tetrahedrons are in green. Table 1. Experimental Values of the Cu2+−O2− Distances, R (Å), and Peak Energies (cm−1) of the Three d-d Transitions for 6− Compounds Containing Square-Planar CuO4 Complexes CaCuSi4O10 (Egyptian Blue) BaCuSi4O10 (Han Blue) BaCuSi2O6 (Han Purple) CaCuO2 Li2CuO2 a space group P4/ncc P4/ncc P41/acd P4/mmm Immm R 1.928b 1.921b 1.945c 1.929d 1.959e → 2 2 f g h i j b2g(xy) b1g(x -y ) 12740 12900 13500 13220 14000 → 2 2 f g h i j eg(xz,yz) b1g(x -y ) 16130 15800 17000 15720 17000 2 2 → 2 2 f g ∼ h i ∼ j a1g(3z -r ) b1g(x -y ) 18520 18800 21000 21370 21900 aSee structure in the Supporting Information. bFrom ref 13. cFrom ref 16. dFrom ref 22. eFrom ref 23. fFrom refs 9, 18−20, and 24. gFrom ref 8. hFrom ref 25. iFrom ref 11. jFrom ref 10. − 6− employed during the Han dynasty (208 BC 220 AD) in Compounds containing square-planar CuO4 complexes − China.3,4 The color of the so-called Han Blue pigment is also have three broad d-d bands (bandwidth ∼2500 cm 1) in the bright blue due to BaCuSi4O10, which has the same gillespite optical region that cover most of the spectrum. Table 1 shows structure as cuprorivaite13 (Figure 1). In contrast, a different experimental structural and optical data in these sys- 9,10,13,16,18−25 color is exhibited by the Han Purple pigment, which was tems. In particular, it contains the peak energies identified in Terracotta Warriors discovered in central China in of the three d-d bands corresponding to Egyptian Blue, Han Blue, and Han Purple pigments as well as those measured for 1974 and is based on BaCuSi2O6. This compound belongs to 16 CaCuO2 and Li2CuO2 compounds, which display a simpler the P41/acd space group (Figure 1) that also involves square- 22,23 6− crystal structure and have interesting electrical and planar CuO chromophores. A similar situation holds for − 4 magnetic properties.26 28 Li CuO and CaCuO compounds, which have d-d transition 2 2 2 As shown in Table 1, when comparing Egyptian and Han energies that are certainly different10,11 than those measured for Blue pigments with the rest of the compounds containing CaCuSi4O10 and BaCuSi4O10, though they also contain square- 6− 6− square-planar CuO4 complexes, we found that the peak planar CuO4 complexes. 2 2 → energy of the band corresponding to the highest a1g(3z -r ) Despite the indisputable importance of the Egyptian Blue 2 2 2 b1g(x -y ) excitation, denoted E(z ), is lowered by approx- pigment in history, art, and archeological studies, as well as in −1 17 imately 3000 cm , thus opening up a gap in the blue region current applications, the actual origin of the unusual blue (∼21500 cm−1) not present in the other systems. Because the color displayed by CaCuSi4O10 and BaCuSi4O10 pigments is rest of the visible spectrum is absorbed by the three broad d-d not yet understood. This Article aims to explain, by means of ab transitions, this gives rise to the characteristic bright blue color initio calculations, why d-d transitions of CaCuSi4O10, CaCuO2, exhibited by such pigments (Figure 1). ff and BaCuSi2O6 are so di erent despite the compounds having Almost without exception, the spectra of transition metal 6− the same CuO4 chromophore. complexes in insulating compounds are analyzed based solely 193 dx.doi.org/10.1021/ic502420j | Inorg. Chem. 2015, 54, 192−199 Inorganic Chemistry Article on the chemical nature of the ligands and their distance to the structures have been relaxed until a maximum force value below 0.02 − − central metal cation.5 11 However, as shown below, this widely eV/Å and a total energy change below 10 7 eV were obtained. used approach is unable to explain the different energies As an example of the reliability of the present periodic calculations, observed for d-d transitions of the historical blue pigments carrying them out on CaCuSi4O10 is found to reproduce the experimental lattice parameters (a = 7.3017 Å, c = 15.1303 Å) and when compared with those of compounds like CaCuO2 or 2+− 2− 6− the Cu O distance (R = 1.929 Å) with errors of <0.7%. Similar CaCuSi2O6, all of which involve square-planar CuO4 ff deviations are found in the calculations on CaCuO2,where complexes. For example, the e ect of chemical substitution of experimental values are a = 3.856 Å, c = 3.180 Å, and R = 1.928 Å. ligands in octahedral complexes is usually accounted for using Calculated lattice parameters of the other compounds are reported in the spectrochemical series.29 In this case, taking the the Supporting Information.
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