Structure Dynamics and Vibronic Coupling in AECuSi4O10 (AE: Ca, Sr, Ba) Compounds

Alkaline earth copper silicate-based pigments are compounds of interest from a historical point of view, thanks to their interesting near-infrared (NIR) photoemission. It is well recognized that the emission originates from the Cu2+ d-d transitions inside the chromophore group [CuO4]6-; however, the optical properties are slightly different, probably related to a variation in the local structure. Here, we report detailed X-ray powder diffraction and pair distribution function analysis of AECuSi4O10 (AE = Ca2+, Sr2+, Ba2+) silicates. Remarkably, our investigation shows that all of these compounds are characterized by a complex structural dynamic involving the [SiO4]4- tetrahedral framework embedding the [CuO4]6- chromophore group. This structure dynamics induces a vibronic coupling, allowing the d-d transitions responsible for the characteristic color of the compounds as well as a strong vibronic broadening of the electronic states. As a consequence, the absorption and emission spectra in the visible and NIR range are influenced by the alkaline earth metal and are characterized by broad bands and display the Stokes shift typically observed in vibronic systems.