Abstract: Raman scattering and differential scanning calorimetry (DSC) measurements have been carried out on four mixed tellurium-zinc oxide (TeO2)(1-x)(ZnO)(x) (x = 0.1, 0.2, 0.3, 0.4) glasses under variable temperature, with particular attention being given to the respective glass transition region. From the DSC measurements, the glass transition temperature T-g has been determined for each glass, showing a monotonous decrease of T-g with increasing ZnO content. The Raman study is focused on the low-frequency band of the glasses, the so-called boson peak (BP), whose frequency undergoes an abrupt decrease at a temperature T-d very close to the respective T-g values obtained by DSC. These results show that the BP is highly sensitive to dynamical effects over the glass transition and provides a means for an equally reliable (to DSC) determination of T-g in tellurite glasses and other network glasses. The discontinuous temperature dependence of the BP frequency at the glass transition, along with the absence of such a behaviour by the high-frequency Raman bands (due to local atomic vibrations), indicates that marked changes of the medium range order (MRO) occur at T-g and confirms the correlation between the BP and the MRO of glasses.
Abstract: The Raman spectra of three (TeO2)(1-x)(ZnO)(x) (x=0.1, 0.2, and 0.4) glasses have been measured under pressure up to 13 GPa. We report an unusual pressure dependence of the Boson peak frequency which is quite different from that observed in relevant experiments for other network glasses or predicted by theory. The unexpected observed effect indicates that tellurite glasses respond like elastic, rather than plastic, media to both compression and decompression treatments. We suggest that, with respect to plastic versus elastic deformation mode, the behavior of glasses under pressure is not universal. Further, we conclude that a medium range order elastic glass-to-glass transition from a rigid to a more flexible network is induced by pressure for two of the glasses (x=0.2, 0.4). This transition is reversible upon decompression, merely exhibiting a small hysteresis.
Abstract: High-pressure modifications in glassy GeS2 have been investigated up to 45 GPa using the diamond anvil cell through x-ray absorption fine structure (XAFS) spectroscopy and x-ray diffraction (XRD). The gradual increase in the Ge-S distance of about 0.1 angstrom between 15 and 25 GPa evidenced by XAFS is interpreted as a signature of a coordination increase around Ge. The negative shift of the germanium K absorption edge position is connected to the progressive metallization of the glass. XRD measurements reveal the amorphous-amorphous nature of the structural transformation. The first sharp diffraction peak loses intensity and shifts toward higher Q values in the first few gigapascal, reflecting changes in the intermediate-range order prior to the local coordination increase. The observed changes are reversible after pressure release with strong hysteresis.
Abstract: We have performed a theoretical and experimental study of the structural stability of terbium phosphate at high pressures. Theoretical ab initio total-energy and lattice-dynamics calculations together with x-ray diffraction experiments have allowed us to completely characterize a phase transition at similar to 9.8 GPa from the zircon to the monazite structure. Furthermore, total-energy calculations have been performed to check the relative stability of 17 candidate structures at different pressures and allow us to propose the zircon -> monazite -> scheelite -> SrUO4-type sequence of stable structures with increasing pressure. In this sequence, sixfold P coordination is attained for the SrUO4-type structure above 64 GPa. The whole sequence of transitions is discussed in association with the high-pressure structural behavior of oxides isomorphic to TbPO4.
Abstract: The Raman spectra of tetragonal RPO /sub 4/ (R = Y, Tb, Er, Tm) phosphates belonging to the zircon-structure class (space group D /sub 4//sub h//sup 19/ ) have been measured at high pressures up to 7.9 GPa (YPO /sub 4/ , ErPO /sub 4/ ) and 15.5 GPa (TbPO /sub 4/ , TmPO /sub 4/ ). Most of the 12 Raman active modes of this crystal class have been observed at high pressures using a Diamond Anvil Cell. A characteristic feature in the spectra of all crystals studied is the anomalous pressure dependence of the B /sub 2//sub g/ symmetry internal bending mode which softens substantially with increasing pressure to the extent that a level crossing occurs between this mode and one normal E /sub g/ rotational mode. These observations imply that an incipient structural phase transition of the crystals may occur at a higher pressure. Such a phase transition has been observed for TbPO /sub 4/ at a pressure of ~9.5 GPa. The abrupt spectral variations observed at this pressure, indicate that TbPO /sub 4/ undergoes a first-order phase transition from tetragonal zircon to a lower symmetry structure which is quenchable upon pressure release.
