|
||||||||
Fluorine-substituted CaTiO3:Pr phosphors were prepared by a solid-state reaction. Rietveld refinements of powder X-ray diffraction patterns revealed that increasing fluorine-substitution leads to the gradual shrinkage of the unit-cell. Enhanced afterglow intensities were observed with fluorine-substitution. Furthermore, the effect of annealing atmosphere was investigated by thermochemical treatment in different atmospheres (Ar, air and NH3). UV-Vis diffuse reflectance spectra and photoluminescence excitation spectra revealed that Pr4+ in the pristine CaTi(O,F)3:Pr phosphor was partially reduced to Pr3+ under NH3 flow leading to an intensity improvement of ca. 450% compared to CaTiO3:Pr. The substantial improvement of afterglow intensity by fluorine substitution and annealing in NH3 is considered to be connected with the generation of oxygen vacancies and the partial reduction of Pr4+ to Pr3+. |
|
||||||||
The phosphor CaTiO3:Pr3+ was synthesized via a solid-state reaction in combination with a subsequent annealing under flowing NH3. Comparatively large off-center displacements of Ti in the TiO6 octahedra were confirmed for as-synthesized CaTiO3:Pr3 by XANES. Raman spectroscopy showed that the local crystal structure becomes highly symmetric when the powders are ammonolyzed at 400 °C. Rietveld refinement of powder X-ray diffraction data revealed that the samples ammonolyzed at 400 °C have the smallest lattice strain and at the same time the largest average Ti-O-Ti angles were obtained. The samples ammonolyzed at 400 °C also showed the smallest mass loss during the thermal re-oxidation in thermogravimetric analysis (TGA). Enhanced photolumincescence brightness and an improved decay curve as well as the highest reflectance were obtained for the samples ammonolyzed at 400 °C. The improved photoluminescence and afterglow by NH3 treatment are explained as a result of the reduced concentration of oxygen excesses with simultaneous relaxation of the lattice strain. |