A study of lattice defects in alkali fluorides by thermoluminescence experiments

Abstract

Summary. --Lattice defects in KF, NaF and LiF are investigated by recording the thermoluminescent emission of X-irradiated samples vs. temperature and wavelength. Experimental results show the presence of cation vacancies and interstitials in KF and NaF and of F (-) vacancies and interstitials in NaF and LiF. In NaF, the ionization energy of VF centres is found to be 6.4 eV, that of H centres 5.6 eV.PACS 78.60.Kn -Thermoluminescence. PACS 61.70.Ey -Other point defects.1. -Introduction.In alkali halides, lattice defects such as vacancies or interstitials localize point charges of both signs which, when crystals are excited at low temperature by X-rays, trap opposite-sign carriers. The thermoluminescent (TL) emission, originated by carrier recombination when crystals are heated, can be utilized to gain information on the presence and nature of these defects [I].On this basis, we recorded the TL emission of KF, NaF and LiF crystals by means of an apparatus operating under a vacuum and capable of spectral resolution within the 375 to 730 nm range [2]. Samples of high purity were utilized to avoid spurious emissions due to chemical impurities. The effect of thermal treatments, up to the crystal melting point, was also investigated since the nature and density of lattice defects, owing to thermodynamic constraints, are affected by temperature. -Experiments.Samples of LiF were supplied by Koch-Light (purity 99.5%) and J. Matthey (purity 99.98%), those of NaF by C. Erba (purity 99.72%) and J. Matthey (purity 99.999%). As for KF, only samples supplied by J. Matthey (purity 98%) were considered, since this material is highly deliquescent, so that only powders with 903