Abstract: The Raman spectra of tetragonal TbPO4 (zircon-type I4(1)/amd structure, D-4h(19) space group) have been measured at ambient conditions and under variable pressure up to 15.5 GPa inside a diamond anvil cell (DAC). Assignment of the Raman active modes of the tetragonal phase has been carried out based on polarized measurements from a single oriented crystal of TbPO4 at ambient conditions. The abrupt Raman mode discontinuities and the appearance of numerous new Raman peaks at a pressure P-c approximate to 9.5 GPa have provided strong evidence for a first-order phase transition to a lower crystal symmetry, most likely monoclinic. The high-pressure structure appears to be more compact compared to the tetragonal one, and is retained upon bringing the crystal to ambient pressure.
Abstract: The Raman scattering spectra of two ternary floppy (Z < 2.4) Ge-As-S chalcogenide glasses have been measured over temperatures ranging from 20 K, through the respective glass transition temperature T-g and up to a temperature close to melting point. Additional Differential Scanning Calorimetry (DSC) measurements have been carried out in these glasses at high temperatures from which values of T-g have been determined. The purpose of this work is twofold: (i) to study, through the Raman spectra and in particular the Boson peak (BP), critical phenomena around the T-g and make comparisons with the results obtained by DSC, and (ii) to make overall comparisons of the present Raman results and conclusions with those reported in a previous Raman study [Boulmetis et al, J. Optoelectron. Adv. Mater. 7, 1209 (2005)] on rigid (Z > 2.4) Ge-As-S glasses. It has been found that the study of the Boson peak is a reliable technique for predicting and determining the onset of the transition to the supercooled liquid phase in floppy chalcogenide glasses.
Abstract: The Raman spectra of binary As-S and ternary Ge-As-S semiconducting chalcogenide glasses have been studied under variable hydrostatic pressure up to 7.0 GPa. Two As-S glasses were investigated, the first at the stoichiometric composition As2S3 (with critical mean coordination number value Z = 2.4) and the other S-rich, non-stoichiometric one belonging to the floppy network regime (AsS4, Z = 2.2), while the ternary Ge-As-S glass was on the rigid network side (Ge25As10S65, Z = 2.6). Reversible pressure-induced effects have been observed in the floppy AsS4 glass manifested by the amorphization of its crystalline sulphur domains. In contrast, the pressure-induced appearance of homopolar Ge-Ge bonds and the formation of "ethanol-like" S3Ge-GeS3 units in the rigid ternary Ge25As10S65 glass are irreversible, after pressure relief.
Abstract: The Raman spectra of DyPO4, under variable hydrostatic pressure at room- and low-temperature (110 K), have been studied. Crossing, with increasing pressure, of two modes in the frequency range of the internal-external border, is observed and discussed. At low temperature, the pressure dependence of the three highest-frequency internal modes is monitored, which exhibit a (d omega/dP)-slope differing, well beyond the experimental error, from the corresponding room temperature slopes. At room temperature, all the expected modes are observed and are divided in two distinct groups on a "logarithmic derivative" vs. "peak-frequency" graph. This division is discussed in terms of the internal-external mode-character.
Abstract: Ancient metal objects react with moisture and environmental chemicals to form various corrosion products. Because of the unique character and high value of such objects, any cleaning procedure should guarantee minimum destructiveness. The most common treatment used is mechanical stripping, in which it is difficult to avoid surface damage when employed. Lasers are currently being tested for a wide range of conservation applications. Since they are highly controllable and can be selectively applied, lasers can be used to achieve more effective and safer cleaning of archaeological artifacts and protect their surface details. The basic criterion that motivated us to use lasers to clean Roman coins was the requirement of pulsed emission, in order to minimize heat-induced damages. In fact, the laser interaction with the coins has to be short enough, to produce a fast removal of the encrustation, avoiding heat conduction into the substrate. The cleaning effects of three lasers operating at different wavelengths, namely a TEA CO2 laser emitting at 10.6 mum, an Er:YAG laser at 2.94 mum, and a 2omega-Nd:YAG laser at 532 nm have been compared on corroded Romans coins and various atomic and nuclear techniques have also been applied to evaluate the efficiency of the applied procedure.
