Valence control of Pr in ZrO2 nanocrystals by aliovalent Gd3+ co-doping

Praseodymium doped ZrO2 materials were prepared via sol-gel route and structurally characterized by X ray powder diffraction (XPD) technique as well as Rietveld refinements. The addition of the Gd3+ codopant gradually changes the zirconia structure from monoclinic to tetragonal, and then to cubic. Intensification of the Pr3+ luminescence was observed with the increasing Gd3+ co-dopant concentration. Emission spectra of the Zr0.99-xGdxPr0.01O2 materials show an initial strengthening of the red emission of Pr3+ (D-1(2)-> H-3(4) transition) with increasing Gd3+ co-doping. However, the luminescence is quenched at the highest Gd3+ concentration-possibly due to strongly increased concentration of the charge compensation defects. The valence change (Pr-IV -> Pr3+) is supported by the XANES results on the L-III edge of Pr. Although predominantly in the Pr3+ form irrespective of the Gd3+ concentration, the contribution from Pr-IV is clearly visible at low (or zero) Gd3+ concentrations leading to the loss of Pr3+ and to poor luminescence output. Though enhancing the emission intensity of Pr3+, the defect clusters engender short Pr3+-Pr3+ distances enhancing the cross-relaxation process coupling the P-3(0)-> D-1(2) relaxation with the H-3(4)-> H-3(6) excitation. This process leads to the high red/blue-green emission ratio by quenching the P-3(0)-> H-3(4) transition in blue-green. Eventually, the increased Gd3+ co-doping dilutes the Pr3+ ions and, the cross-relaxation process becomes non-operational; the quenching of the P-3(0)-> H-3(4) transition is reversed. (C) 2015 Elsevier B.V. All rights reserved.