Photoluminescence and cathodoluminescence of BaAl₂O₄:Eu²⁺ and undoped BaAl₂O₄: evidence for F-centres

In this article the photoluminescence (PL) and cathodoluminescence (CL) of undoped BaAl₂O₄ and BaAl₂O₄ doped with 500 ppm and 3 mol% Eu²⁺ is described. The most important results from the CL measurements are: (1) Undoped BaAl₂O₄ manifested intrinsic CL at 460 nm, which increased at low temperature and did not change significantly upon exposure to the e-beam; (2) Doping BaAl₂O₄ with Eu²⁺ changed the character of the intrinsic luminescence band: it became more sensitive to temperature variations and the band experienced a blue shift to ∼425 nm; (3) electron beam (e-beam) exposure of Ba_0.97Eu_0.03Al₂O₄ at low temperature increased the 425 nm band strongly while the Eu²⁺ emission at ∼500 nm decreased by about 70%. The Eu²⁺ emission band was symmetric, indicating that BaAl₂O₄:Eu has changed to the P6₃22 phase upon e-beam exposure at low temperature; (4) We have identified the 460 nm band in undoped BaAl₂O₄ and the 425 nm band in BaAl₂O₄:Eu²⁺ with F-centre luminescence, corresponding to the F-centre emission in α-Al₂O₃. The evidence for the assignment of the 425 nm band in BaAl₂O₄:Eu²⁺ is the spectacular increase of the spectral radiance at 425 nm by e-beam exposure at 200 keV and low temperature. A preliminary model is presented that explains the results. The PL from BaAl₂O₄:Eu²⁺ quenched at the rather low temperature of 140°C; this observation is explained in terms of thermal ionization of the Eu²⁺ ion.