On the character of the optical transitions in closed-shell transition metal oxides doped with Bi3.

A criterion is introduced to achieve the assignment of the optical features observed in the excitation spectra of Bi3+ ions incorporated in closed-shell transition metal oxides. The model is based on the calculation of the energy associated with the lowest 1S0 → 3P1 intra-ionic transition of Bi3+ (A-like transition), the metal-to-metal charge transfer (D-like transition) and the Stokes shift of the corresponding emission.

Energy levels in CaWO4:Tb(3+) at high pressure.

The luminescence properties of Tb(3+) in CaWO4 crystals are investigated under a hydrostatic pressure of up to 200 kbar, i.e. across scheelite-to-fergusonite phase transition. It is shown that the typical blue ((5)D3) and green ((5)D4) emissions in this material are progressively quenched at room temperature as pressure is increased. This quenching is caused by a downshift of the charge transfer (or impurity trapped exciton) state that is formed between Tb(3+) and nearby W(6+) cations in conjunction with a pressure-induced increase of the lattice relaxation experienced by this excited state. An empirical model is introduced to calculate the evolution of the (Tb(3+)-W(6+)) charge transfer energy with pressure. Combined with the pressure dependence of the energy bandgap in CaWO4, the model allows locating the 4f levels of Tb(3+) relative to the fundamental host lattice for any pressure in the range 0-200 kbar.

NaNbO(3):Pr(3+): a new red phosphor showing persistent luminescence.

The Pr(3+)-doped NaNbO(3) perovskite is introduced as a new red phosphor excitable in the near UV region at around 350 nm. A bright single red emission is observed at room temperature and ascribed to transitions between the (1)D(2) excited state and the ground state (3)H(4) of Pr(3+) ions. This peculiar behavior is related to the presence of a low-lying intervalence charge transfer state that contributes to quench the emission from the otherwise emitting (3)P(0) level. Red afterglow is also evidenced in NaNbO(3):Pr(3+). A preliminary model is discussed to clarify the afterglow mechanism.

High pressure evolution of YVO(4):Pr(3+) luminescence.

Photoluminescence and time-resolved photoluminescence spectra of YVO(4) doped with Pr(3+) obtained at high hydrostatic pressure up to 76 kbar applied in a diamond anvil cell are presented. At pressures lower than 60 kbar the steady state emission spectra consist of sharp lines related to the [Formula: see text] transition in Pr(3+). At pressures above 68 kbar the Pr(3+) emission intensity decreases and the corresponding bands are replaced by a broad band peaking at 19 500 cm(-1) attributed to perturbed VO(4)(3-) host luminescence. The quenching of the [Formula: see text] emission has been attributed to nonradiative transition to the charge transfer exciton trapped at Pr(3+) ion. The recovering of the VO(4)(3-) host luminescence at high pressure has been attributed to energy transfer from a Pr(3+) trapped exciton (PTE) to the host YVO(4). The kinetics of such a process is analyzed using the model of PTE considered as a Pr(4+) + electron bound by the Coulomb potential at the delocalized Rydberg states.

Photon conversion in Bi(3+)/Pr(3+)-codoped CaTiO(3).

The luminescence properties of CaTiO(3) both singly and doubly doped with Bi(3+) and Pr(3+) have been measured in different experimental conditions in order to investigate the de-excitation processes active in these materials and in particular to understand the mechanism responsible for the significant increment of the red emission observed in the codoped system upon near-ultraviolet (NUV) excitation. The steady state spectra and the decay profiles have been analyzed in the light of the possible interactions between active ions and host lattice. The general model applied to the analysis of the temporal profiles of the emission allows us to assess which are the main mechanisms involved in the Bi(3+)  â†’  Pr(3+) sensitization process.

Luminescence properties of K1/2Bi1/2TiO3:Pr3+ and Na1/2Bi1/2TiO3:Pr3+.

The luminescence properties of K(1/2)Bi(1/2)TiO(3):Pr(3+) and Na(1/2)Bi(1/2)TiO(3):Pr(3+) powders are investigated in the temperature range 10-600 K. The experimental data are interpreted on the basis of metal-to-metal charge transfer processes and by considering Bi(3+)-to-Pr(3+) sensitization effects.