Under irradiation with a CW ultra-violet laser (325nm) at room temperature in vacuum or an O gas atmosphere, film, microcrystalline and powder samples of Eu2O3 exhibited a reversible photoluminescence spectral change between a red sharp-line structure and a white broad one. After ceasing ultra-violet irradiation, the white photoluminescence lasted for more than several months at room temperature under room lighting; in spite of changes in atmosphere. The reversible phenomena observed were interpreted as being the result of valence-number changes (Eu3+ → Eu2+) and O-vacancy formation.

Valence-Change- and Defect-Induced White Luminescence of Eu2O3. S.Mochizuki, Y.Suzuki, T.Nakanishi, K.Ishi: Physica B, 2001, 308-310, 1046-9