Intrinsic luminescence in alkali metal sulfates

For the first time, the nature of intrinsic luminescence of Na2SO4 and LiKSO4 crystals at 80 and 300 K excited by ultraviolet photons with energy of (4-6.2 eV) is studied. During the excitation of sulfates by low-energy photons, the electrons on the bonds become excited. These electrons can be found in the conduction band formed from the free states of cations or 3a1* and 4t2* orbitals of SO42− anion, also placed in the conduction band. As a result of the electrons transfer, holes are formed in the bonds or orbitals, which move from the valence band to the bandgap as SO4− radicals. Localized holes correspond to different local states. A special feature of this study is the appearance of several bands within the broad radiation band (4-2 eV), efficiently excited at photon energies (5.5-6.2 eV). It is supposed that the observed excitation spectra in spectral regions around 5, 5.5 and (6-6.2) eV of intrinsic luminescence of LiKSO4 crystal correspond to transitions of electrons from e terms 1t1, 3t2, e, 2t2 of the ground state of the SO42− ion. Intrinsic luminescence with several maxima occurs as a result of recombination of the excited electron in the conduction band with localized holes SO4−.