Structural, electronic and optical characterization of substitutional Ag defect in Li2B4O7 scintillator

The DFT based full-potential linear augmented plane-wave method has been employed to simulate computationally the Li2B4O7 scintillator with isolated Ag1+ impurity substituting for the Li1+ ion. With the objective to investigate the physics of the Ag1+ defect in pre- and post-irradiated compound, the unit cell of the Li2B4O7:Ag has been differently charged. It was found that additional electrons and holes were captured exactly by the Ag1+, confirming the experimental fact that this defect acts as an electron and a hole trapping center. It has been concluded that the Ag impurity can assume the Ag1+, Ag-0, and Ag2+ charge states. The lattice position and the relaxed local structure around each of them have been determined and chemical bonding with neighboring atoms carefully analyzed. The Ag1+ defect is found to be dislocated from the Li site, while the Ag2+ is stabilized approximately at it, both being bonded only to neighboring oxygens. The Ag-0 defect is dislocated from the Li site by approx. 1.0 angstrom and bonded to the oxygens and one boron ion, a fact which classifies it as an interstitial defect. Electronic structure and absorption coefficient spectra of the compound containing each of the 3 charged defects have been calculated. A comparison of the obtained results with experimental data shows that the 4d(10) -> 4d(9)5s(1) transitions at the sole Ag1+ defect are responsible for the optical response of the pre-irradiated compound (peak centered on 6.0 eV). The Ag2+ and Ag-0 defects contribute to the additional absorption bands which appear after the X-ray irradiation of the compound. The absorption peak centered on 1.9 eV is due to 4d -> 4d electronic transitions at the Ag2+ centers, while the Ag-0 center contributes to the creation of the broad absorption band spread between 2.5 and 5.5 eV via the 5s -> 5p and the 4d -> 5s transitions. The present study interprets the results of the recent experimental studies and substantiates, in the largest part, their conclusions. Additionally, it predicts a shift of the 1.9 eV absorption peak to lower energies as a response to the light polarized along the crystalline c-axis.