Ana Regina Blak

Optical absorption and electron spin resonance in blue and green natural beryl

A comparative study of blue and green beryl crystals (from the region of Governador Valadares, Minas Gerais, Brazil) using electron paramagnetic resonance (EPR) and optical absorption (OA) spectroscopy is reported. The EPR spectra show that Fe3+ in blue beryl occupies a substitutional Al3+ site and in green beryl is localized in the structural channels between two O6 planes. On the other hand the infrared spectra show that the alkali content in the blue beryl is mostly at substitutional and/or interstitial sites and in green beryl is mostly in the structural channels. The OA spectra show two types of Fe2+. Thermal treatments above 200° C in green beryl cause the reduction of Fe3+ into Fe2+ accompanied by a change of color to blue. The blue beryl color does not change on heating. The kinetics of the thermal conversion of Fe3+ into Fe2+ is composed of two first order processes; the first one has an activation energy ΔE 1=0.30 eV and the second one has an activation energy ΔE 2=0.46 eV.

Defect simulation in MgAl2O4 spinels

Abstract Computer modelling methods, based on the lattice energy and the defect energy minimization, have been used to study some properties of the MgAl2O4 spinels. Calculations have been carried out using defect simulation procedures, programmed in the “General Utility Lattice Program – GULP” following the Mott and Littleton strategy. Intrinsic defects which originate from cation antisite disorder due to the partial exchange of the positions of the divalent and trivalent cations have been studied. It was found that the probability of Al3+ replacing Mg2+ is higher than the inverse process. The results of the simulation studies indicate that the configuration energy is lower for Al3+ being replaced by trivalent impurities such as Cr3+, Mn3+ and Co3+, than for the replacement of Al3+ and Mg2+ by divalent impurities such as Cr2+, Mn2+ and Co2+. Another type of defect formed by two Al3+ replacing two Mg2+ and one Mg2+ vacancy has been studied. In this case the two Al3+ were also substituted by Cr3+, Mn3+ and ...

OPTICAL ABSORPTION (OA) AND THERMALLY STIMULATED DEPOLARIZATION CURRENTS (TSDC) IN BRAZILIAN AMETHYST

Abstract Amethyst species of Brazilian natural quartz were studid by means of Thermally Stimulated Depolarization Currents (TSDC) and Optical Absorption (OA). The absorption spectra show three main peaks in the visible region (10 000, 18 000 and 27 500 cm−1) attributed to iron in different valence states. In the infrared region the bands at 3430 and 3580 cm−1 indicated the presence of OH−1 centers. A change to yellowish colour is observed by heating the samples from 673 to 873 K. An increase in the absorption spectra in the ultraviolet region is then observed. The TSDC spectra show two bands at 235 and 275 K attributed to defects of dipolar origin. Thermal treatments between 673 and 873 K drastically reduce the two peaks that were related to substitutional Fe3+-interstitial H+ in two different configurations. This dipole was also correlated to the 18 000 cm−1 OA band.

Dipole defects in MgAl2O4 spinel crystals

Abstract In order to study defects of dipole origin in normal and inverse spinel structures, Thermally Stimulated Depolarisation Current (TSDC) measurements have been carried out in MgAl2O4 spinel crystals doped with Fe, Cr, Mn and Co. The TSDC spectrum of the as-received sample labelled R shows two bands at 160K and 290K. Temperatures above 700K destroy the 290K band and γ-irradiation up to 1,500kGy restores the peak. In another sample labelled UC, the 290K band, of dipole origin, is only detected after γ-irradiation. The dipole origin of the 160K band has already been verified. This band is affected neither by γ-irradiation nor by thermal treatment. The reduction of the 290K band is observed when heating the samples between 350K and 480K for three minutes at each temperature. The correlation between the thermal reduction of the 290K band and the study of V-centres in MgAl2O4 spinels indicates the possible attribution of the band to V-type centres.

Radiation-Induced Defects in Calcium Aluminate Glasses

In this work, MgF2 and CaF2 doped calcium aluminate glass samples have been investigated in order to describe the intrinsic defects that are formed in UV irradiated glasses. It is well known that calcium aluminate, when UV-illuminated, changes colour from blue to green. Thermoluminescence (TL), Optical Absorption (OA) and Electron Paramagnetic Resonance (EPR) measurements have been carried out and the results obtained were associated with the structural properties of the CaO—Al2O2—SiO2 system, in order to provide a defect model. Thermal treatments up to 150°C were performed for OA and TL measurements but the thermal decay curves could not be adjusted by an exponential function. The TL experimental results were fitted, according to a continuous model, with a Gaussian distribution function centred at 0.6eV and a frequency factor of 109s−1.

