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The temperature dependence of the infrared absorption spectra of two Verneuil-grown corundum samples is investigated in the OH stretching range. The spectra display three main bands at 3184, 3232 and 3309 cm− 1, belonging to the so-called “3309 cm− 1 series”, as well as two additional bands at 3163 and 3278 cm− 1 previously reported in some synthetic corundum samples. The anharmonic behavior of the observed bands is analyzed using the pure dephasing model of Persson and Ryberg and depends on the local geometry of the OH defects, which are all associated with Al vacancies. The unexpected increase with temperature in the absorbance of a weak band at 3209 cm− 1 supports a revised interpretation of both the 3209 and 3232 cm− 1 bands. These two bands are interpreted as resulting from the low-temperature equilibrium between two Ti-associated OH defects, enabled by the possibility of hydrogen hopping within the Al vacancy. The temperature-dependent properties of the 3278 cm− 1 band are similar to those of the other Al-vacancy related defects and a comparison with the theoretical properties of selected OH defects suggests that this band corresponds to the association of the H atom with a non-dissociated Al Frenkel pair. Finally, the properties of the band at 3163 cm− 1 are consistent with its previously proposed association with Si for Al substitution in corundum. [ABSTRACT FROM AUTHOR]

Epitaxial cubic (100) 3C-SiC films on a (100) silicon wafer were irradiated at room temperature with 2.3-MeV Si+ or 3.0-MeV Kr+ ions up to a fluence of 1 × 1016 cm−2. The evolutions of the epilayer and the substrate were followed as a function of ion fluence by using micro-Raman spectroscopy, optical absorption, and diffuse reflectance spectroscopy in the UV-visible and near infrared range. Raman spectra evidence the amorphization of SiC films at an estimated dose of about 0.1 displacement per atom (dpa) for both ion irradiations. The narrow peaks of the Raman-allowed TO and LO modes of SiC and Si are recorded in the virgin sample, together with few peaks assigned to zone-edge modes of SiC arising from the intrinsic disorder in the strained films. Those crystal phonon peaks broaden or disappear with increasing fluence. The spectra finally exhibit broad extra peaks assigned to the formation of Si–Si and C–C wrong homonuclear bonds in the local order of the amorphous phase. The optical transmission and diffuse reflectance spectra feature interference fringe patterns in the SiC film that are smoothened out with irradiation due to the matching of refractive indices of the amorphous SiC film and Si substrate. The evolution of the refractive index of SiC and optical gap of Si are deduced from those spectra. The respective roles of ballistic effects and electronic excitations in the radiation damage of both SiC and Si are discussed for those two ions with about the same electronic stopping power and about one order-of-magnitude difference in nuclear stopping power. The damage is dominated by the nuclear collision processes and rather well correlated with the estimated irradiation dose in dpa. Optical spectra show that electronic excitations induce damage recovery of the amorphized substrate below the SiC/Si interface. Raman spectra and optical absorption/reflection spectra yield complementary pictures of the radiation damage. [ABSTRACT FROM AUTHOR]

The FTIR absorption bands of virgin and electron-irradiated CeO2 single crystals were measured from 20 K to 500 K between 4000 cm−1 and 12,000 cm−1 (~830 nm to 2500 nm). Three broad bands centered at about 6100 cm−1 (~0.75 eV), 7000 cm−1 (~0.87 eV), and 10,500 cm−1 (~1.3 eV) were recorded above 100 K for the 2.5 MeV electron energy. Two smaller bands at about 4300 cm−1 (~0.53 eV) and 5500 cm−1 (~0.68 eV) were also recorded below 100 K. Similar broad bands centered at about 4100 cm−1 (~0.52 eV), 6400 cm−1 (~0.79 eV), 7600 cm−1 (~0.94 eV), and 10,500 cm−1 (~1.3 eV) are also found for the 1.4 MeV electron energy above 300 K. The evolution of these absorption bands was followed as a function of temperature. The plots of band intensity ratios show a thermally activated process corresponding to the ionization of the deep electronic levels of point defects in the band gap of ceria of ~26,000 cm−1 (~3.2 eV). These five bands are assigned to the different charge states (0, −1, −2, −3, −4) of the Ce vacancies produced by elastic collisions above 1.0 MeV. [ABSTRACT FROM AUTHOR]

