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 Sn2+ 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 C4 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 C2 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 Sn2+ ions with cation vacancies vc: associates with C4V symmetry (the type I centres) and associates with C2v symmetry (the type II centres) in the ground electronic state.