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Natural single-crystal specimens of the montebrasite/amblygonite series from Brazil, with general formula LiAlPO4(F,OH), were investigated by electron microprobe, Raman spectroscopy, X-ray diffraction, and infrared absorption. Since little is known about impurities and their local symmetries, electron paramagnetic resonance (EPR) was applied. Six different paramagnetic impurities and radiation defects were detected by EPR. Three of them, all substituting for Al3+ions, namely, iron (Fe3+), vanadium (V4+), and niobium (Nb4+) impurities were characterized in this work. The Fe3+(3d5)-related EPR spectra and angular dependencies show occupation of low-symmetry sites that are revealed in the high asymmetry parameter of the electronic fine structure, E/D = 0.27. Vanadium and niobium impurities are identified through their typical strong hyperfine interactions. Both form interesting examples for which the properties of 3d1ion (V4+) and 4d1ion (Nb4+) in the same host matrix can be compared. It is shown that both ions form complex defects of type VO2+(vanadyl) and NbO2+(niobyl), showing superhyperfine interaction with two equivalent hydrogen ions and not to fluorine. The EPR rotation patterns are analyzed in detail for three mutually perpendicular crystal planes. Spin Hamiltonian parameters are calculated and discussed.

Tourmaline with the general formula XY 3 Z 6(BO 3) 3Si 6O 18(OH,O) 3(OH,F) and the trigonal space group R3 m ( C 3 v 5) is known as a gemstone with great variety of colors. Some color centers are related to transition metal ions, and others to electron or hole traps. In this paper we report on a combined study using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and the optical detection of EPR (ODEPR) on a yellow color center produced by ?-irradiation in colorless Li-bearing elbaite tourmaline from Brazil. The color center is an O - hole trap center, which is stabilized within the plane spanned by three Y sites, and is located in the structural channels formed by Si 6O 18. We suggest that two of the Y sites are substituted by 27Al and the other by 6,7Li. During the irradiation process atomic hydrogen H 0 is also produced, which shows the same thermal stability as the hole center (250 °C). Therefore, we assign H 0 to be the local charge compensator for the hole trap. From the ODEPR measurements we conclude that the yellow color is caused by the O - hole center. The large negative isotropic Al superhyperfine interaction of the O - hole trap center is consistent with a calculation of the transferred hyperfine interactions by exchange polarization supporting the proposed defect model of an O - at the O 1 sites, whereby the O - is relaxed into the plane formed by three Y ions.

AbstractElectron Paramagnetic Resonance (EPR) and Raman Spectroscopy were used to study the low temperature phase transition of pulled Li3ThF7single crystals, occurring at 281 K. In both cases, the room temperature spectra were very broad, owing to the statistical disorder and high ionic mobility of the lithium ions, which occupy only 3/4 of their structural sites. The results are compatible with a first order ferroelastic transition from the room temperature orthorhombic D2h22phase to a monoclinic C2h(x)one. The symmetry rules are well respected assuming a model with four lithium ions in average per chemical formula. The EPR spectra show also the appearing of additional ferroelastic domains.

AbstractCerium doped fluoride materials are known for their fast scintillating properties. Large LuF3single crystals have been grown by the Czochralski method. Electron paramagnetic resonance (EPR) has been used to characterize Ce3+and residual paramagnetic impurities. At 300 K Gd3+has been detected, which substitutes for Lu3+in LuF3. From the analysis of the EPR angular dependence the b20parameter for Gd3+was determined to be 1.846 GHz, which is about times larger than the corresponding parameter in LaF3hosts structure. At 10 K the EPR spectra show the presence of Ce3+and other still undetermined rare earth impurities. The analysis of the EPR angular dependence of Ce3+shows a strong nearly axial symmetry of the g tensor with gxx= 0.29, gyy= 0.60 and gzz= 3.374. The Y axis of the g tensor is parallel to the crystallographic bdirection, whereas the X and Z axes are rotated in the acplane by 25°.