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

Abstract

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.

From Bulk to Nano: The Effect on the Persistent Luminescence

“…However, it represents the situation at long-range that may be different from that on the local scale. Indeed, a common situation is the preservation of the crystal structure observed at the local range but with variation respect to the average situation considering the whole material. , In the NCs, this effect may be emphasized, and it is common to observe strong fluctuations of the local structure. , Consequently, determining the real situation shown at the nanoscale becomes fundamental to develop accurate theoretical models that can describe the real situation. Indeed, several techniques are useful to reach this goal, spanning from synchrotron analyses such as pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) to nuclear magnetic resonance (NMR) and electron paramagnetic resonance spectroscopy (EPR).…”