Water-related Defects Research Articles

Interaction between water and point defects inside volume-constrained α-quartz: An ab initio molecular dynamics study at 300 K

Quartz-based minerals in earth’s crust are well-known to contain water-related defects within their volume-constrained lattice, and they are responsible for strength-loss. Experimental observations of natural α-quartz indicate that such defects appear as hydroxyl groups attached to Si atoms, called Griggs defect (Si-OH), and molecular water (H2O) located at the interstitial sites. However, factors contributing to the formation of Griggs and interstitial H2O defects remain unclear. For example, the role of point defects like vacancy sites (O2− and Si4+), and substitutional (Al3+) and interstitial (Li+, K+, Ca2+, Mg2+, etc.) ions has remained largely unexplored. Here, we performed ab initio molecular dynamics at 300 K to examine the energetics and structure of water-related defects in volume-constrained α-quartz. Several configurations were systematically interrogated by incorporating interstitial H2O, O2− and Si4+ vacancies, substitutional Al3+, and interstitial Li+, Ca2+ and Mg2+ ions within α-quartz. Interstitial H2O defect was found to be energetically favorable in the presence of Substitutional Al3+, and interstitial Ca2+, Mg2+, and Li1+. In the presence of O2− and Si4+ vacancies, H2O showed a strong tendency to dissociate into OH—to form Griggs defect—and a proton; even in the presence of substitutional and interstitial ions. These ions distorted the α-quartz lattice and, in the extreme case, disrupted long-range order to form local amorphous domains; consistent with experimental reports. Our study provides an initial framework for understanding the impact of water within the crystal lattice of an anhydrous silicate mineral such as quartz. We provide not only thermodynamic and process-related information on observed defects, but also provides guidelines for future studies of water’s impact on the behavior of silicate minerals.

Oxygen vacancy-induced plasmonic and polaronic excitations in rubidium- and potassium-doped hexagonal tungsten bronzes

Potassium- and rubidium-doped hexagonal tungsten bronzes (K- and Rb-HTBs) were examined in comparison with cesium-doped hexagonal tungsten bronze (Cs-HTB) for their crystal structure and dielectric functions via spectrophotometry, spectroscopic ellipsometry, X-ray diffraction, and X-ray photoelectron spectroscopy. A substantial amount of oxygen vacancy (VO) was detected in all HTBs via chemical analysis, which increased depending on the dopant K → Rb → Cs. A minute (201) peak appeared for K- and Rb-HTBs, indicating symmetry transition from P63/mcm to P6322 due to the pseudo Jahn–Teller distortion; this suggests that K- and Rb-HTBs contain lesser conduction electrons than Cs-HTB. Double-peaked loss functions were decomposed using the Drude−Lorentz theory, and the associated plasma frequency, polaron energies, and W5+ fractions increased as the dopants changed, K → Rb → Cs, with increasing VO content. The origin of the decreased VO content in K- and Rb-HTBs was discussed in relation to water-related defects on prismatic planes.

Lubrication of dislocation glide in MgO by hydrous defects

Water-related defects, principally in the form of protonated cation vacancies, are potentially able to weaken minerals under high-stress or low-temperature conditions by reducing the Peierls stress required to initiate dislocation glide. In this study, we use the Peierls–Nabarro (PN) model to determine the effect of protonated Mg vacancies on the 1/2 {110} and 1/2 {100} slip systems in MgO. This PN model is parameterized using generalized stacking fault energies calculated using plane-wave density functional theory, with and without protonated Mg vacancies present at the glide plane. It found that these defects increase dislocation core widths and reduce the Peierls stress over the entire pressure range 0–125 GPa. Furthermore, 1/2 {110} slip is found to be more sensitive to the presence of protonated vacancies which increases in the pressure at which {100} becomes the easy glide plane for 1/2 screw dislocations. These results demonstrate, for a simple mineral system, that water-related defects can alter the deformation behavior of minerals in the glide-creep regime by reducing the stress required to move dislocations by glide. (Mg, Fe)O is the most anisotropic mineral in the Earth’s lower mantle, so the differential sensitivity of the major slip systems in MgO to hydrous defects has potential implications for the interpretation of the seismic anisotropy in this region.

