The quasi-binary phase diagram BaF2-BaBr2 and its relation to the x-ray storage phosphor BaFBr : Eu2+

Abstract

In order to understand the formation and stoichiometry of the x-ray storage phosphor BaFBr : Eu2+, the phase diagram of the quasi-binary BaF2-BaBr2 system has been investigated. The phase diagram was obtained by means of differential thermal analysis and temperature controlled x-ray diffraction experiments. The resulting phase diagram indicates that BaFBr forms a compound with no detectable solid solubility for neither BaF2 nor BaBr2. Experiments to obtain non-stoichiometric BaFBr via the synthesis route using BaF2 and NH4Br as proposed in the literature could not be verified. It will be shown that the type of colour centre created during x-ray irradiation is related to the non-stoichiometry of the starting compositions before sintering. A surplus of either barium fluoride or barium bromide during sintering allows the controlled formation of F(Br-)- and F(F-)-centres, respectively.

Crystal Chemistry in the Barium Fluoride Chloride System

“…On the fluoride-rich side, the eutectic point is found at 940 °C for X(F) = 0.73. This phase diagram is very similar to the one reported for the BaBr 2 –BaF 2 system (see Figure S2). Other phase diagrams of MH 2 –MX 2 with M = Ca, Sr, Ba and X = Cl, Br, I also show (Figure S3) the intermediate MHX with the matlockite structure and two eutectic points. − …”

New synthesis of high-quality storage phosphors

“…Most likely the excess of fluoride ions following from Eq. (2) forces the system to form the F þ (Br)-centers in BaFBr (Kolb et al, 2002). The vacancy is charge compensated through the replacement of a fluorine ion by a O 2À ion according to Eq.…”

“…An excess of fluoride causes more bromide vacancies and therefore, following irradiation, more F(Br À )-centers, while an excess of bromide leads to more F(F À )-centers (Zimmermann et al, 2004;Hesse and von Seggern, 2004). Other studies have found, however, that the ratio of fluoride to bromide in BaFBr:Eu 2þ is always macroscopically stoichiometric, regardless of the starting composition, and any excess halide appears as an additional phase (Kolb et al, 2002).…”

Storage efficiency of BaFBr:Eu2+ image plates irradiated by swift heavy ions

“…The nature of the electron-trap centers is understood in detail, being mainly F centers in the fluorine and bromine sublattices (F(F À ) and F(Br À )) [2]. The holes are localized close to the Eu 2+ ions, but the microstructure of the hole storage centers and processes linked to the PSL center formation are still under discussion [3,4]. In BaFBr:Eu 2+ the highest PSL signal can be achieved by photo-stimulation of the F(Br À ) color centers.…”