Polarized luminescence and the mössbauer effect of tin impurity centres in NaCl crystals

An investigation is made of various types of impurity centres in NaCl;Sn and KCl:Sn crystals using the Mössbauer effect and polarized luminescence. Two types of Sn 2+ centres are found in NaCl: Sn. The centres of type I have polarized emission at 2.78 eV due to electronic transitions allowed along the C 4 axis of the crystal. A small quadrupole splitting (<0.4 mm/s) of the Mössbauer absorption line at +4.32 mm/s is observed for these centres. The centres of type II show the polarized emission at 2.3 eV due to electronic transitions allowed along the C 2 axis of the crystal. For these centres a considerable value of quadrupole splitting Δ = 0.97 mm/s of the Mössbauer lines with the isomer shift + 3.75 mm/s is found. Only the centres of the type I are found in KCl: Sn crystals. It is supposed, that the investigated centres in NaCl:Sn represent two types of associates of Sn 2+ ions with cation vacancies v c : associates with C 4V symmetry (the type I centres) and associates with C 2v symmetry (the type II centres) in the ground electronic state.