169,712 research outputs found
The pressure-induced phase transition(s) of ZrSiO 4 : revised: Experimental proof for the existence of a new high-pressure polymorph of zircon
No full textThe existence of a new high-pressure low-symmetry (HPLS) ZrSiO 4 phase (space group I4 ̄ 2 d), which has been predicted by density-functional-theory (DFT) calculations (Stangarone et al. in Am Mineral, 2019b), is experimentally confirmed by in situ high-pressure Raman spectroscopic analysis up to 25.3 GPa. The new ZrSiO 4 polymorph is developed from zircon via a soft-mode-driven displacive phase transition. The Cochran-law-type pressure dependency of the soft-mode wavenumber reveals a zircon-to-HPLS critical pressure pc = 20.98 ± 0.02 GPa. The increase in the phonon compressibilities of the zircon hard mode near 202cm-1 at p> pr= 10.0 GPa as well as of the reidite hard mode near 349cm-1 at p< pr marks the pressure above which zircon becomes thermodynamically metastable with respect to reidite; the experimentally determined value of pr is in good accordance with the equilibrium zircon–reidite transition pressure derived from DFT simulations. However, at room temperature, there is not enough driving force to rebuild the atomic linkages and the reconstructive transition to reidite happens ∼ 1.4 GPa above pc, indicating that at room temperature, the HPLS phase is a structural bridge between zircon and reidite. The pressure dependencies of the phonon modes in the range 350--460cm-1 reveal that the reconstructive phase transition in the ZrSiO 4 system is triggered by energy resonance and admixture of hard modes from the parent and resultant phase
Experimental and calculated Raman spectra in Ca-Zn pyroxenes and a comparison between (CaxM2+1-x)M2+Si2O6 pyroxenes (M2+= Mg, Co, Zn, Fe2+)
No full textThe Raman spectra of the end member pyroxenes CaZnSi2O6 and Zn2Si2O6 are calculated by quantum mechanical modeling and compared with the experimental ones measured in synthetic (CaxZn1−x)ZnSi2O6 pyroxenes with x = 0, 0.2, 0.3, 0.5, 0.7, 1. The calculated spectra correctly predict the intensity and peak positions of the spectra recorded on the end members. The model provides also useful hints for the mode assignment at the intermediate compositions. The experimental peak positions are compared in (CaxM2+1−x)M2+Si2O6 pyroxenes, with M2+ = Mg, Co, Zn, Fe2+. These pyroxenes share a common charge and different mass and ionic radius; the relative contributions of the mass and ionic radius in the experimental spectrum are discussed in four of the most intense peaks. We found that the positions of the strongest peaks are related to the average bond distances of the polyhedra which most affect a given mode. Ca–Zn pyroxenes provide an exception, whereas the CaZnSi2O6 end member fits quite well in the bond-distance/peak positions relations found in other pyroxenes, and the same does not occur as Zn exchanges for Ca. Peak broadening occurs in Zn pyroxenes in intermediate compositions; it is related to the presence of polyhedral local configurations around Zn and Ca atoms in the M2 polyhedron. The broadening is higher in the ~ 1010 cm−1 peak (ν19), which, among the strongest peaks, shows the highest difference in the Raman wavenumber between end members. The different behaviours of Zn pyroxenes with respect to Mg, Co, and Fe2+ ones are likely related to the partially covalent bonding in the M2 cavity shown by Zn pyroxenes
New insights into the zircon-reidite phase transition
No full textThe structure, the elastic properties, and the Raman frequencies of the zircon and reidite polymorphs of ZrSiO4 were calculated as a function of hydrostatic pressure up to 30 GPa using HF/DFT ab initio calculations at static equilibrium (0 K). The softening of a silent (B1u) mode of zircon leads to a phase transition to a "high-pressure-low-symmetry" (HPLS) ZrSiO4 polymorph with space group I42d and cell parameters a = 6.4512 Å, c = 5.9121 Å, and V = 246.05 Å3 (at 20 GPa). The primary coordination of SiO4 and ZrO8 groups in the structure of zircon is maintained in the high-pressure phase, and the new phase deviates from that of zircon by the rotation of SiO4 tetrahedra and small distortions of the ZrO8 dodecahedra. The new polymorph is stable with respect to zircon at 20 GPa and remains a dynamically stable structure up to at least 30 GPa. On pressure release, the new phase reverts back to the zircon structure and, therefore, cannot be quenched in experiments. In contrast, the transformation from zircon to reidite is reconstructive in nature and results in a first-order transition with a volume and density change of about 9%. The calculated energies from the DFT simulations yield an equilibrium transition pressure of 9.13(1) GPa at 0 K. Simulations of the Raman spectra of the three polymorphs at 20 GPa show how they can be distinguished. In particular, the peak due to the lowest-energy A1 mode with a calculated wavenumber of 94 cm-1 is diagnostic of the HPLS phase because it does not overlap with any of the peaks of zircon or reidite
Measurement of strains in zircon inclusions by Raman spectroscopy
No full textWe have carried out ab initio hybrid Hartree-Fock/Density Functional Theory simulations to determine the structure and vibrational modes of zircon, ZrSiO4, as a function of different applied strains. The changes in phonon-mode wavenumbers are approximately linear in the unit-cell strains, and have been fitted to determine the components of the phonon-mode Grüneisen tensors of zircon which reproduce the change in measured Raman shifts with pressure. They can therefore be used to convert Raman shifts measured from zircon inclusions in metamorphic rocks into strains that in turn can be used to determine the metamorphic conditions at the time that the inclusion was trapped. Due to the strong anisotropy in the thermal pressure of zircon, the phonon-mode Grüneisen tensor is not able to reproduce the temperature-induced changes in Raman shifts. Because zircon inclusions are normally measured at room conditions this does not prevent the calculation of their entrapment conditions
