In-situ Raman spectroscopic measurements of the deformation region in indented glasses

This paper describes the design and integration of a custom-built optical instrument for in-situ Raman microscopy suitable for collecting high-quality spectroscopic data during the indentation of glass materials. It will further show that the reported experimental setup enables meaningful in-situ spectroscopic observations during indentation of fused silica at forces in the millinewton range. The aim of the paper is to demonstrate the vital importance of matching the analysis volume of the Raman microscope with the indentation-induced deformation volume to capture the full extent of the related spectral alterations by minimizing spectral contributions from the unperturbed bulk material (in-situ and ex-situ) and indenter probe (in-situ only). In this context, the paper will also touch upon possible pitfalls in ex-situ and in-situ Raman measurements on indented glasses in cases where the analysis and deformation volumes are not well matched and describe the misinterpretations that may result.

Multiple pathways in pressure-induced phase transition of coesite

High-pressure single-crystal X-ray diffraction method with precise control of hydrostatic conditions, typically with helium or neon as the pressure-transmitting medium, has significantly changed our view on what happens with low-density silica phases under pressure. Coesite is a prototype material for pressure-induced amorphization. However, it was found to transform into a high-pressure octahedral (HPO) phase, or coesite-II and coesite-III. Given that the pressure is believed to be hydrostatic in two recent experiments, the different transformation pathways are striking. Based on molecular dynamic simulations with an ab initio parameterized potential, we reproduced all of the above experiments in three transformation pathways, including the one leading to an HPO phase. This octahedral phase has an oxygen hcp sublattice featuring 2 × 2 zigzag octahedral edge-sharing chains, however with some broken points (i.e., point defects). It transforms into α-PbO₂ phase when it is relaxed under further compression. We show that the HPO phase forms through a continuous rearrangement of the oxygen sublattice toward hcp arrangement. The high-pressure amorphous phases can be described by an fcc and hcp sublattice mixture.

Ion irradiation induced structural modifications and increase in elastic modulus of silica based thin films

Ion irradiation is an alternative to heat treatment for transforming organic-inorganic thin films to a ceramic state. One major shortcoming in previous studies of ion-irradiated films is the assumption that constituent phases in ion-irradiated and heat-treated films are identical and that the ion irradiation effect is limited to changes in composition. In this study, we investigate the effects of ion irradiation on both the composition and structure of constituent phases and use the results to explain the measured elastic modulus of the films. The results indicated that the microstructure of the irradiated films consisted of carbon clusters within a silica matrix. It was found that carbon was present in a non-graphitic sp²-bonded configuration. It was also observed that ion irradiation caused a decrease in the Si-O-Si bond angle of silica, similar to the effects of applied pressure. A phase transformation from tetrahedrally bonded to octahedrally bonded silica was also observed. The results indicated the incorporation of carbon within the silica network. A combination of the decrease in Si-O-Si bond angle and an increase in the carbon incorporation within the silica network was found to be responsible for the increase in the elastic modulus of the films.

Extending the single-crystal quartz pressure gauge up to hydrostatic pressure of 19 GPa

In situhigh-pressure diffraction experiments on single-crystal α-quartz under quasi-hydrostatic conditions up to 19 GPa were performed with diamond-anvil cells. Isotropic pressures were calibrated through the ruby-luminescence technique. A 4:1 methanol–ethanol mixture and the densified noble gases helium and neon were used as pressure media. The compression data revealed no significant influence of the pressure medium at room temperature on the high-pressure behavior of α-quartz. In order to describe its compressibility for use as a pressure standard, a fourth-order Birch–Murnaghan equation of state (EoS) with parametersKT0 = 37.0 (3) GPa,KT0′ = 6.7 (2) andKT0′′ = −0.73 (8) GPa−1was applied to fit the data set of 99 individual data points. The fit of the axial compressibilities yieldsMT0 = 104.5 (8) GPa,MT0′ = 13.7 (4),MT0′′ = −1.04 (11) GPa−1(aaxis) andMT0 = 141 (3) GPa,MT0′ = 21 (2),MT0′′ = 8.4 (6) GPa−1(caxis), confirming the previously reported anisotropy. Assuming an estimated standard deviation of 0.0001% in the quartz volume, an uncertainty of 0.013 GPa can be expected using the new set of EoS parameters to determine the pressure.

