Abstracts

X-rays were invented in 1895 by observing luminescence excited by this radiation. By the next year, CaWO₄ powder screens were introduced and dominated the medical X-ray diagnostics markets until 1970s – for 75+ years. A new phosphor, Eu²⁺ doped BaF(Cl,Br,I) took then gradually over and the X-ray excited Photoluminescence emitted by CaWO₄ shifted to Photostimulated Luminescence (PSL) of Eu²⁺. At the same time, the process became much more complicated, though. The X-ray image was now possible to be stored for up to weeks by trapping energy in defects of the host lattice. Other improvements took place as well: strong Eu²⁺ band band emission replaced the weaker emission of CaWO₄ and, more important, the considerable afterglow of CaWO₄ causing blurring of the X-ray images was removed. The change of phosphors occurred without much struggle though the Tb³⁺ doped LaOBr (structurally isomorphic to matlockite PbFCl) was initially considered as well. The candidate of Fuji Corp. (BaF(Cl,Br,I:Eu²⁺) overcame finally in 1990s (money talks). As the chemical formula of the new phosphor suggests, the extensive substitution of Cl with Br and even I would mean that the defect structure of this BaFCl phosphor is quite complex. In this report, the preparation, thermal stability and PL/PLE/PSL/TL luminescence of LaOBr:Tb³⁺ are described to solve the defect structure, luminescence, PSL/TL properties as well as the luminescence mechanisms in detail.The LaOBr:Tb³⁺ powders were prepared by heating La₂O₃:Tb³⁺ with 55 % excess of NH₄Br @ 800 C. Annealing at 1000+ C in air leads to a partial substitution of bromide (Br^-) by oxide (O^2-). To achieve the global neutrality of the doped compound, the charge compensation (CC) of excess negative charge due to the said substitutions is compulsory and was carried out by oxidation of Tb³⁺ to Tb^IV which changed the material’s color from white to yellow. Despite the faint colouring of the material, the anionic substitution enabled the compulsory formation of negatively charged traps which are capable to trap holes (h⁺). On the other hand, the Tb^IV in the Tb³⁺ site, acts as a positively charged trap which is capable to trap electrons (e^-). As a result, the oxidation of Tb³⁺ resulted in an unexpected change in the Tb³⁺ emission: the blue emission from the ⁵D₃ levels was seriously quenched changing the emission colour from bluish green to green composed essentially of the emission from the ⁵D₄ levels, The quenching mechanisms include the self-absorption by the Tb^IV impurities at energies above 2.5 eV (500 nm). Alternatively, the cross-relaxation between the ⁵D₃ and ⁵D₄ as well as the ⁷F₆ and ⁷F₀ levels has a similar effect, The structural effects of doping and defect formations are studied and the exact PersLum and PSL mechanisms will be eventually studied.