Does the blue luminescence of Eu-doped β-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.