Structure and photostimulated luminescent properties of Eu-doped M 2 Ba X 4 ( M = Cs , Rb; X = Br , Cl) CsBr: Eu 2+ has recently been investigated as a photostimulable x-ray storage phosphor with great potential for application in high-resolution image plates. In a recent paper Hackenschmied et al. ͓J. Appl. Phys. 93, 5109 ͑2003͔͒ suggested that segregations of CsEuBr 3 or Cs 4 EuBr 6 formed within CsBr: Eu 2+ during annealing are responsible for an increase in the photostimulated luminescence ͑PSL͒ yield. In this work single crystals of CsEuBr 3 were prepared by a one step synthesis and identified by x-ray diffraction ͑XRD͒ analysis as single phase perovskites. It was concluded that, after preparation, CsEuBr 3 degrades in normal atmosphere into at least two phases, one of which is the orthorhombic structure of Cs 2 EuBr 5 · 10H 2 O. The XRD powder diffraction pattern of this compound is very similar to that of the segregations observed within CsBr: Eu 2+ and reported by Hackenschmied et al. However, the increased PSL yield in CsBr: Eu 2+ after annealing cannot be due to the segregations, because the trivalent nature of the europium in the segregations renders them PSL inactive.

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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.

The poor radiation hardness of the otherwise excellent x-ray storage phosphor CsBr:Eu2+ constitutes a problem for its commercial application in medical diagnostics. X-ray induced vacancy centers such as M-centers enhance the diffusion of Eu2+ activators resulting in a formation of photostimulated luminescence (PSL) inactive europium clusters or second phases of europium compounds. The present study investigates the influence of Li-codoping on the radiation hardness of CsBr:Eu2+. It is reported that the integration of Li+ into the CsBr:Eu2+ suppresses the generation of M-centers during x-irradiation and thereby partially improves the radiation hardness.