Luminescence and excited state dynamics in Bi3+ -doped LiLaP4O12 phosphates

Photo- and X-ray-excited luminescence characteristics of Bi-doped LiLaP4O12 phosphates with different bismuth contents (from 1 to 25 at% in the melt) are investigated in the 4.2-300 K temperature range and compared with the characteristics of the undoped LiLaP4O12 phosphate. The broad 2.95 eV emission band of LiLaP4O12:Bi excited around 5.4 eV is found to arise from the bismuth dopant. Relatively large FWHM and Stokes shift of the emission band and especially the data on the low-temperature decay kinetics of the 2.95 eV emission and its temperature dependence, indicating a very small spin-orbit splitting energy of the corresponding excited state, allow the conclusion that this emission arises from the radiative decay of the triplet state of an exciton localized around a Bi3+ ion. No spectral bands are observed, arising from the electron transitions between the energy levels of Bi3+ ions. Phenomenological model is proposed for the description of the excited state dynamics of the Bi3+-related localized exciton in LiLaP4O12:Bi and the parameters of the triplet localized exciton state are determined.