Full Length ArticleCenters responsible for the TL peaks of willemite mineral estimated by EPR analysis
Introduction
Silicate minerals are found abundantly in the Earth crust. They are ionic crystals in which anions can be tetrahedral SiO4 or complex molecules formed by coupling of SiO4 tetrahedra. One, two, up to five valence cations can be bonded to produce a very large variety of crystals of silicate. The natural minerals of silicate during their formation have incorporated large number of impurities that influence profoundly the proprieties of minerals. These impurities together with thermodynamically produced point defects are, of course, basic of mineral physical proprieties and how to correlate them is an important and not easy task to be carried out.
Willemite, Zn2SiO4, is isomorphous with phenakite (Be2SiO4) and phenakite belongs to point group. The rhombohedral unit cell has parameters 0.768 nm and 108° 1′ and the hexagonal a=1.242 nm and c=0.821 nm [1]. Willemite is a silicate of individual unlinked silicon tetrahedra. Each zinc is also at the center of the oxygen tetrahedral of almost the same size as the silicon tetrahedron. These tetrahedral are linked together in a chain running parallel to the hexagonal c axis. Each oxygen has two zinc and one silicon atoms at the corners of an equilateral triangle.
In a perfect lattice, Si and Zn atoms are expected to be located in their respective sites. However, antisite cation exchange is likely to exist in Zn2SiO4 wherein Si atoms will partially replace Zn sites. These replacements, called as cation exchange disorder, are a point defect in crystal lattices. The occurrence of such defects has been predicted by theoretical calculations [2]. X-ray diffraction [3] and X-ray absorption fine structure [4] investigations support this prediction. In recent years, it has also been possible to directly observe these defects by advanced electron microscopy [5]. This mineral has been investigated experimentally by many authors due to its luminescence properties. Luminescence properties may be affected by these defects and a recent study of Cr3+ doped AB2O4 spinels has demonstrated the effects of such defects [6]. The luminescence of the synthetic willemite doped with Eu3+, Mn2+(Tb3+), or Ce3+ covers the red, green, and blue portions of the visible spectrum, respectively [7], [8], [9], [10]. A process of irradiation and thermal treatment can change the luminescent properties of the crystals [11], [12], [13]. Due to their high luminescence, the sample natural willemite can be used in the dosimetry by TL and EPR.
As far as we know, no work has been published on TL and EPR of willemite crystal up to the present. The objective of the present work is to study the nature of the luminescence and paramagnetic centers in natural willemite, measuring the effects of gamma irradiation and thermal treatments through electron paramagnetic resonance (EPR) technique in an attempt to understand some physical properties and seeking possible applications in the area of ionizing radiation dosimetry of this mineral.
Section snippets
Materials and methods
Willemite of chemical formula Zn2SiO4 here investigated is a natural silicate mineral from Mexico, purchased through stone dealer Luis Menezes Minerals Ltd. in Belo Horizonte, MG. This mineral together with olivine, phenakite and zircon belongs to the group of olivine.
The TL measurements were carried out in a nitrogen atmosphere with a model 4500 Harshaw TL reader equipped with two photomultiplier tubes, which could record luminescence independently. The reader was controlled by WinRems
Results and discussion
The diffractograms of the willemite crystal is shown in Fig. 1, together with that of a standard willemite crystal. Comparing the powder XRD pattern to the JCPDS files, all the peaks of the crystal are identified as belonging to willemite (JCPDS card, No. 37-1485), as shown in Fig. 1. An analysis of the main oxide components of the willemite crystal was obtained by X-ray fluorescence (XRF). Results are presented in Table 1. This analysis was performed to identify the chemical elements in the
Conclusions
Willemite mineral exhibits TL glow peaks at about 160, 225, 260, 310 and 400 °C. Four defect centers have been identified in the irradiated mineral. These centers are tentatively assigned to O− ions, O2− ion and F+ center. O− ion (1) correlates with the 160 °C TL peak while O− ion (2) is also associated with this TL peak. A broad decay is exhibited by the O2− ion which relates to the TL peak at 160 °C and also may contribute to the main dominant peaks at 225 and 260 °C. The F+ center appears to act
Acknowledgments
The authors wish to thank Ms. E. Somessari and Mr. C. Gaia, Instituto de Pesquisas Energeticas e Nucleares (IPEN), Brazil, for kindly carrying out the irradiation of the samples. To FAPESP (Process number 2014/03085-0) for partial financial support and to CAPES (Process number BEX-9612130) for fellowship to T.K. Gundu Rao.
References (34)
- et al.
Mater. Chem. Phys.
(2001) - et al.
Thin Solid Films
(2000) - et al.
Chem. Phys. Lett.
(2002) - et al.
J. Phys. Chem. Solids
(2003) - et al.
Spectrochim. Acta A
(2015) - et al.
J. Lumin.
(2011) - et al.
J. Lumin.
(2015) - et al.
Chem. Phys. Lett.
(1971) - et al.
J. Magn. Magn. Mater.
(2016) - et al.
J. Phys. Chem. Solids
(1989)
Appl. Catal.
J. Catal.
Phys. Crystallogr.
J. Phys.: Condens. Matter
Appl. Phys. Lett.
Phys. Rev. B
ACS Appl. Mater. Interfaces
Cited by (8)
Thermoluminescence (TL) and electron paramagnetic resonance (EPR) of dumortierite sensitized by heat treatments
2024, Physica B: Condensed MatterUnveiling the luminescence of α-Zn<inf>2</inf>SiO<inf>4</inf> phosphor: Profound influence of sintering temperatures
2023, Radiation Physics and ChemistryEstimating trap distribution and intertrap charge transfer in SrZnO<inf>2</inf> nanoparticles
2020, Journal of Physics and Chemistry of SolidsCitation Excerpt :The lack of maxima at any radiation dose indicated that trap B had sufficient space to capture the charge carriers from trap A (as shown in Fig. 4(b)). For other material systems, the glow curve in the temperature range from 200 to 350 °C was assigned to doubly or singly ionized oxygen vacancies [36–38]. Janotti et al. [2] also reported that oxygen vacancies can create deep donor levels in systems.
Thermoluminescence and defect centers in β-CaSiO<inf>3</inf> polycrystal
2020, Journal of LuminescenceCitation Excerpt :They have observed that the TL intensity increases with radiation dose, a property desired by all TL dosimeters. In recent years, defect centers in other silicate minerals, natural and synthetic, were investigated using the EPR technique in order to study their correlations with the peaks of the TL glow curve emission [8,9,20–23]. As far as we know, no work has been published on the studies carried out on the role of defect centers in TL of the wollastonite.
Thermoluminescence and defect centers in synthetic diopside
2019, Journal of LuminescenceCitation Excerpt :Recently, some natural and synthetic silicates have been widely investigated due to their significance for fundamental research and their high potential for application in radiation dosimetry using the TL technique [4–8]. Even though these materials present interesting luminescence properties, few studies have been carried out on the defects responsible for the TL emission of these materials [9–13]. Radiation-induced defects in phosphors and their role in their thermoluminescence properties are well established.
Synthetic polycrystals of CaSiO<inf>3</inf> un-doped and Cd, B, Dy, Eu-doped for gamma and neutron detection
2018, Journal of LuminescenceCitation Excerpt :The development and study of new radiation dosimeters have been published in the past [1–16].