Peter Lazor

Raman spectroscopic study of magnetite (FeFe2O4): a new assignment for the vibrational spectrum

Raman spectroscopic study on magnetite (FeFe2O4): a new assignment for the vibrational spectrum.

Experimental and theoretical identification of a new high-pressure phase of silica

Following the discovery of stishovite (the highest-pressure polymorph of silica known from natural samples), many attempts have been made to investigate the possible existence of denser phases of silica at higher pressures. Based on the crystal structures observed in chemical analogues of silica, high-pressure experiments on silica and theoretical studies, several possible post-stishovite phases have been suggested. But the likely stable phase of silica at pressures and temperatures representative of Earths lower mantle remains uncertain. Here we report the results of an X-ray diffraction study of silica that has been heated to temperatures above ∼2,000 K and maintained at pressures between 68 and 85 GPa. We observe the occurrence of a new high-pressure phase which we identify with the aid of first-principles total-energy calculations. The structure of this phase (space group Pnc2) is intermediate between the α-PbO2 and ZrO2 structures, and is denser than other known silica phases.

Measurement of melting temperatures of some minerals under lower mantle pressures

Melting temperature measurements of six minerals (stishovite (SiO2), corundum(Al2O3), diopside (CaMgSi2O6), and three perovskites (MgSiO3, CaSiO3, Mg3Al2Si3O12)) at high pressures were carried out in a YAG laser-heated diamond anvil cell with rhenium meta

Temperatures in Earth's Core Based on Melting and Phase Transformation Experiments on Iron.

Experiments on melting and phase transformations on iron in a laser-heated, diamond-anvil cell to a pressure of 150 gigapascals (approximately 1.5 million atmospheres) show that iron melts at the central core pressure of 363.85 gigapascals at 6350 � 350 kelvin. The central core temperature corresponding to the upper temperature of iron melting is 6150 kelvin. The pressure dependence of iron melting temperature is such that a simple model can be used to explain the inner solid core and the outer liquid core. The inner core is nearly isothermal (6150 kelvin at the center to 6130 kelvin at the inner core-outer core boundary), is made of hexagonal closest-packed iron, and is about 1 percent solid (MgSiO3 + MgO). By the inclusion of less than 2 percent of solid impurities with iron, the outer core densities along a thermal gradient (6130 kelvin at the base of the outer core and 4000 kelvin at the top) can be matched with the average seismic densities of the core.

Stability of Perovskite (MgSiO3) in the Earth's Mantle

Available thermodynamic data and seismic models favor perovskite (MgSiO3) as the stable phase in the mantle. MgSiO3 was heated at temperatures from 1900 to 3200 kelvin with a Nd-YAG laser in diamond-anvil cells to study the phase relations at pressures from 45 to 100 gigapascals. The quenched products were studied with synchrotron x-ray radiation. The results show that MgSiO3 broke down to a mixture of MgO (periclase) and SiO2 (stishovite or an unquenchable polymorph) at pressures from 58 to 85 gigapascals. These results imply that perovskite may not be stable in the lower mantle and that it might be necessary to reconsider the compositional and density models of the mantle.

Experimental Evidence for a New Iron Phase and Implications for Earth's Core

Iron is known to occur in four different crystal structural forms. One of these, the densest form (ε phase, hexagonal close-packed) is considered to have formed Earths core. Theoretical arguments based on available high-temperature and high-pressure iron data indicate the possibility of a fifth less dense iron phase forming the core. Study of iron phase transition conducted between pressures of 20 to 100 gigapascals and 1000 to 2200 Kelvin provides an experimental confirmation of the existence of this new phase. Thee ε iron phase transforms to this lower density phase before melting. The new phase may form a large part of Earths core.

High pressure melting and equation of state of aluminium

Abstract Melting curve of aluminium has been determined up to 50 GPa in a diamond anvil cell with laser heating. Using wavelength independent emissivity very good agreement was found with previous melting study, theoretical calculations and shockwave experiments. Analysis of the data using Lindemann and Simon melting equations suggests that melting occurs close to, but not exactly at the same pressure as measured at the cold state. Grueneisen parameter γ scales with volume as γ/γ0=V/V0, where γ0=2.2 ± 0.1, a value close to 2.14 from adiabatic decompression experiments. The melting curve, when transformed to normalized coordinates, verifies the prediction of work of Schlosser et al. [Phys. Rev. B, 40 (1989) 5929] for the linear relationship between normalized melting temperature and normalized pressure. Fit of the combined data from melting, thermal expansion, room compression, and shockwave studies to the Mie–Grueneisen thermal equation of state gives γ0=2.42 ± 0.02; Anderson’s thermal pressure model yields −0.032 ± 0.001 and −0.001 ± 0.001 GPa/K for the temperature derivative of isothermal bulk modulus at constant pressure and constant volume, respectively.

X-ray diffraction and Raman spectroscopic study of nanocrystalline CuO under pressures

Compressibility of an oxide does not necessarily change due to decrease in particle size. This is found in nanocrystalline CuO, an important semiconductor. We studied nanometric CuO with high energy synchrotron radiation and Raman spectroscopic techniques to pressures of 47 GPa. Our results indicate that nanometric CuO has the same bulk modulus as observed in the macrometric CuO. Combination of results obtained from MgO, Ni, and e-Fe indicates that the contribution of the size-induced surface energy to total internal energy has very little effect on the high pressure behavior of this type of nanomaterials, in which their bulk counterparts exhibit the structural stability over a wide range of pressure. This result is contrary to some observations, showing that the reduction of particle size significantly leads to increased bulk modulus.

Melting of wüstite and iron up to pressures of 600 kbar

The effect of pressure on melting temperature of wüstite and iron has been measured with laser-heated diamond anvil cell. The temperature was determined by measuring the thermal radiation emitted by the sample as a function of wavelength in the range from 600 nm to 900 nm to which Plancks radiation function was fitted; the pressure was measured by ruby-fluorescence technique. The melting curve of wüstite in this study when extrapolated to low pressures agrees with Lindsleys (1966) data. Our data are similar to the recent data of Boehler (1992) and close to that of Ringwood and Hibberson (1990) at pressure of 160 kbar, but the melting temperature does not rise as rapidly with increasing pressure as reported by Knittle and Jeanloz (1991). If tungsten emissivity is used in the temperature calculation, the melting curve of iron matches those of Boehler et al. (1990). Use of emissivity of iron in the temperature calculation results in somewhat higher temperatures than those reported by Boehler et al. (1990).

Laser-heated diamond anvil cell experiments at high pressure: Melting curve of nickel up to 700 kbar

Laser-heated experiments in a diamond anvil cell have been performed on high pressure melting of nickel up to 700 kbar. The laser heating system consists of diamond anvil cell, Nd:YAG and argon lasers, spectrograph with diode array, computer with software, CCD camera with monitor and optics. Experiments on melting of tungsten, nickel and platinum at 1 bar outside the diamond anvil cell and melting of nickel below 80 kbar in the cell were carried out to check the system for pressure and temperature measurements. The results show that for solid pressure medium the uncertainties in measurements of pressure at the experimental spot vary between ±5 kbar at 100 kbar and ±25 kbar at 660 kbar. Spectroradiometrically determined temperature is reliable within ±70 K. Melting was detected in situ by visual observation. The melting point of nickel at 660 kbar has been found to be 2557 ±66 K.