Raman Spectroscopy Under Simultaneous High Pressure - High Temperature in a Laser Heated Diamond Cell: Thermal Pressure, Heat Capacity, Entropy, Thermal Expansivity, and Anharmonic

University Of Washington, Seattle WA

Investigators

Abstract

The proposed work consists of the measurement of vibrational modes of minerals at various pressure and temperature conditions, which allows thermodynamic characterization of this mineral through statistical methods or vibrational modeling. Useful information obtained so far using vibrational modeling includes the pressure derivatives of heat capacities, entropies, thermal expansivities, and sound velocities. One important result has not yet been fully exploited: Thermal pressure at a given P and T may be derived using vibrational modeling of internal energy, from PTH = gEvib/V. Thus, vibrational studies at simultaneous high pressure and temperature allow computation of the equation of state, which will support the many x-ray diffraction studies of the thermal equation of state of minerals. Vibrational modeling will also help provide a means for extrapolation beyond the range of the measurements of all thermodynamic properties. Magnetic and electronic contributions to heat capacity and entropy for iron-bearing minerals (or any others with transition metals) will be explored. The proposed laboratory work consists of the development of Raman spectroscopy in the laser heated diamond anvil cell. The efforts will concentrate on the correlation between vibrational spectra and the equations of state of key mantle phases, but will also include the other key thermoelastic parameters. A small, single monochromator system for in situ measurements of temperature is requested. We propose to extend the current Raman spectroscopic capabilities in our laboratory at UW to allow simultaneous high temperatures and pressures using CO2 laser heating in the diamond anvil cell. Once the instrument is in place, geophysically important minerals whose thermal equations of states have been determined by x-ray diffraction will be studied, including MgSi perovskite and forsterite and its high pressure poly-morphs as well as the other MgSiO3 phases. In addition to the first-order goal of obtaining Raman at simultaneous high pressures and temperatures, in situ laser heating in the diamond anvil cell will provide other useful data: phase boundaries where appropriate will be determined in situ and any unquenchable phases can be detected, with probable structures or space groups identified based on Raman spectral patterns. Volume is the parameter against which many other properties are scaled in many temperature, compositional and dynamical models of the Earth's interior. V(P,T) at all deep Earch conditions as well as heat capacity and entropy at pressure will be invaluable to workers in many branches of geoscience and physics, including geophysics, geodynamics, mineralogy, and petrology as well as condensed matter physics. The PI has hosted six of undergraduate researchers, including two women, to participate in research in the laboratory. The PIs current graduate student is a Native American woman, one of the very few to obtain a Ph.D. in any field.