Elsevier

Journal of Luminescence

Volume 177, September 2016, Pages 139-144
Journal of Luminescence

Full Length Article
Centers responsible for the TL peaks of willemite mineral estimated by EPR analysis

https://doi.org/10.1016/j.jlumin.2016.04.026Get rights and content

Abstract

The mineral willemite (Zn2SiO4) exhibits five thermoluminescence (TL) peaks approximately at 160, 225, 260, 310 and 400 °C. Electron paramagnetic resonance (EPR) studies were carried out to study the defect centers induced in the mineral by gamma irradiation and also to identify the centers responsible for the TL process. Room temperature EPR spectrum of irradiated mineral is a superposition of at least four distinct centers. One of the centers (center I) with an isotropic g factor 2.0114 is attributable to an intrinsic O type center and the center correlates with the TL peak at 160 °C. Center II exhibiting hyperfine lines is also tentatively assigned to an O ion and is related to the low temperature TL peak at 160 °C. Center III is characterized by an axially symmetric g-tensor with principal values g||=2.0451 and g=2.011 and is identified as an O2 ion. This center appears to be related to 160, 225 and 260 °C TL peaks. Center IV with principal g-values g||=2.0025 and g=2.0088 is attributed to an F+-type center (singly ionized oxygen vacancy) and is the likely recombination center for TL peaks between 160 and 310 °C.

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 C3h2(R3) 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.

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