소프트 베이킹 온도가 용액기반 Zn-Sn-O 박막 트랜지스터의 전기적 특성에 미치는 영향

In this study, the effects of soft baking temperature on the solution derived ZTO (Zn-Sn-O) TFTs (thin-film transistors) as a In-free oxide semiconductor were investigated. In spite of the same hard baking at high temperature(), the electrical properties of ZTO TFT was greatly changed by a small difference in soft baking temperature(). The performance of TFT was deteriorated as the soft baking temperature increased. Therefore, it is important to remove the water-related defects well as organic impurities from the ZTO films during soft baking for fabrication of solution-derived high performance of TFTs.

Electrical conductivity during incipient melting in the oceanic low-velocity zone.

A low viscosity layer in the upper mantle, the Asthenosphere, is a requirement for plate tectonics1. The seismic low velocities and the high electrical conductivities of the Asthenosphere are attributed either to sub-solidus water-related defects in olivine minerals2-4 or to a few volume percents of partial melt5-8 but these two interpretations have shortcomings: (1) The amount of H2O stored in olivine is not expected to be higher than 50 ppm due to partitioning with other mantle phases9, including pargasite amphibole at moderate temperatures10, and partial melting at high temperatures9; (2) elevated melt volume fractions are impeded by the too cold temperatures prevailing in the Asthenosphere and by the high melt mobility that can lead to gravitational segregation11,12. Here we determined the electrical conductivity of CO2-H2O-rich melts, typically produced at the onset of mantle melting. Electrical conductivity modestly increases with moderate amounts of H2O and CO2 but it dramatically increases as CO2 content exceeds 6 wt% in the melt. Incipient melts, long-expected to prevail in the asthenosphere10,13-15, can therefore trigger its high electrical conductivities. Considering depleted and enriched mantle abundances in H2O and CO2 and their effect on the petrology of incipient melting, we calculated conductivity profiles across the Asthenosphere for various plate ages. Several electrical discontinuities are predicted and match geophysical observations in a consistent petrological and geochemical framework. In moderately aged plates (>5Ma), incipient melts most likely trigger both the seismic low velocities and the high electrical conductivities in the upper part of the asthenosphere, whereas for young plates4, where seamount volcanism occurs6, higher degree of melting is expected.

Intrinsic doping and gate hysteresis in graphene field effect devices fabricated onSiO2 substrates

We have studied the intrinsic doping level and gate hysteresisof graphene-based field effect transistors (FETs) fabricated overSi/SiO2 substrates. It was found that the high p-doping level of graphene in some as-prepareddevices can be reversed by vacuum degassing at room temperature or abovedepending on the degree of hydrophobicity and/or hydration of the underlyingSiO2 substrate. Charge neutrality point (CNP) hysteresis, consisting of the shift of thecharge neutrality point (or Dirac peak) upon reversal of the gate voltage sweepdirection, was also greatly reduced upon vacuum degassing. However, another type ofhysteresis, consisting of the change in the transconductance upon reversal of thegate voltage sweep direction, persists even after long-term vacuum annealing at200 °C, whenSiO2 surface-bound water is expected to be desorbed. We propose a mechanismfor this transconductance hysteresis that involves water-related defects,formed during the hydration of the near-surface silanol groups in the bulkSiO2, that can act as electron traps.

Protonation in germanium equivalents of ringwoodite, anhydrous phase B, and superhydrous phase B

