The Nature of Anomalous Red Luminescence in K₂Al₂B₂O₇ : Fe Nonlinear Optical Crystals

Spectra of anomalously intense luminescence at 1.675 eV (740 nm, FWHM = 0.173 eV) have been experimentally recorded for the first time in wide-band (180–3600 nm) K₂Al₂B₂O₇ nonlinear optical crystals upon excitation in the spectral region of 4–5 harmonics of the fundamental frequency of Nd³⁺ radiation. It is shown by low-temperature (7–80 K) luminescence–optical spectroscopy under selective excitation by synchrotron radiation that red luminescence arises at the intracenter ⁴T₁(⁴G) → ⁶A₁(⁶S) transitions in Fe³⁺ impurity ions with a concentration below 2 ppm. The high excitation efficiency in the exciton region is due to partial overlap of the fundamental absorption edge of the crystal, where mobile excitons are excited, and wide O–Fe charge-transfer absorption band peaking at 6.5 eV. The luminescence spectrum and temperature dependence are caused by the superposition of the luminescence centers of two types (activation energies 9 and 20 meV) based on Fe³⁺ impurity ions located at regular Al³⁺ sites of two nonequivalent Al₂O₇ clusters.