The damage recovery of an electron-irradiated CeO2 single crystal was studied by in situ UV–visible absorption spectroscopy during isochronal thermal annealing up to 873 K. The spectrum of the as-irradiated crystal shows band tailing below the fundamental absorption edge for photon energy near 3.2 eV. Two broad Gaussian bands centered at about 0.9 and 1.4 eV are also recorded. The edge region is treated by including the Urbach edge contribution. The variation of bandgap energy and Urbach energy is followed as a function of the annealing temperature. The absorption edge was red-shifted with annealing temperature then returned to the starting value after cooling back to room temperature. The disorder contribution to the Urbach edge is deduced by subtracting the temperature contribution. It shows a decay of the disorder contribution following a first-order kinetics process with an onset of recovery at 373 K and activation energy of about 0.15 eV. Full recovery is achieved at 873 K, whereas a partial recovery of the two absorption bands (at 0.9 and 1.4 eV) is found. The low activation energy is interpreted in reference to the available literature data on the energy barriers for point defect migration, and assigned to the migration of oxygen interstitials associated to electronic levels near the top of the valence band edge. A recombination mechanism is suggested, involving both electron hopping and interstitial oxygen diffusion. [ABSTRACT FROM AUTHOR]

Reflection spectra of cerium dioxide sintered samples were measured in the UV-visible range after irradiation with various heavy ions (2.4-MeV Cu, 5-MeV W, 10-MeV W, 36-MeV W, 100-MeV Kr, and 200-MeV Xe). Differential reflectance spectra of irradiated samples after subtraction of the reference sample spectrum are fit with six broad Gaussian bands centered at about 1.2, 2.2, 2.8, 4.0, 4.8, and 6.2 eV. The growth curves of most bands show a saturation behavior vs ion fluence. Reflection spectra are consistent with the UV-visible absorption spectra of electron-irradiated ceria single crystals for photon energies lower than 3.2 eV, corresponding to the optical gap. The spectra are tentatively analyzed on the basis of charge transfer bands and the 4f-5d transitions related to the Ce3+ ions in the distorted environment formed by irradiation. More insight into defect formation is given by the reflection spectra rather than the absorption ones due to the limitation in the absorption of the optical gap. These data are also discussed with reference to the radiation damage processes by electronic excitations and nuclear collisions in ceria. [ABSTRACT FROM AUTHOR]

We have studied the formation of color centers in magnesium aluminate spinel (MgAl2O4) by X-ray and electron irradiations near room temperature (RT). For this purpose, Cu Kα radiation and three electron energies (1.0, 1.4, and 2.5 MeV) were used for variable fluences (up to 4.2 × 1018 cm−2). Off-line UV-visible absorption spectra were recorded at RT as well as at low temperature down to 27 K after electron irradiation. The dependence of the production rate of F centers (oxygen vacancies) on the electron energy yields a threshold displacement energy of 190 ± 10 eV for oxygen atoms at RT, which is much higher than the determinations by molecular-dynamics simulations. Such a discrepancy is discussed on the basis of available migration data of point defects in spinel. Equipartition of F0 and F+ centers, i.e., the neutral (VOx) and singly ionized (VO.) oxygen vacancies, is reached for high electron fluences. Moreover, the evolution of the width of color-center absorption bands versus temperature is interpreted with the classical theory for F centers (neutral halogen vacancies) in alkali halides. The Stokes shifts are deduced from the temperature dependence of the absorption bandwidths of color centers like for alkali halides and alkaline-earth oxides. Finally, isothermal annealing data for long annealing time show a non-zero asymptotic behavior for both F0 and F+ centers. This uncommon behavior is interpreted by charge exchange processes leading to an equilibrium state between those two color centers. [ABSTRACT FROM AUTHOR]

Micro-Raman spectroscopy, X-band electron paramagnetic resonance (EPR) spectroscopy, and UV-visible optical absorption spectroscopy were used to study the damage production in cerium dioxide (CeO2) single crystals by electron irradiation for three energies (1.0, 1.4, and 2.5 MeV). The Raman-active T2g peak was left unchanged after 2.5-MeV electron irradiation at a high fluence. This shows that no structural modifications occurred for the cubic fluorite structure. UV-visible optical absorption spectra exhibited a characteristic sub band-gap tail for 1.4-MeV and 2.5-MeV energies, but not for 1.0 MeV. Narrow EPR lines were recorded near liquid-helium temperature after 2.5-MeV electron irradiation; whereas no such signal was found for the virgin unirradiated crystal or after 1.0-MeV irradiation for the same fluence. The angular variation of these lines in the {111} plane revealed a weak g-factor anisotropy assigned to Ce3+ ions (with the 4f¹ configuration) in a high-symmetry local environment. It is concluded that Ce3+ ions may be produced by a reduction resulting from the displacement damage process. However, no evidence of F+ or F0 center or hole center formation due to irradiation was found from the present EPR and optical absorption spectra. [ABSTRACT FROM AUTHOR]

