[en] Tourmaline is known as a gemstone with great variety in color and composition. Some color centers are related to transition metal ions, others to radiation-induced electron-hole traps. The knowledge concerning the microscopic structure of the color centers is very poor. Therefore, color enhancing processes by irradiation and/or thermal treatments in tourmaline are performed empirically. By electron paramagnetic resonance and electron nuclear double resonance a yellow color center produced by γ-irradiation in colorless Li-bearing tourmaline (elbaite) has been identified. The color center is proposed to be a hole trap of type O^- showing mainly superhyperfine interaction with two Al nuclei. During the irradiation process atomic hydrogen H⁰ is also produced which shows the same thermal stability as the hole trap (250 deg. C)

[en] Natural and γ-irradiated pink and colorless beryl from Brazil has been investigated by electron paramagnetic resonance (EPR), optical absorption and infrared absorption. Beryl has the chemical formula Be₃Al₂Si₆O₁₈ and is hexagonal with space group P6/m cc. Electron microprobe analysis of the different samples shows that the beryl samples are rich in Cs (3.30 wt.%) and contain low concentrations of transition-metal ions, in total ∼0.03 wt.% Fe and ∼0.05 wt.% Mn. In addition to the transition-metal ions, beryl accommodates many molecules and alkalis in its channels, which lie parallel to the c-axis and which have large diameters with a maximum of 5.1 A. Infrared absorption measurements in the different samples indicate the presence of type-I and type-II water together with OH^-. Other molecules in the channels with not yet associated infrared absorptions are NO₃⁰ and CO₃^- which have been observed by EPR. In this work we focus on the EPR identification of the different molecules and discuss their relation to color

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[en] In this paper we identify, with optically-detected magnetic resonance via the magnetic circular dichroism of the absorption, the NO₃ and CO₃^- molecular radicals trapped in the structural channels of gamma-irradiated beryl. We confirm that, along with the CO₃^-, the NO₃ is also responsible for the blue color by introducing several absorption peaks between 14500 cm^-1 and 18000 cm^-1 (720 nm down to 540 nm). Their individual contributions to the overall many-peak optical spectra have been established and most of the transitions could be explained in terms of a combination of crystal-field and spin-orbit split ²A₂' → ²E' transition plus vibronic replicas. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

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[en] Single crystals of YF₃ grown by the Czochralski method have been investigated by electron paramagnetic resonance (EPR). In this paper we present an EPR analysis of residual Gd³⁺ impurities. We found that the Gd³⁺ rare-earth ion substitutes for Y³⁺ in YF₃. Detailed analysis of the EPR spectra allowed us to determine the fine structure parameters of Gd³⁺ in monoclinic C_s site symmetry. The EPR angular dependence is dominated by the electronic quadrupole tensor; the g-factor is slightly axial. The b⁰₂ parameter found for Gd³⁺ in YF₃ is unexpectedly high when compared to Gd³⁺ in the hosts of LaF₃ structure where the coordination number is the same. The results let us conclude that YF₃ is a promising laser host crystal. (author)