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Ab initio simulations and experimental Raman spectra of Mg2SiO4 forsterite to simulate Mars surface environmental conditions
No full textIn this study, we present the full Raman vibrational spectrum of forsterite (Mg2SiO4), computed from first principles, employing a hybrid HF/density functional theory Hamiltonian (WC1LYP) as implemented in the CRYSTAL14 code, at static equilibrium and at the temperatures of 0, 300 and 1000 K. The simulations are compared with the available literature data, confirming the accuracy of the calculations, and to experimental Raman spectra taken at room temperature on a natural sample of forsterite (Mg1.76 Fe0.22 SiO4), to test the effect of compositions on the reliability of a comparison between a computed spectrum for the end member and the experimental spectrum on a real sample having a slightly different compositions. The comparison with the experimental data at room temperature shows a very good agreement (an average discrepancy of 7 cm-1), and it allows a reliable symmetry assignment of Raman signals to specific vibrational modes. Spectra are also simulated by changing the mass of the nuclei for each of the six symmetry-independent species within the unit to quantify the contributions of each element to the overall vibration. The aim is not only to relate the major experimental peaks to specific structural features but also to link them to the Raman shifts observed because of both temperature and composition variation. Moreover, so as to foresee the possible response of Raman spectra to the different environmental conditions occurring on planetary surfaces, i.e. Mars, we calculate full Raman spectra at 0, 300 and 1000 K including zero point vibrational effects, within the limit of the quasi-harmonic approximation. These results may be useful to widen a Raman database and provide new clues to improve the interpretation of data acquisitions during the 2020 ExoMars mission, which will carry on board a Raman Laser Spectrometer
Tackling the North Korea Problem: Nukes, Human Rights, and Sanctions
No full textAs we search for a tenure-track professor of Korean Studies, the Croft Institute is honored to host three speakers from the Korea Economic Institute of America for a panel discussion about American policy towards North Korea. The event will be held on Tuesday, March 28, from 4 to 5:30 p.m. in the Joseph C. Bancroft Conference Room (Croft 107). William Brown is a Senior Advisor to the East Asia and North Korea Mission Managers at the Office of the Director of National Intelligence. Scott Snyder is Senior Fellow for Korea Studies and Director of the program on U.S.-Korea Policy at the Council on Foreign Relations (CFR), where he had served as an Adjunct Fellow from 2008 to 2011. Troy Stangarone is the Senior Director of Congressional Affairs and Trade at Korea Economic Institute of America (KEI). He oversees KEI’s trade and economic related initiatives, as well as the Institute’s relations with Capitol Hill and the Washington, DC trade community.https://egrove.olemiss.edu/croft_spe/1024/thumbnail.jp
Mitomycin C in highly myopic eyes - Author reply
No full textOphthalmology. 2005 Feb;112(2):208-18; discussion 219.
Mitomycin C modulation of corneal wound healing after photorefractive keratectomy in highly myopic eyes.
Gambato C, Ghirlando A, Moretto E, Busato F, Midena E.
SourceRefractive Surgery Service and Antimetabolite Therapy Research Unit, Department of Ophthalmology, University of Padova, Padova, Italy.
Abstract
PURPOSE: To evaluate the role of topical mitomycin C in corneal wound healing (CWH) after photorefractive keratectomy (PRK) in highly myopic eyes.
DESIGN: Prospective, double-masked, randomized clinical trial.
PARTICIPANTS: Seventy-two eyes of 36 patients affected by high (>7 diopters) myopia.
METHODS: In each patient, one eye was randomly assigned to PRK with intraoperative topical 0.02% mitomycin C application, and the fellow eye was treated with a placebo. Postoperatively, mitomycin C-treated eyes received artificial tears (3 times daily, tapered in 3 months), whereas the fellow eye was treated with fluorometholone sodium 2% and artificial tears (3 times daily, tapered in 3 months).
MAIN OUTCOME MEASURES: Uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA), contrast sensitivity, manifest refraction, and biomicroscopy. Contrast sensitivity was determined using the Pelli-Robson chart. Corneal confocal microscopy documented CWH.
RESULTS: Mean follow-up was 18 months (range, 12-36). No side effects or toxic effects were documented. At 12-month follow-up examination, UCVAs (logarithm of the minimum angle of resolution) were 0.4+/-0.48 and 0.5+/-0.53 (P = .03) in mitomycin C-treated eyes and corticosteroid-treated eyes, respectively. At 1 year, corneal haze developed in 20% of corticosteroid-treated eyes, versus 0% of mitomycin C-treated eyes. At 12, 24, and 36 months, corneal confocal microscopy showed activated keratocytes and extracellular matrix significantly more evident in untreated eyes (Ps = 0.004, 0.024, and 0.046, respectively).
CONCLUSION: Topical intraoperative application of 0.02% mitomycin C can reduce haze formation in highly myopic eyes undergoing PRK.
Comment in
Ophthalmology. 2006 Feb;113(2):357; author reply 357-8
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
A Multi-Language Comparison of Influences on Author Verification using Character N-Grams
No full textWe create a new multi-language corpus for author verification based on Wikipedia talkpages, and evaluate the influence that differences in topic and time have on character n-gram author profiles. Topic alignment between two texts is found to increase author verification precision, and an authors writing style is found to change over time, but not more significantly after 3 years than after 1 year.Information ArchitectureWISElectrical Engineering, Mathematics and Computer Scienc
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