The powder-transmission infrared spectra of OH-stretching modes in well-ordered samples of kaolinite and dickite are recorded from 270 to 10 K. Effective medium modeling of the kaolinite spectrum confirms the coupled motion of non-equivalent OH groups in the inter-layer OH-stretching modes. A full assignment of OH-stretching bands in dickite is proposed. In particular, the band observed at 3685 [cm.sup.-1] in room-temperature spectra is attributed to a kaolinite-like defect, whereas that at 3706 [cm.sup.-1] is related to OH3 groups of the dickite structure. The changes in phonon frequency and line shape are analyzed as a function of temperature. The results evidence the quasi-harmonic nature of the OH-stretching modes in kaolinite-group minerals. Coupling of OH-stretching modes to each other can be related to the macroscopic electrostatic properties of the composite medium, made of mineral particles and KBr matrix, and not to microscopic anharmonic mechanisms. The low saturation temperatures of vibrational frequencies and linewidths indicate that the OH-stretching modes probe the average thermal bath of the crystal, without coupling to a specific decay channel. Keywords: Infrared spectroscopy, kaolinite, dickite, thermodynamic, phonons DOI: 10.2138/am.2010.3447

The temperature dependence of the infrared absorption spectra of two Verneuil-grown corundum samples is investigated in the OH stretching range. The spectra display three main bands at 3184, 3232 and 3309 cm− 1, belonging to the so-called “3309 cm− 1 series”, as well as two additional bands at 3163 and 3278 cm− 1 previously reported in some synthetic corundum samples. The anharmonic behavior of the observed bands is analyzed using the pure dephasing model of Persson and Ryberg and depends on the local geometry of the OH defects, which are all associated with Al vacancies. The unexpected increase with temperature in the absorbance of a weak band at 3209 cm− 1 supports a revised interpretation of both the 3209 and 3232 cm− 1 bands. These two bands are interpreted as resulting from the low-temperature equilibrium between two Ti-associated OH defects, enabled by the possibility of hydrogen hopping within the Al vacancy. The temperature-dependent properties of the 3278 cm− 1 band are similar to those of the other Al-vacancy related defects and a comparison with the theoretical properties of selected OH defects suggests that this band corresponds to the association of the H atom with a non-dissociated Al Frenkel pair. Finally, the properties of the band at 3163 cm− 1 are consistent with its previously proposed association with Si for Al substitution in corundum. [ABSTRACT FROM AUTHOR]

The damage induced by heavy-ion irradiation has been studied in yttrium iron garnet (Y3Fe5O12or YIG) films, doped with Ca, Tb and Tm, grown by liquid-phase epitaxy on gadolinium gallium garnet (Gd3Ga5O12or GGG) substrates. Irradiations of doped-YIG epitaxial films and GGG substrates with 36-MeV183W and 12-MeV197Au ions were applied for fluences between 1 × 1013and 3 × 1015 cm–2near room temperature. The radiation damage was monitored by micro-Raman spectroscopy and UV–visible optical absorption spectroscopy. Raman spectra revealed that amorphisation was achieved in YIG for both ions, whereas a high lattice disorder was induced in GGG without reaching amorphisation for the Au ion irradiation. Raman spectra also showed that a major damage of the tetrahedral sites was induced in GGG, as previously found for YIG. It is concluded that with such ions reaching the stopping power threshold of track formation in YIG and GGG the observed rate of amorphisation may result from a combination of electronic and nuclear energy losses as calculated using the unified thermal spike model. [ABSTRACT FROM PUBLISHER]

We propose a microscopic model of the dominant carbonate for phosphate substitution in fluor- apatite. A well-crystallized sedimentary fluorapatite sample containing ~2.3 ± 0.8 wt% of carbonate was investigated using Fourier transform infrared spectroscopy (FTIR) and 13C and 19F magic angle spinning nuclear magnetic resonance (MAS NMR). About 75% of the carbonate groups replace the phosphate group ("B-site"), whereas a lesser contribution from carbonate groups located in the structural channels ("A-site") is observed. Beside the dominant 19F NMR signal of channel ions at ~ -102 ppm, an additional signal corresponding to ~8% of fluoride ions is observed at -88 ppm. 19F double quantum-single quantum (DQ-SQ) MAS NMR and 13C{19F} frequency-selective Rotational Echo DOuble Resonance (REDOR) experiments prove that this additional signal corresponds to isolated fluoride ions in the apatite structure, located in close proximity of substituted carbonate groups. Density functional theory (DFT) calculations allow us to propose a composite carbonate-fluoride tetrahedron defect model accounting for these experimental observations. The planar carbonate ion lies in the sloping face of the tetrahedron opposite a fluoride ion occupying the remaining vertex, together replacing the tetrahedral phosphate ion. This "francolite-type" defect leads to a diagnostic narrow IR absorption band at 864 cm-1 that could be used as a guide to, e.g., detect the incipient transformation of fossil bone and teeth samples. [ABSTRACT FROM AUTHOR]