To gain insight into hydroxyl solubilities and possible hydration mechanisms of mantle silicates, as well as to test the utility of germanium analog models in studies of water-related defects, our present work is focused on the protonation of germanium analogs of silicates. For this purpose Ge-analogs of ringwoodite, anhydrous phase B (anhB), and for the first time, superhydrous phase B (shyB), were synthesized in a piston cylinder device at 2 GPa and 950-1000 °C under water-excess conditions. Electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and X-ray diffraction as well as Raman and infrared (IR) spectroscopy were used to characterize the experimental products. Ge-ringwoodite incorporates from 900 to 2200 ppm H 2 O by weight, which is much less than Smyth et al. (2003) observed for the Si-equivalent synthesized at 22 GPa and 1500 °C, but 200× more than published for γ-Mg 2 GeO 4 by Hertweck and Ingrin (2005). In addition to this discrepancy, the incorporation mechanism of H in Ge-ringwoodite also differs from that of Si-ringwoodite. Ge-anhB, which is currently believed to be anhydrous in the Si-system, contains from 2400 to 5300 ppm water by weight. A hydration model for germanate anhB was constructed based on single-crystal X-ray diffraction analysis and IR spectroscopy, in which OH is incorporated via the hydrogarnet substitution [V Ge ·4(OH) O ] x and via vacant Mg sites [V Mg ·2(OH) O ] x . For Ge-shyB the water concentration and incorporation mechanism obtained in this study are identical to results reported for the silicate phase synthesized at 22 GPa and 1200 °C (Koch-Müller et al. 2005). Thus, germanates are good low-pressure analogs for hydrous mantle silicates in which protonation is controlled by stoichiometry. However, for nominally anhydrous minerals we cannot recommend the use of germanates as high-pressure models in water-related studies. In these Ge-analogs, which are usually synthesized at much lower pressures, i.e., lower water fugacities, OH incorporation seems to differ from the high-pressure silicate equivalents qualitatively and quantitatively, as hydroxyl solubility is governed by other factors such as water fugacity and intrinsic defects.

A computer simulation study of OH defects in Mg2SiO4 and Mg2GeO4 spinels

Classical atomistic simulation techniques have been used to investigate the energies of hydrogen defects in Mg2SiO4 and Mg2GeO4 spinels. Ringwoodite (γ-Mg2SiO4) is considered to be the most abundant mineral in the lower part of the transition zone and can incorporate large amounts of water in the form of hydroxyls, whereas the germanate spinel (γ-Mg2GeO4) corresponds to a low-pressure structural analogue for ringwoodite. The calculated defect energies indicate that the most favourable mechanisms for hydrogen incorporation are coupled either with the reduction of ferric iron or with the creation of tetrahedral vacancies. Hydrogen will go preferentially into tetrahedral vacancies, eventually leading to the formation of the hydrogarnet defect, before associating with other negatively charged point defects. The presence of isolated hydroxyls is not expected. The same trend is observed for germanate, and thus γ-Mg2GeO4 could be used as a low-pressure analogue for ringwoodite in studies of water-related defects and their effect on physical properties.

Transmission electron microscopy of coesite inclusions in the Dora Maira high-pressure metamorphic pyrope-quartzite

Inclusions composed of relict coesite and retrograde quartz in garnet of a pyrope-quartzite from the Dora Maira massif were examined in transmission electron microscopy. Although coesite has suffered very little plastic deformation, the Burgers vectors of dislocations could be identified. They are [100], [001], [110] (i.e., a, c, and a+b), and other symmetrically almost identical vectors. The (110) plane could be identified as a slip plane. The dislocations are very straight, pointing to a high Peierls stress. Some small prismatic dislocation loops with Burgers vector [010] are observed, possibly formed by water-related defects. Retrograde quartz occurs at the margin of coesite with a palisade texture and as veins within coesite. Palisade quartz exhibits a high density of dislocations pinned on bubbles, and Brazil twins parallel to {1011} planes. The quartz veins are aligned parallel to the (100) and (021) composition planes of twins in coesite crystals. These observations strongly suggest that back-transformation of coesite into quartz starts at grain and twin boundaries, and that coesite loses water to quartz during transformation.

Water-related defects and oxygen diffusion in albite: a computer simulation study

Computer simulation techniques have been used to investigate the energetics of defect formation in Albite and to calculate O and (OH) migration activation energies. We find that the Na Frenkel defect has the lowest formation energy, whilst interstitials associated with impurities are the most favourable O defects. Water can be accommodated in the albite structure as both OH groups and as H2O molecules with solution energy of 0.73 eV and 0.9 eV respectively. The activation energies for O migration is reduced by up to 50% when the O is migrating as part of an (OH) group. In addition, we find a marked diffusional anisotropy for both O and (OH) in albite.

