Does the blue luminescence of Eu-doped <i>β</i>-NaYF4 originate from Eu2+ ions?

Fluorides, such as β-NaYF4, are important hosts for a variety of lanthanide dopants that are useful for luminescence applications. Phosphor hosts that can accommodate both Eu3+ and Eu2+ ions offer flexibility for luminescence applications. However, hosts based on Y3+ cations generally (with some exceptions) exhibit only luminescence from Eu3+ ions. Therefore, reports attributing the blue luminescence from β-NaYF4 to Eu2+ ions are of great interest. This blue luminescence, together with characteristic red luminescence from Eu3+ ions, was confirmed using samples made by hydrothermal synthesis for cathodoluminescence and photoluminescence. Although x-ray photoelectron spectroscopy confirmed the presence of both Eu2+ and Eu3+ ions on the sample surface, the conclusion that the blue luminescence originated from the Eu2+ ions could not be supported. This was because the measured lifetime of the blue luminescence (∼3 ns) was two orders of magnitude less than typical for Eu2+ ions, while the blue luminescence was also found for undoped samples. Further work is required to identify whether this blue luminescence originated from host defects or other impurities, while the present work shows that care must be taken in identifying Eu2+ luminescence, as well as assuming that Eu2+ ions identified on the surface of samples by x-ray photoelectron spectroscopy are luminescent and representative of the bulk material.