Dual-Anion Vacancies in Apatite Systems

In this work, we report on a theoretical, computational, and experimental approach for evaluating dual-vacancies in (halo-)oxide systems focusing on application to vacancy defects in a variety of apatite structures, i.e., structures of the form A10(BO4)6X2 (A = Ca/Sr/Ba/La; B = P/Si; X = F/Cl/Br/O), which are prepared via solid-state reaction. Anion vacancy defects (particularly, oxygen, VO) are often intentionally introduced through varying atmospheric conditions to design inorganic multifunctional materials, and this work demonstrates a process of discovering controllable synthesis routes that will yield desired properties. Theoretically, calculating defect formation energies is not a straightforward endeavor due to confusion resulting in the ambiguous bounds for the chemical potentials of the multicomponents in a complex compound. Therefore, we have paid careful attention to the corrections of the chemical potential references for all anionic components in multianion compounds (VO and VX defects with X = F, Cl, Br, etc.) and the effect of environmental conditions on the chemical potentials. We find from our results that it is easier to form VO rather than VX defects within the M5(PO4)3X (MPOX; M = Ca, Sr, Ba) apatite systems. However, for silicon apatite systems, La4Ba6(SiO4)6F2 (LBSOF) and La4Sr6(SiO4)6O (LSSOO), other anion VX defects can be more readily prepared, albeit VO defects are the prevalent vacancies. Our conclusions are supported by photoluminescence spectra where, in the MPOX systems, we note a single emissive center, and by contrast, two emissive centers are observed in silicate LMSOF systems.