Site occupancy and mechanisms of thermally stimulated luminescence in Ca9Ln(PO4)7 (Ln=lanthanide)

Abstract The structural properties, X-ray-excited radioluminescence and thermally stimulated luminescence (TSL) of Ca 9 Ln (PO 4 ) 7 ( Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were investigated. (La–Nd) occupied mainly the largest M 3 site, whereas the smallest lanthanides (Yb, Lu) adopted the smallest octahedral M 5 site. The intermediate-sized ions (Sm–Tm) occupied the medium-sized sites M 1 and M 2. Only the Ln 3+ ions with the highest Ln 3+/4+ ionization energy ( Ln = Pr, Nd, Tb, Dy) were able to trap holes and become luminescent centers for thermally stimulated Ln 3+ 4 f –4 f emission. For low Ln 3+/4+ ionization potential ( Ln = La, Gd, Yb, Lu), no hole trapping at Ln 3+ was possible and Mn 2+ impurities at the M 5 sites were the effective hole traps and luminescent centers for thermally stimulated Mn 2+ 3 d –3 d red emission. Very broad TSL peaks were attributed to thermally assisted detrapping of electrons from positive traps, possibly Ln 3+ at Ca 2+ sites, tunneling back towards the hole trap/luminescent center. Ca 9 Ln (PO 4 ) 7 compounds doped with Ln = Tb or Lu were found to be efficient X-ray-excited long-lasting phosphorescence materials, emitting in the green and in the red, respectively.