Fourier transform infrared spectroscopy determinations of intragranular water content in quartz-bearing rocks: implications for hydrolytic weakening in the laboratory and within the earth

Fourier transform infrared (FTIR) spectroscopy has been used to measure intragranular water contents of quartz (and feldspar) within fine-grained quartzites, granite, and naturally-deformed mylonites. Calibrations and tests of methods developed for this application using natural and synthetic quartz standards indicate that hydrogen concentrations down to 30 ppm are detectable (within apertured regions 100–200 μm in diam.) and concentrations of ≥ 400 ppm measurable to an accuracy of 30%. Quartzite and novaculite starting materials, used in previous studies of mechanical properties, contain substantial intragranular water contents (2400–3900 ppm), much larger than water concentrations commonly found in large clear natural crystals and more comparable to those of wet synthetic crystals. However, unlike molecular water within rapidly-grown synthetic quartz, most of this water is freezable, resembling fluid inclusion water of natural milky quartz crystals. Likewise, quartz and feldspar grains within granite and mylonites deformed at greenschist to upper amphibolite facies conditions contain large amounts of water (from 500 to 8000 ppm) as fine (often submicron size) fluid inclusions and IR spectra are dominated by a freezable O-H absorption band. Comparisons between dry natural and wet synthetic quartz crystals have formed the basis for our understanding of hydrolytic weakening. However, mechanisms by which water-related defects access dislocation cores within quartzites deformed in the laboratory and mylonites deformed within natural shear zones may differ from those within idealized natural and synthetic single crystals, and their mechanical properties may not be directly comparable.

Dislocation nucleation and multiplication in synthetic quartz: Relevance to water weakening

Recent observations suggest that the water-related defects associated with the so-called water weakening of single crystals of “wet” synthetic quartz are high-pressure clusters of molecular water. The microstructures which evolve in these crystals during both creep and constant strain-rate experiments and by heating alone were observed by TEM and show that the clusters act as highly efficient sources of the glissile dislocations which must be nucleated before plastic flow can be induced. These microstructural observations, together with simple microdynamical concepts based on the Orowan equation, are used to rationalize the creep behaviour and all the main features of the stressstrain curves observed in “wet” synthetic quartz crystals with a wide range of bulk water-contents, without postulating any direct influence of water on dislocation glide. It is proposed, therefore, that the relatively low yield stress of “wet” synthetic quartz is primarily due to the ease with which fresh glissile dislocations are nucleated, rather than to an enhanced glide of hydrolysed dislocations as is generally assumed in most models of water weakening.

Behavior of the Si/SiO2 interface observed by Fowler-Nordheim tunneling

Thin-oxide (40–50 Å) metal oxide semiconductor (MOS) structures are shown to exhibit, prior to large levels of electron tunnel injection, the near-ideal behavior predicted for a uniform trapezoidal barrier with thick-oxide properties. The oscillatory field dependence due to electron-wave interference at the Si/SiO2 interface indicates an abrupt, one-monolayer barrier transition (∼2.5 Å) consistent with earlier work. After tunnel injection of 1017 –5×1018 electrons/cm2, the barrier undergoes significant degradation leading to enhanced tunneling conductance, with reproducible behavior observed among different samples. This effect is consistent with the generation of positive states in the region of the oxide near the Si/SiO2 interface (<20 Å), where the tunneling electrons emerge into the oxide conduction band. Densities of positive-charge and interface-state buildup are also observed from capacitance-voltage (C-V) measurements and are found to be consistent with the observed tunneling dependence on positive-state generation. We suggest that the interface-state buildup may be identical to the positive-state generation observed by tunneling. The generated oxide states are shown to anneal slowly at room temperature, and more rapidly at 100 °C. Comparisons are made between wet, wet/annealed, and dry oxidation processes, with wet oxides exhibiting the largest densities of state generation, and dry oxides the smallest. The results are consistent with other work on the effect of water-related defects in oxides, and with x-ray photoelectron spectroscopy results, showing that breaking of strained Si-O-Si bonds is responsible for state generation during stress. The tunneling data also indicate that, prior to stress, very few positive states reside in the oxide (?1010 cm−2), even though appreciable interface-state desities are observed from C-V data. We therefore conclude that these initial interface states must be associated primarily with the monolayer transition layer and disorder in the underlying silicon, and are not distributed into the oxide near the interface.