Effects of vacancies on valence stabilities of europium ions in β-Ca2SiO4: Eu phosphors

The formation energies of oxygen and calcium vacancies (V-o and V-Ca) in beta-Ca2SiO4 are derived from density functional theory (DFT) calculations performed on constructed supercells, revealing the thermodynamic stabilities of intrinsic defects under oxygen-poor conditions. On the basis of DFT calculations with HSE06 hybrid functional, defect states produced by vacancies (V-o and V-Ca), dopants Eu (Eu-Ca) and Eu-related defect complexes (Eu-Ca+V-o and Eu-Ca+V-Ca) in the band gap of beta-Ca2SiO4 host have been further distinguished. The calculated results indicate that the neutral and single negatively charged V-Ca (i.e., V-Ca(x) and V-Ca') and the V-o in positive charge states may act as trapping centers for electrons. When approaching to Eu2+ ions, V-Ca(x) and V-Ca' may capture the electrons of Eu2+ ions and stabilize Eu3+ ions at Ca2+ sites in beta-Ca2SiO4, but not the neutral and positively charged V-o. This work demonstrates that electronic structure calculations combined with thermodynamic analyses can be utilized to study the interactions between europium ions and their neighboring defects, and further study the tuning mechanisms of the valence stabilities of europium ions in inorganic compounds. (C) 2016 Elsevier B.V. All rights reserved.