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

The structural properties, X-ray-excited radioluminescence and thermally stimulated luminescence (TSL) of Ca(9)Ln(PO4)(7) (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were investigated. (La-Nd) occupied mainly the largest M3 site, whereas the smallest lanthanides (Yb, Lu) adopted the smallest octahedral M5 site. The intermediate-sized ions (Sm-Tm) occupied the medium-sized sites M1 and M2. 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+) 4f-4f emission. For low Ln(3+/4+) ionization potential (Ln = La, Gd, Yb, Lu), no hole trapping at Ln(3+) was possible and Mn2+ impurities at the M5 sites were the effective hole traps and luminescent centers for thermally stimulated Mn2+ 3d-3d red emission. Very broad TSL peaks were attributed to thermally assisted detrapping of electrons from positive traps, possibly Ln(3+) at Ca2+ sites, tunneling back towards the hole trap/luminescent center. Ca(9)Ln(PO4)(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. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.