Optical excitation processes in the near band-edge region of and feldspar

Rare earth ion Tb3+ doped natural sodium feldspar (NaAlSi3O8) Luminescent properties and energy transfer

In this study, Tb³⁺-doped natural sodium feldspar (NaAlSi₃O₈) phosphors have been successfully prepared using high-temperature solid-state method with natural sodium feldspar as a substrate. Energy-dispersive X-ray spectrometry analysis (EDX) of NaAlSi₃O₈ showed that 0.03 wt% of Eu element was present, and elemental distribution mapping analysis showed that the distribution of trace Eu in minerals was aggregated. The crystal structure and luminescence properties of the natural sodium Eu-containing feldspar and synthetic sodium feldspar NaAlSi₃O₈:Eu³⁺, Tb³⁺ phosphors are discussed in detail. The crystal structure analysis of the samples showed that the Na⁺ in the natural matrix was partly replaced by the doped Tb³⁺. Studies on the photoluminescence properties of the samples indicate that Eu does not form a luminescent center in the natural mineral, however, the strong characteristic peak of Eu³⁺ at 615 nm appears after doping with Tb³⁺ and the peak at 615 nm increases with the increase of Tb³⁺ concentration. According to the above spectral results, the energy transfer from Tb³⁺ to Eu³⁺ is obtained. Through the measurement and analysis of color coordinates, it is found that with the increase of Tb³⁺ concentration, the luminescence color of the samples can be regulated in the green to red region. NaAlSi₃O₈:Eu³⁺ Tb³⁺ phosphors has potential application value.

Stimulated luminescence emission from localized recombination in randomly distributed defects

We present a new kinetic model describing localized electronic recombination through the excited state of the donor (d) to an acceptor (a) centre in luminescent materials. In contrast to the existing models based on the localized transition model (LTM) of Halperin and Braner (1960 Phys. Rev. 117 408-15) which assumes a fixed d → a tunnelling probability for the entire crystal, our model is based on nearest-neighbour recombination within randomly distributed centres. Such a random distribution can occur through the entire volume or within the defect complexes of the dosimeter, and implies that the tunnelling probability varies with the donor-acceptor (d-a) separation distance. We first develop an 'exact kinetic model' that incorporates this variation in tunnelling probabilities, and evolves both in spatial as well as temporal domains. We then develop a simplified one-dimensional, semi-analytical model that evolves only in the temporal domain. An excellent agreement is observed between thermally and optically stimulated luminescence (TL and OSL) results produced from the two models. In comparison to the first-order kinetic behaviour of the LTM of Halperin and Braner (1960 Phys. Rev. 117 408-15), our model results in a highly asymmetric TL peak; this peak can be understood to derive from a continuum of several first-order TL peaks. Our model also shows an extended power law behaviour for OSL (or prompt luminescence), which is expected from localized recombination mechanisms in materials with random distribution of centres.

Direct evidence for the participation of band-tails and excited-state tunnelling in the luminescence of irradiated feldspars

The significance and extent of band-tail states in the luminescence and dosimetry properties of natural aluminosilicates (feldspars) is investigated by means of studies using low temperature (10 K) irradiation and optically stimulated luminescence (OSL) stimulation spectroscopy, and thermoluminescence (TL) in the range 10-200 K, made in comparison with high temperature (300 K) irradiation and photo-transferred OSL and TL investigations undertaken at low temperature. These measurements allow mappings of the band-tails to be made; they are found to be ∼0.4 eV in extent in the typical materials studied. Furthermore, by populating charge trapping centres at high temperature (300 K) and monitoring the OSL stimulation spectra at temperatures in the range 10-300 K, clear evidence is presented for the presence of both thermally activated and non-thermally activated OSL processes; it is argued that the former result from thermally activated hopping through the band-tail states, whilst the latter are due to tunnelling processes, either from the excited state of the OSL centres or through the tail states. The spectral measurements are supported by analysis of the temporal dependence of the OSL signals, which correspond to either tunnelling or general order kinetic decay processes.