Role of crystal defects in red long-lasting phosphorescence of CaMgSi2O6:Mn diopsides

CaMgSi2O6:Mn diopsides are used for in vivo long-lasting phosphorescence (LLP) imaging. Trapping defects involved in the LLP mechanism were investigated. On annealing Mn-doped diopsides at 1100 degrees C in an Ar-H-2 atmosphere, Ca EXAFS/XANES and electron paramagnetic resonance (EPR) evidenced paramagnetic oxygen vacancies while X-ray diffraction, Mn XANES and EPR revealed SiO2 formation and significant evaporation of the Mn dopant. A thermally stimulated luminescence (TSL) peak at 475 K ascribed to electron trapping at oxygen vacancies was found responsible for LLP at RT. Most intense red LLP suitable for in vivo imaging was achieved by a trade-off between a high Mn-II content (favourable to Mn-Mg(II) red over Mn-Ca(II) orange luminescence) and the formation of oxygen vacancies favourable to LLP and luminescence light yield. Mn XANES revealed an effective charge larger than 2+ for Mn-II, in line with the role of hole trap in the LLP mechanism. Compounds annealed at lower temperatures (800 degrees C and 900 degrees C) in Ar-H-2 showed smaller particle size (60-70 nm) and maximum Mn content but poor luminescence and LLP due to surface quenching defects.