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

Abstract

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 Mn^II content (favourable to Mn^II_Mg red over Mn^II_Ca 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 °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.