Barium Vacancies as the Origin of Triboluminescence in Hexacelsian Ceramics: An Ab Initio and Experimental Investigation

We describe the triboluminescence response of undoped (BaAl2Si2O8, h-BAS) and Eu-doped (h-BAS:Eu) barium hexacelsian powders and show that the triboluminescence behavior is dependent on the formation of barium vacancies. X-ray photoelectron spectroscopy of the h-BAS:Eu powders confirms the presence of Eu3+ and Eu2+ in the compound, leading to the formation of significant vacancy point defects in excess of those found in h-BAS as a result of the charge imbalance caused by the substitution of Eu3+ in Ba2+ sites. From electron paramagnetic resonance measurements and density functional theory (DFT) calculations, we demonstrate that the vacancy defects correspond to singly ionized barium vacancies. DFT-calculated thermodynamic transitions and electronic structure calculations reveal deep energy levels within the compound's energy band gap, with a strong emission at 3.33 eV correlated to an electron exchange between the conduction band minimum and a barium vacancy center. Time-resolved triboluminescence spectra show that the increased concentration of barium vacancies in h-BAS:Eu enhances the signal by about 75\% compared to the signal from h-BAS. These results play an important role in the understanding of fundamental mechanisms behind the triboluminescence response of ceramic materials as well as the role of different types of defects in this process.