12 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
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
Emissivity of Powdered Silicates in TIR Spectral Range (7-14 µm) Under Simulated Daytime Surface Conditions of Mercury and Their Detection from the Orbit
No full textIntroduction: The mid infrared (MIR) spectral region is especially sensitive to the abundance of Si-O, unlike the visible-near infrared spectral region. Though the geochemical suite on the NASA MESSENGER spacecraft to Mercury revealed compositionally diverse crustal materials [1], the spectrometer suite (MASCS; VIS-IR) could not reveal the silicate mineralogy of crustal materials due to the Fe2+-poor nature of the silicate minerals on the surface of Mercury. On October 20, 2018, ESA/JAXA’s BepiColombo mission was successfully launched to Mercury. MERTIS (Mercury Radiometer and Thermal Imaging Spectrometer) onboard BepiColombo will be the first thermal infrared (TIR) hyperspectral imager (7 – 14 μm) and radiometer (7 – 40 μm) to orbit Mercury mapping global spectral emissivity and surface temperature of Mercury at a spatial resolution of ~500 m/pixel and ~2 km/pixel respectively [2]. MERTIS will therefore provide spatially resolved information on mineralogy of various geological terrains including hollows and pyroclastic deposits, rock abundance, grain size, thermal inertia, and surface temperature [3]. Studying the thermal emissivity measurements of silicates at Mercury surface temperatures up to 450°C and under vacuum will help us to create the standard spectral library for MERTIS data analysis. Sample selection and preparation: Over a decade, the Planetary Spectroscopy Laboratory in the Department of Planetary Laboratories at the Institute for Planetary Research, DLR, Berlin has been undertaking huge efforts in collecting natural silicate endmembers from various sources in preparation to MERTIS data science [4]. These silicates are suggested by groundbased observations of Mercury and indirect mineralogy derived from NASA MESSENGER geochemistry suite [e.g., 1,5] and they include; a) olivine: forsterite, b) pyroxenes: enstatite, diopside, c) plagioclase feldspar: hypersthene, anorthite, labradorite, andesine, oligoclase, orthoclase, and d) nepheline. Here we present the emissivity of these silicates (at grain size of <25μm) at 7-14 μm as a function of temperature under vacuum conditions. Facility and Methods: A Bruker Vertex 80V instrument with MCT HgCdTe detector (cooled by liquid nitrogen) and KBr beamsplitter is used at PSL to measure the thermal infrared (TIR) emission spectra of the samples. This spectrometer is attached to an external chamber where the samples are placed in steel cups which are heated up to Mercury’s peak daytime temperatures via induction technique under vacuum (Fig. 1). Each sample is heated from 100° to 500°C (step 100°C) at medium vacuum (0.7 hPa) and then cooled down in vacuum. Radiance from the heated samples is collected by a gold (Au) coated 90° off-axis parabolic mirror and reflected into the spectrometer. It samples the thermal emission spectra of the silicates at wavelength intervals of 7-14 μm at spectral resolution of 4 cm-1 (Fig. 1). The spectra are calibrated following the standard PSL calibration procedure. A blackbody target with a known emissivity spectrum is measured at the same geometry and temperature as the samples. Emissivity is derived by dividing the sample signal by the blackbody signal correcting for the emissivity spectrum of the calibration target. Figure 2 shows the resulting emissivity measurements at temperatures of 100°C, 200°C, 300°C, 400°C, and 500°
A New Facility for the Planetary Science Community: The Planetary Sample Analysis Laboratory (SAL) at DLR
No full textNew 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
Sistema Nacional de Inovação: o desenvolvimento tecnológico da Coreia do Sul
No full textTCC (graduação) - Universidade Federal de Santa Catarina. Centro Sócio-Econômico. Economia.O presente trabalho tem como objetivo compreender o papel das políticas tecnológicas na criação e no desenvolvimento do Sistema Nacional de Inovação sul-coreano. Este processo envolveu a rápida transformação de um dos países mais pobre do mundo em uma economia industrializada e moderna. Dentre as forças propulsoras da dinâmica de transição da imitação à inovação do país, destacam-se o forte papel governamental e o aprimoramento dos recursos humanos. O objetivo foi cumprido recorrendo-se a pesquisa bibliográfica sobre o referencial analítico dos Sistemas Nacionais de Inovação, do Paradigma Tecnoeconômico, para contextualizar o processo histórico de mudança shumpeteriana e, finalmente, das Janelas de Oportunidade, foram apresentadas como momentos específicos a serem observados por países em desenvolvimento. Com base nisso, o processo histórico de desenvolvimento da Coréia do Sul foi apresentado e discutido. Complementarmente, indicadores estatísticos, disponibilizados pela Organisation for Economic Cooperation and Development (OECD) e United Nations Conference on Trade and Development (UNCTAD), foram utilizados. Como resultado, o crescente aprimoramento da capacidade tecnológica do país pode ser observado no tempo, destacando-se o papel das políticas industriais e de inovação no processo. Finalmente, a pesquisa sobre o foco das ações recentes revelou as ideias de economia criativa e economia verde, de baixa emissão de carbono, como o sentido da estratégia tecnológica para desenvolvimento da economia sul-corean
