Annealing effects on cathodoluminescence of zircon

Abstract

Color cathodoluminescence (CL) images of zircon from southern Malawi (MZ) show mottled yellow CL emissions on a dull luminescent background by the annealing below 600°C, and relatively homogeneous blue emissions above 800°C. CL spectroscopy of the samples reveals a change of the luminescent features with an annealing temperature, and explains a variation of their colors and luminance observed by a CL microscope. Emission bands at 310 and 380 nm in an ultraviolet (UV) to blue CL are assigned to intrinsic centers in host lattice, narrow peaks at 475 and 580 nm to Dy 3+ impurity centers, and broad bands at 500 to 650 nm to Frenkeltype defects and SiO m n− groups. The former two show increases in emission intensities against annealing temperature, but the latter exhibits an increase in intensity up to 300°C and a decrease above 300 to 700°C. An increase in an annealing temperature leads to a reproduction of the intrinsic centers, which is responsible for an increase in UV-blue emission, in host lattice accompanied with a recrystallization from metamict state. An increase in the intensities of narrow peaks activated by Dy 3+ impurities may result possibly from a recovery of ionization due to the self-radiation and an energy transfer of other REEs to Dy 3+ activator. A gradual increase in yellow emission bands up to 300°C might be caused by a migration of the hole around a thermally-instable lattice defect and/or activated impurities into a more stable site related to Frenkel-type defects and SiO m n− groups, whereas the yellow should be subsequently reduced due to an elimination of these defects.

Cathodoluminescence of synthetic zircon implanted by He⁺ ion

“…This might be attributed to structural defect centers related to radiation-induced damage. According to Gaft et al (1998) and Finch et al (2004), the luminescence centers detected from IL and PL in a yellow region are assigned to structural defects referred from the SiO m n-groups and Frenkel-type defects, which were also confirmed by Tsuchiya et al (2015). The natural zircon (U: 241 ppm and Th: 177 ppm) from Malawi has two emission components at 1.97 eV and 2.20 eV in the yellow region (Tsuchiya et al, 2015), which are obtained by the deconvolution of remaining CL-spectral data after deduction of the emission components derived from the REEs such as Dy (Fig.…”

“…These components can be assigned to the "intrinsic" defect centers of a 3 , a 4 , a 5 and a 6 , respectively, as was proposed by Cesbron et al (1995). The same emission centers were recognized in annealed natural zircon with various degree of metamictization (Tsuchiya et al, 2015). According to Cesbron et al (1995) and Rémond et al (1995), the emission centers of a 1 to a 6 obtained in synthetic zircon are closely related to the structural defects in a host lattice associated with specified atomic-bonding formed during crystallization, so termed "intrinsic" defects.…”

“…2), that is too weak to be recognized in its color image. Yellow emission has been extensively reported in various types of natural zircon, and assigned to radiation-induced defects by the disintegration of U and Th or to impurity activation of (UO 2 ) +2 (Götze et al, 1999;Nasdala et al, 2002;Gaft et al, 2002;Tsuchiya et al, 2015). According to Tsuchiya et al (2014), young natural zircon extracted from the Takidani granodiorite aged at ~1.4 Ma does not have an obvious emission bands in a yellow region possibly due to very low radiation damage in its structure.…”

“…Therefore, a spectral deconvolution should be required for the characterization of constituent emission components in zircon CL. Recently, Tsuchiya et al (2015) have demonstrated that a spectral deconvolution of natural zircon CL can be successfully carried out using a Gaussian function in energy units by the peak-fitting method proposed by Stevens-Kalceff (2009) and Kayama et al (2010). We deconvolute the CL spectra of unimplanted and He + -ion implanted samples with initial peak energies determined by Tsuchiya et al (2015) using the peak-fitting software (Peak Analyzer) implemented in Origin Pro 8.1J SR2.…”

“…Furthermore, a process of metamictization in radioactive minerals (e.g., zircon) has been estimated by luminescence methods for ionimplanted samples as a simulation of radiation-induced damage on minerals (e.g., Weber et al, 1994;Lian et al, 2003;Ewing et al, 2003;Finch et al, 2004;Nasdala et al, 2011). Annealing effects on the CL derived from radiation-induced defect in zircon have been investigated in detail by Raman and CL spectroscopies, and CL color imaging (e.g., Nasdala et al, 2002;Tsuchiya et al, 2015).…”