Electron paramagnetic resonance and thermally stimulated depolarisation currents in natural brazilian amethyst

Abstract EPR measurements have been performed in powder and crystal samples of natural Brazilian amethyst for two magnetic field directions. The dominant features of the EPR spectrum are provided by iron centres. Thermal treatments performed in powder samples show that the reduction of the spectrum is larger for increasing temperatures and for longer time intervals. The thermally treated samples were X and γ-irradiated and UV-illuminated. The spectra of the samples bleached by heating could not be recovered. TSDC measurements in amethyst crystals show four bands of dipole origin. The dipoles were attributed to Fe3+ in a Si4+ site adjacent to an alkali-ion. The difference in the temperature positions of the TSDC peaks is originated from the unequal distribution of the Fe3+ among the three equivalent sites.

Simulation models of defects in MgAl2O4:Fe2+, Fe3+spinels

Abstract A static computer simulation using the GULP code has been used to study dipole defects in iron doped spinels in normal and inverse structures. The occupation of tetrahedral (Mg2+) and octahedral (Al3+) sites in the normal structure was simulated and the results indicate a preferential replacement of Mg2- by an Al3+ justified by the observed stoichiometric deviation in synthetic spinels. Besides this, Al2O3 excess produces Mg2+ vacancies and favours the hole centre production. Antisite disorder of the form [Mg2+]Al 3+ and [Al3+]Mg 2+ is one of the possible mechanisms responsible for the existence of dipole defects in these materials. The obtained energy for the interchange between Al3+ and Mg2+ was 0.52eV. The calculated energies indicate that Fe2+ replaces Mg2+ and Fe3+ replaces Al3+. The presence of Fe3+ in octahedral sites takes into account the oxidation process of Fe2+ in these sites. Computer simulation has also been carried out on the inverse structure where the stability of the lattice was obtained for the Imma space group.

Thermally stimulated depolarization currents in natural beryl

Abstract Thermally Stimulated Depolarization Current (TSDC) spectra of untreated pink, green and blue beryl samples show a single peak centered at 200 K, unaffected by heat and linearly dependent on the polarization field, a typical behaviour of defects with a dipolar origin. When the samples are thermally treated above 1000 K in air two new peaks appear at 170 K and 220 K. Above 1200 K water is removed from the channels of the beryl, the Fe2+ is reduced into Fe2+ and the formation of iron dipoles is then enhanced, giving rise to the observable bands. An increase in the peak height of the 170 K and the 220 K bands is observed when the samples are ultraviolet (UV) illuminated after the heat treatments. UV illumination of the samples leads to the photodissociation of hydroxyls followed by the displacement of the hydrogen to isolated sites, thereby contributing to the formation of iron dipoles.

The role played by hydrogen in the ionic thermocurrents of beryl

Abstract Ionic thermocurrents (ITC) were measured in pink (morganite) and colourless (goshenite) samples of natural beryl from 77K up to 250K at a constant rate b = 0.1 Ks−1. The spectra present three distinct bands, at 170K, 200K and 220K. The band at 200K predominates in the untreated crystals. After thermal treatments between 700K and 1300K, the ITC bands at 170K and 220K become prominent, masking the 200K band. The reduction of Fe3+ into Fe2+ in two distinct positions in the beryl lattice justify this observation. Infrared (IR) spectra show that treatments above 1000K remove water from the channels of the beryls. Ultraviolet (UV) irradiation of these thermally treated samples enhance the two bands at 170K and 220K.

Radiation effects on TL and EPR of sodalite and application to dosimetry

Sodalite, Na8Al6Si6O24Cl2, a natural silicate mineral, has been investigated concerning its thermoluminescence (TL) and electron paramagnetic resonance (EPR) properties. Samples, both natural and heat treated at 500°C for 30min, present an EPR signal at g=2.01132. The irradiation induces two signals at g=2.0008 and a group of 11 lines due to an O− ion in an intermediate position with respect to two adjacent Al nuclei, that are superimposed on the signal at g=2.0008. An annealing at 500 °C/30min produces a broad signal around g~2.0, probably due to Fe3+ magnetic dipole interactions. The samples were annealed at 500 °C/30min and then irradiated with gamma-ray doses varying from 1 Gy to 20 kGy. All the samples have shown TL peaks at 110, 230 and 270 °C. Similar results were obtained for natural samples. The accumulated dose (AD) was determined to be AD~2.5 kGy and the annual dose of (0.595±0.001) mGy/year was obtained from the concentration of U, Th, and K2O in the sample. Therefore, the age obtained for natural sodalite was (4.2±0.2)106 years.