TL in green tourmaline: Study of the centers responsible for the TL emission by EPR analysis

Abstract

Electron paramagnetic resonance (EPR) studies have been carried out to identify the defect centers responsible for the thermoluminescence (TL) peaks in the mineral tourmaline. The mineral exhibits three TL peaks approximately at 170, 250 and 310 °C. The EPR spectrum of the green tourmaline sample pre-heated to 500 °C presented a large signal around g = 4.3 due to Fe³⁺ ion. Room temperature EPR spectrum of irradiated green tourmaline shows the formation of two defect centers in the region of g = 2.0. One of the centers (center II) with a g factor equal to 1.96 is identified as an F⁺-center and is related to the observed high temperature 250 and 310 °C TL peaks. Center I exhibiting a doublet is due to hydrogen atoms (H⁰), stable in the crystal lattice at room temperature and this center correlates with the TL peak at 170 °C of the green tourmaline. An optical absorption measurement also was carried out. Bands at around 430, 730 and 1100 nm have been observed.

Introduction

The silicate minerals are, in general, excellent thermoluminescent materials, some of them with high sensitivity to low as well as high radiation doses [1], [2], [3], [4], [5], [6], [7]. Hence, they are candidates for radiation dosimetry.

There is one group of silicates called ring silicates or cyclosilicate to which belong beryl, cordierite and tourmaline. The tourmaline structure is typically rhombohedral with space-group R3m [8], [9], [10], although some studies report lower symmetry such as orthorhombic, monoclinic or triclinic [11], [12], [13]. The structure is characterized by groups of XO₉, YO₆, TO₄, and BO₃ polyhedra connected to each other through ZO₆ octahedra. The latter are arranged in a 3-D framework and are linked to the YO₆ octahedron through the O3–O6 edge. The tourmaline has a complex formula, XY₃Z₆(T₆O₁₈)(BO₃)₃V₃W, where X, Y and Z sites can be occupied by different ions [8], [9], [10]. Therefore, about 12 varieties of tourmaline are formed in nature. According to several authors [14], [15], [16], [17], the following ions fit into the following structural sites: X = Na, Ca, ο (= vacancy), K; Y = Al, Fe³⁺, Cr³⁺, V³⁺, Mg, Fe²⁺, Mn²⁺, Cu²⁺, Zn, Li, Ti⁴⁺, ο; Z = Al, Fe³⁺, Cr³⁺, V³⁺, Mg, Fe²⁺; T = Si, Al, B, Be; B = B, (ο); W(O1) = OH, F, O; V(O3) = OH, O.

Tourmaline is a well known silicate mineral because some of its varieties have high gemological value [18], [19].

The tourmaline crystal has been widely investigated by many authors through spectroscopic methods such as Mössbauer spectroscopy, UV–Vis spectroscopy, Raman spectroscopy and other spectroscopic techniques due to its color and gemological value [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36].

Several authors [37], [38], [39], [40], [41], [42] measured the effects of thermal treatments and irradiation on optical absorption spectra of natural tourmaline of different color and discussed the crystal field effect on the energy levels of transition ions, mostly Mn²⁺, Mn³⁺, Fe²⁺, Fe³⁺ and Ti⁴⁺, usually responsible for the coloration of the crystal. In contrast, studies on luminescence properties and the identification of the defects responsible for emission of the TL peaks of the tourmalines crystal so far are few.

The process of irradiation and thermal treatment can change some physics properties of the mineral that are dependent on point defects, such as luminescence and electron paramagnetic resonance. These properties make tourmaline crystal an interesting material for some applications like dosimetry. However, although it has been a subject of some experimental studies, an investigation of defect centers created by ionizing radiation responsible for TL properties of tourmaline is still lacking. The identification and characterization of these centers form an essential step in understanding the mechanisms of TL emission. In this context, EPR provides a convenient and sensitive technique for such a study, as it helps in providing support and further identification of the paramagnetic species by EPR technique.

In the present work a green tourmaline sample was studied using the TL and EPR techniques to investigate the centers responsible for their TL properties and possible applications in gamma radiation dosimetry. No published papers in the literature were found related to study here carried out. Optical absorption measurements also have been carried out.