Spectroscopic study of red-light-emitting centers in K2Al2B2O7: Fe single crystals

We report on spectroscopic study of red-light-emitting centers in K2Al2B2O7 (KABO) single crystals containing ca. 2 ppm of Fe3+. Owing to the low Fe3+-concentration, KABO does not show noticeable absorption due to Fe3+d–d-transitions in the visible spectral region, but it exhibits the charge-transfer (CT) UV-absorption bands O–Fe at 4.7, 5.7 and 6.5 eV. The red photoluminescence at 1.675 eV (FWHM = 0.173 eV) is due to intracenter 4T1 (4G) → 6A1 (6S) transitions in Fe3+ ions. Because of partial overlapping of the fundamental absorption edge of the crystal, where mobile excitons are created, and a broad CT absorption band at 6.5 eV, the most intensive red emission occurs at 7 K upon excitation in the excitonic energy region. The presence of two nonequivalent Al2O7 clusters in KABO lattice provides two different types of red-light-emitting centers in the form of Fe3+ ion occupied the Al3+ tetrahedral site. Superposition of their luminescence bands determines both the spectrum and temperature dependence of red emission in KABO at T = 7–80 K: two bands with the ratio of intensities of ca. 2:1 are 20 meV-shifted relative to each other; two-stage thermal quenching obeys the Mott law with ET = 9 and 20 meV.

► We carried out spectroscopic study of red-light-emitting centers in KABO.
► We measured optical absorption and low-temperature luminescence spectra.
► Red emission of KABO is due to lowest d–d transitions in Fe (III) ions.
► Two-stages temperature quenching occurs at 19 and 31 K (E = 9 and 20 meV).
► The dominant PLE at 6.5 eV is due to excitonic energy transfer to Fe (III).