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

Abstract

We report on spectroscopic study of red-light-emitting centers in K2Al2B2O7 (KABO) single crystals containing ca. 2ppm 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 6.5eV. The red photoluminescence at 1.675eV (FWHM=0.173eV) 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 6.5eV, the most intensive red emission occurs at 7K 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=7–80K: two bands with the ratio of intensities of ca. 2:1 are 20meV-shifted relative to each other; two-stage thermal quenching obeys the Mott law with ET=9 and 20meV.