Understanding the metastable states in K-Na aluminosilicates using novel site-selective excitation-emission spectroscopy

Abstract

Optically stimulated luminescence (OSL) and thermoluminescence (TL) from the metastable states in solids are widely used in luminescent phosphors, dosimetry, geochronology, and thermo- and photo-chronometry. OSL and TL result from a combination of three different processes (charge detrapping, transport, and recombination) and are, therefore, not ideal for characterizing the charge trapping states. Therefore, despite many decades of research, the OSL and TL kinetics and the associated defect systems remain poorly understood in natural minerals. Recently, a radio-photoluminescence (RPL) signal has been discovered in feldspar (K-Na-Ca aluminosilicates occupying > 50% of Earth’s crust) which helps overcome this limitation. This site-selective signal termed infrared photoluminescence (IRPL) arises from radiative relaxation of the excited state of the main electron trapping center (principal trap). In this study, IRPL excitation and emission spectroscopy at cryogenic temperatures reveals two distinct electron-trapping centers (i.e. two principal traps) in feldspar, and helps to determine their trap depths and the excited-state energies. The two trapping centers show the same electron capture cross-sections and the excited-state relaxation lifetimes, but different ground- and excited-state energies. Based on this peculiar combination of trap characteristics, we conclude that that the principal traps consist of the same defect residing at two different crystal sites. The differences in the energy levels of the two principal traps explain their distinct optical and thermal bleaching behavior.

Site-selective characterisation of electron trapping centres in relation to chemistry, structural state and mineral phases present in single crystal alkali feldspars

“…12 4.1.2 Intensities and energies of IRPL and IR-RL emissions Kumar et al (2018Kumar et al ( , 2020a showed the presence of two IRPL and IR-RL emissions with peak maxima around 1.3 eV and 1.41 eV in their samples. In the present study the emission spectra were fitted in R using the nls function (Bates and DebRoy, 2018), which uses a non-linear least squares approach to iteratively find the most suitable fitting parameters for the given equation.…”

“…Poolton et al 1995). Kumar et al (2020a) found that the electron trap depth varied from sample to sample and between the two measured IRPL emissions, with a minimum trap depth of 1.90 eV (IRPL 1.3 centre) and a maximum trap depth of 2.38 eV (IRPL 1.41 centre).…”

“…Poolton et al 1995). Kumar et al (2020a) found that the electron trap depth varied from sample to sample and between the two measured IRPL emissions, with a minimum trap depth of 1.90 eV (IRPL 1.3 centre) and a maximum trap depth of 2.38 eV (IRPL 1.41 centre).Since feldspars are complex framework silicates, properties inherent to the sample's crystal structure, in addition to the chemistry, could influence electron trapping centres in feldspars.Despite previous efforts, it remains to be established whether (i) electron traps in feldspars are associated with K + ions (Erfurt, 2003), (ii) if they occupy tetrahedral sites (Short, 2004), (iii) whether the same crystal defect which captures electrons in the feldspar crystal lattice is indeed located in two different crystallographic environments resulting in two different trapping centres (Kumar et al, 2018(Kumar et al, , 2020a or (iv) if neither of these suggestions holds true and further possibilities need to be considered.Recently, Kumar et al (2020a) described an approach for constraining electron trap ground and excited state energies using low temperature infrared-photoluminescence (IRPL) excitation-emission spectroscopy. In this paper we apply this technique to a range of chemically and structurally…”

“…Et refers to the transition from the ground state of the electron trap to the edge of the conduction band. Data for R28 and K8 are from Kumar et al (2020a). 1 The association of R28 as orthoclase is solely based on information given in Poolton et al (2009), however, these authors do not present any analytical data (i.e.…”

A novel coupled RPL/OSL system to understand the dynamics of the metastable states

“…Throughout the text, the IRPL (955 nm) and IRPL (880 nm) emissions are denoted as IRPL 955 and IRPL 880 , respectively. Kumar et al 16,37 demonstrated that these signals do not represent the two excited states of the same defect site but instead two different sites; the respective (unknown) principal traps that give rise to these signals are referred to as the 880 or 955 nm (emission) centres.…”