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

• Eu oxidation state in CsSrI3:Eu scintillator was studied using X-ray absorption spectroscopy.
• Both raw materials and grown crystals contain Eu(II), which is desirable for scintillation.
• The hygroscopic nature of Eu-containing halides results in oxidation of Eu(II) to Eu(III).

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 CsSrI3:Eu scintillator using X-ray absorption spectroscopy. Single crystals of CsSr0.93Eu0.07I3 were grown from the melt via the Bridgman method using EuI2 powder as a source of divalent Eu activators. The X-ray absorption spectra on the Eu M4 and M5 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 EuI2 starting material and the CsSr0.93Eu0.07I3 scintillator crystals. However, a small presence of Eu(III) points to the partial oxidation of europium, especially at the sample surface.