The europium oxidation state in CsSrI3:Eu scintillators measured by X-ray absorption spectroscopy

Abstract Divalent Eu is a common luminescence activator in inorganic scintillators due to its allowed 5d–4f transitions. Since Eu can exist in both divalent and trivalent states, but only luminescence from divalent Eu is suitable for scintillation applications, it is important to determine the potential presence of trivalent Eu in the crystal. We report measurements of Eu(II) and Eu(III) in the new CsSrI 3 :Eu scintillator using X-ray absorption spectroscopy. Single crystals of CsSr 0.93 Eu 0.07 I 3 were grown from the melt via the Bridgman method using EuI 2 powder as a source of divalent Eu activators. The X-ray absorption spectra on the Eu M 4 and M 5 edges were obtained at beamline 4.0.2 at the Advanced Light Source synchrotron. The results were compared to model samples of trivalent and divalent europium, which provided clear signatures of the two oxidation states. The measured electronic structure of Eu confirms predominant Eu(II) in both the EuI 2 starting material and the CsSr 0.93 Eu 0.07 I 3 scintillator crystals. However, a small presence of Eu(III) points to the partial oxidation of europium, especially at the sample surface.