Cathodoluminescence characterization of tridymite and cristobalite: Effects of electron irradiation and sample temperature

Abstract

Cathodoluminescence (CL) spectra of tridymite and cristobalite have broad peaks around 430 and 400 nm, respectively, both of which can be assigned to the [AlO 4 /M + ] 0 defect. The CL intensities of these spectral peaks in the blue region decrease with prolonged exposure to electron irradiation, similar to the short-lived luminescence observed in quartz, although quartz shows a lower decrease in CL intensity compared to these minerals. Cristobalite has a higher CL intensity reduction rate during irradiation than does tridymite. Irradiation of these minerals at low temperatures results in a more rapid decay of CL emission, whereas that of quartz shows no apparent change in the rate of CL emission at similar temperatures. Confocal micro-Raman spectroscopy of the electron irradiated surface of these minerals reveals the amorphization caused by the interaction of the electron beam with the surface layer to a depth of several micrometers. This suggests that such structural destruction diminishes the activity of CL emission centers related to the [AlO 4 /M + ] 0 defects by migration of monovalent cations associated with exchanged Al in the tetrahedral sites. Both samples present a considerable reduction of their CL intensities at higher temperature, suggesting a temperature quenching phenomenon. The activation energy in the quenching process was evaluated by a least-squares fitting of the Arrhenius plots, assuming the Mott-Seitz model. The result implies that the energy of non-radiative transition in this process might be transferred to lattice vibrations as phonons in two different manners. This might be related to different irradiation responses of the CL with a change in sample temperature.

Effect of impurities on cathodoluminescence of tridymite and cristobalite

“…Tridymite from Vechec, Gunma, and Kumamoto and cristobalite from Shizuoka and Kagoshima used in a previous study (Kayama et al, 2009) were considered as reference samples and their CL data were compared with the CL data of cristobalite from Coso Peak, California, and Utah. Hand -picked crystals were embedded in a small dimple (φ: 2 mm) on a brass disk using a nonluminescent epoxy resin and were polished and mirror -finished using diamond powder with particle size of 1 μm.…”

“…Hence, a low beam current has to be maintained to prevent the saturation of the photomultiplier detector. Therefore, CL spectral measurements on all tridymite and cristobalite samples are performed in the scanning mode across an area of 220 μm × 165 μm at an accelerating voltage of 15 kV and a beam current of 0.1 nA; these conditions are different from those used in our previous study (Kayama et al, 2009). The change in the CL intensity at the peak positions with an increase in electron irradiation time is also monitored under a low current intensity of 1.0 nA in the focused mode (φ: 1 μm) for 600 s. Panchromatic CL images are obtained at high magnification using a CL detector (Gatan: MiniCL) that can be used for SEM -CL analysis; the spectral response of this detector has a maximum in the blue region and gradually decreases with an increase in the wavelength up to 800 nm.…”

“…The change in the CL intensity at the peak positions with an increase in electron irradiation time is also monitored under a low current intensity of 1.0 nA in the focused mode (φ: 1 μm) for 600 s. Panchromatic CL images are obtained at high magnification using a CL detector (Gatan: MiniCL) that can be used for SEM -CL analysis; the spectral response of this detector has a maximum in the blue region and gradually decreases with an increase in the wavelength up to 800 nm. The equipment construction and the analytical procedure are strictly followed from the reports of Ikenaga et al (2000) and Kayama et al (2009). Electron microprobe analysis was carried out to determine impurity contents in all samples using JEOL: JXA -8900R.…”

“…Recently, Kayama et al (2009) ) captured by the defects are easily diffused in the lattice. This CL behavior, i.e., short -lived blue luminescence, has been also reported in a previous CL study on hydrothermal quartz (Zinkernagel, 1978;Ramseyer et al, 1988;Ramseyer and Mullis, 1990;Perny et al, 1992).…”

“…This CL behavior, i.e., short -lived blue luminescence, has been also reported in a previous CL study on hydrothermal quartz (Zinkernagel, 1978;Ramseyer et al, 1988;Ramseyer and Mullis, 1990;Perny et al, 1992). Raman spectroscopy revealed that the partial structural destruction of cristobalite and tridymite by electron bombardment promoted the diffusion of monovalent cations, resulting in short -lived luminescence (Kayama, et al, 2009). Kayama et al (2009) also suggested that the impurity concentrations of aluminum and monovalent cations might be related to the reduction process of the CL intensity of both minerals; however, there is no quantitative evidence supporting such a scheme.…”