Shedding Light on Cuprorivaite, the Egyptian Blue Pigment: Joining Neutrons and Photons for a Computational Spectroscopy Study

Abstract

The unique properties of Egyptian blue, a cuprorivaite pigment, were herein studied through a holistic computational and experimental approach. A reliable model of the crystal of cuprorivaite was obtained through periodic density functional theory calculations, allowing for the elucidation of its lattice dynamics, including assessment of structural, electronic, and vibrational properties. From this model, a sound assignment of the inelastic neutron scattering spectrum was obtained, along with estimated values of heat capacity and Debye temperature, band-gap values, and magnetic properties of the crystal. Inelastic neutron scattering and diffuse reflectance infrared Fourier transform spectroscopies provided enlightenment on the chemical surfaces of the pigment Egyptian blue─which was missing hitherto and is critical for potential applications of the pigment, such as those involving host–matrix interactions or requiring surface derivatization─while confirming the simulation results. On that account, it was found that the intensity of the dangling νSiOd mode is ca. 8–13% of the total νSiO modes. Moreover, and regarding the electronic properties, the band structure confirmed that CaCuSi4O10 is a direct band-gap semiconductor, with the valence band maximum and the conduction band minimum located at the Γ-point, in both the alpha (spin-up) and beta (spin-down) density bands.