High-pressure phase equilibria studies provide important constraints on the mineralogy and composition of the earth's interior. However, most of these high-pressure experimental studies have concentrated upon the magnesiumrich simple system. It is generally acknowledged that the phase change of the basaltic crust and surrounding mantle plays significant role in mantle dynamics. In the present study, the cell parameters of minerals in the basaltic (morb) and peridotitic (klb-1) compositions have been determined by a laserheated diamond anvil cell technique and in situ X-ray method at the synchrotron beam line bl10xu, Spring-8, Japan. The morb and peridotite composition crystallized into assemblages of Mg-perovskite + Ca-perovskite + stishovite + aluminous phase and of Mg-perovskite + Ca-perovskite + magnesiowustite phase, respectively. The results of phase relation are generally consistent with previous studies by multi-anvil press and diamond-anvil cell experiments. The estimated densities of morb were denser than those of the seismic observations (e.g. Prem). Therefore, the oceanic crust may subduct into the base of the lower mantle. This result does not consist with the previous studies (Kesson et al, 1998; Ono et al., 2001). In previous studies, the thermoelastic parameters of minerals of simple compositions were used to estimate the mineral volumes. In this study, however, the mineral volumes were directly determined by in situ X-ray methods. Therefore, the compositional effect of the thermoelastic parameters should be considered to investigate the densities of high-pressure minerals in the multicomponent systems.
CaSiO 3 perovskite. The commonly assumed cubic structure was found to be stable at high temperatures and unstable at low temperatures at all pressures. On the basis of these results, we predict that the low temperature structure of CaSiO 3 perovskite is tetragonal space group I4/mcm. This phase would transform into an orthorhombic Imma structure under non-hydrostatic conditions. It is also obtained by fast quenching of cubic CaSiO 3 perovskite. This Imma structure explains hitherto puzzling experimental X-ray powder diffraction patterns.CaSiO 3 perovskite is thought to comprise between 6 and 12 wt% of the lower half of the Earth's transition zone and lower mantle. Its structure throughout this regime is generally assumed to be cubic [1,2] because temperature generally increases symmetry. At lower temperature deviations towards a tetragonal structure were found [1,4].Using VASP code in the PAW frame the CaSiO 3 cubic structures were first optimized at 0 K for pressures of 0, 50, 100, 150 GPa . For MD simulations we used an N-V-T-ensemble with Nosé thermostat [5]. Temperatures of 500, 1500, 2500 and 3500 K were simulated. The lattice parameters were not relaxed but on the difference of stress in the three spacial directions we could observe a phase transition from the cubic phase at high temperatures to a tetragonal phase at low temperatures. As we observe a significant difference in stress components, we predict that transition takes place between 1500K and 500K.[1] Ono S., Ohishi Y., Mibe K., Am. Mineral., 2004Mineral., , 89, 1480Mineral., -1485 Recent progress in high-pressure experiments has greatly expanded the accessible pressure and temperature conditions, and has proven to be a powerful approach for materials design. However, the characterization of new high-pressure phases is still challenging especially at elevated temperatures. For example, compressing hydrogen to the megabar pressure range is already accessible with laser heated Diamond Anvil Cell (DAC) techniques, yet, it has proven extremely difficult to measure the structural changes upon melting. On the other hand, ab-initio calculation methods, in principle, do not have limitations on the investigation of structural properties under high pressure and temperature conditions. To date, lithium hydride is only the alkali hydride, for which a B2 phase has not yet been found experimentally. The B1-B2 phase boundary at 0 K suggested by previous ab-iniito calculations are around 4 megabar, which is still out of reach for DAC experiments, however, the temperature axis has not yet been explored. We demonstrate, by using an ab-initio two-phase simulation method, that the B1-B2 phase boundary near the melting line is as low as 1.5 megabar, which is within the reach of the laser heated DAC technique. rhombohedral -to-orthorhombic -to-tetragonal -to-cubic phase. The cubic phase is paraelectric, and the other three phases are all ferroelectric. The polymorphic structure transitions of KNbO 3 were studied under high pressure using synchrotron radiation at BL-18...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.