Luminescence from self‐trapped excitons in BaFI and BaFBr_1–xI_x crystals

Abstract

Abstract Luminescence from BaFI and BaFBr 1– x I x crystals has been investigated using vacuum ultraviolet photons from synchrotron radiation in the temperature range of 10–300 K. In BaFI, two kinds of luminescence bands are observed under band‐to‐band excitation: one is the 4.52 eV band appearing at low temperatures, and the other is the 3.44 eV band appearing as the 4.52 eV band weakens with increasing temperature. From the excitation spectra, the 4.52 eV band is attributed to the radiative decay of a self‐trapped exciton (STE), and the 3.44 eV band to the extrinsic luminescence coming from an STE trapped by an I – vacancy. The change of luminescence spectra studied in BaFBr 1– x I x suggests that the STE configuration of on‐center type is responsible for the 4.52 eV intrinsic band, and the off‐center instability hardly occurs in BaFI. The temperature dependence of the 4.52 eV and 3.44 eV band intensities is well explained by assuming an STE relaxation mechanism whereby the STE relaxes through phonon‐assisted tunneling and hopping diffusion processes at high temperatures. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)