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 °C in an Ar–H2 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 MnII content (favourable to MnIIMg red over MnIICa 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 MnII, in line with the role of hole trap in the LLP mechanism. Compounds annealed at lower temperatures (800 °C and 900 °C) in Ar–H2 showed smaller particle size (60–70 nm) and maximum Mn content but poor luminescence and LLP due to surface quenching defects.