Origin of Bi3+-related luminescence centres in Lu3Al5O12:Bi and Y3Al5O12:Bi single crystalline films and the structure of their relaxed excited states

Single crystalline films (SCFs) of Y3Al5O12:Bi and Lu3Al5O12:Bi with different Bi3+ contents are studied at 1.7300?K by the time-resolved luminescence spectroscopy methods under excitation in the 2.420?eV energy range. The ultraviolet (UV) emission of these SCFs is shown to arise from the radiative decay of the metastable and radiative minima of the triplet relaxed excited state (RES) of a single Bi3+ centre, which are related to the 3P0 and 3P1 levels of a free Bi3+ ion, respectively. At T?<?80?K, the radiative decay of the metastable minima is only observed resulting in the appearance of the low-energy component of the triplet emission. At T?>?100?K, both the high-energy shift of the maximum and the shortening of the decay time of the UV emission with increasing temperature are caused by thermally stimulated non-radiative transitions between the metastable and radiative minima of the triplet RES. Their excitation bands located around 4.6, 5.2 and 5.95?eV are assigned to the 1S0???3P1, 1S0???3P2 and 1S0???1P1 transitions, respectively, in free Bi3+ ions. The luminescence of dimer Bi3+Bi3+ centres is not detected in the SCFs studied. The lower-energy (similar to 2.6?eV) visible (VIS) emission of these SCFs is due to an exciton, localized near a single Bi3+ ion, while the higher-energy (2.75?eV) VIS emission, an exciton, localized near a dimer Bi3+Bi3+ centre. The phenomenological models are proposed to describe the excited-state dynamics of all the luminescence centres studied. Application of the two- or three-excited-level models on the temperature evolution of the luminescence decay times has allowed determination of characteristic parameters of the corresponding RES: the energy separations between the excited states and the rates of the radiative and non-radiative transitions from these states.