Assignment of spectroscopic properties in praseodymium-doped and praseodymium/ytterbium-co-doped ternary K2LaX5 (X = Cl, Br, I) Halides

The potassium lanthanum ternary halides are appropriate host lattices for various luminescent materials including laser devices. The lanthanum ternary halide lattice, co-doped by different lanthanide ions, can exhibit up-conversion emission in a wide spectral range from blue to red. For this purpose (Pr3+) and (Yb3+) ions and also co-doping by other Ln ions are promising systems. In this lecture the excited states dynamics in the praseodymium and praseodymium co-doped by ytterbium ion ternary halides will be discussed basing on the high resolution absorption, emission and emission excitation spectra at 293, 77 and 4 K as well as the decay times of the crystals with different concentrations from 2 mol% to stoichiometric of the optically active ions. Changes of the spectral properties in the range of 4f–5d and 4f–4f transitions are observed for the series of halides (Cl, Br, I).The differences in emission behavior can be explained by changes of the crystal field strengths, subtle variations of electron–phonon coupling, decrease of the phonon energies and increase of covalency. Some features in the emission vary with excitation energy and in some cases exciton emission and strong re-absorption was detected. Lowering of the emission efficiency in the concentrated systems can be due to the concentration quenching and energy migration to traps. On the other hand, formation of the polymeric chains in the structure is responsible for strong ion–ion interaction, which can promote the up-conversion process. Excitation into 1D2 of the Pr3+ state leads to up-conversion emission from 3P0 state, whereas 940 nm pumping line used in excitation of 2F5/2 level of the Yb3+ in K2LaCl5:5% Pr3+, 2% Yb3+ crystal is responsible for both emission from 1D2 and 3P0 of Pr3+ levels. Non-radiative energy transfers between Pr3+ ions and Pr3+–Yb3+ ions are observed and up-conversion mechanisms are proposed.