Lattice dynamics of coesite

The lattice dynamics of coesite has been studied by a combination of diffuse x-ray scattering, inelastic x-ray scattering and ab initio lattice dynamics calculations. The combined technique gives access to the full lattice dynamics in the harmonic description and thus eventually provides detailed information on the elastic properties, the stability and metastability of crystalline systems. The experimentally validated calculation was used for the investigation of the eigenvectors, mode character and their contribution to the density of vibrational states. High-symmetry sections of the reciprocal space distribution of diffuse scattering and inelastic x-ray scattering spectra as well as the density of vibrational states and the dispersion relation are reported and compared to the calculation. A critical point at the zone boundary is found to contribute strongly to the main peak of the low-energy part in the density of vibrational states. Comparison with the most abundant SiO2 polymorph--α-quartz--reveals similarities and distinct differences in the low-energy vibrational properties.

Mixed threefold and fourfold carbon coordination in compressed CO2

Carbon dioxide (CO2) has been recently reported to possess an amorphous form, named "carbonia," structurally similar to other group-IV oxide glasses. By combining ab initio constant pressure molecular dynamics, density-functional perturbation theory, and experimental IR spectra, we show that carbonia, and possibly also phase VI, is not SiO2-like, and that instead it is partially tetrahedral containing also a sizable amount of carbon in threefold coordination, but no sixfold octahedral coordination. Enthalpic considerations suggest that carbonia is a metastable intermediate state of the transformation of molecular CO2 into fully tetrahedral phases.

Molecular dynamics study of CaSiO(3)-MgSiO(3) glasses under high pressure

The pressure-induced structural evolutions of CaSiO(3)-MgSiO(3) glasses have been examined by means of molecular dynamics simulation. Our calculations revealed that Si coordination remained unchanged up to 15 GPa, while modifier cations caused significant changes in the short-range order structure. In the present study, we conclude that the main compression mechanisms for CaSiO(3)-MgSiO(3) glasses are: (1) the Si-O-Si angle reduction, (2) the coordination increase of Ca and Mg cations, and (3) the compaction in the medium-range scale. Furthermore, small changes in the Q(n) distribution suggest pressure-induced disproportionation reactions. Similar pressure responses between CaSiO(3)-MgSiO(3) glasses may imply that the structural changes of SiO(4) framework units are more significant than those of interstitial cations, Ca and Mg.

Amorphous silica-like carbon dioxide

Among the group IV elements, only carbon forms stable double bonds with oxygen at ambient conditions. At variance with silica and germania, the non-molecular single-bonded crystalline form of carbon dioxide, phase V, only exists at high pressure. The amorphous forms of silica (a-SiO2) and germania (a-GeO2) are well known at ambient conditions; however, the amorphous, non-molecular form of CO2 has so far been described only as a result of first-principles simulations. Here we report the synthesis of an amorphous, silica-like form of carbon dioxide, a-CO2, which we call 'a-carbonia'. The compression of the molecular phase III of CO2 between 40 and 48 GPa at room temperature initiated the transformation to the non-molecular amorphous phase. Infrared spectra measured at temperatures up to 680 K show the progressive formation of C-O single bonds and the simultaneous disappearance of all molecular signatures. Furthermore, state-of-the-art Raman and synchrotron X-ray diffraction measurements on temperature-quenched samples confirm the amorphous character of the material. Comparison with vibrational and diffraction data for a-SiO2 and a-GeO2, as well as with the structure factor calculated for the a-CO2 sample obtained by first-principles molecular dynamics, shows that a-CO2 is structurally homologous to the other group IV dioxide glasses. We therefore conclude that the class of archetypal network-forming disordered systems, including a-SiO2, a-GeO2 and water, must be extended to include a-CO2.

High-pressure infrared and FT-Raman investigation of a dental composite

Composite resins are often used as filling materials on load-bearing surfaces of teeth. As masticatory stresses can be high, here, we study the effect of pressure on the behaviour of a dental composite. Using a polymerized wafer, the IR and FT-Raman spectra of a zirconia-containing proprietary composite (Z100, 3M, Minneapolis, MN, USA) were recorded. The high-pressure IR spectra were also recorded. Band assignments were made for the main peaks of both organic and inorganic components. Breaks in the pressure dependences (dv/dP) of the organic components were found at 22 kbar. Different pressure dependences for different vibrational modes of inorganic components were also observed. These data suggest that the network structure of the composite is compacted under high pressure and that both the atomic distance and bonding angles in the network are altered.