Recent developments in the experimental techniques using the laser heated diamond anvil cell combined with third generation synchrotron X-ray sources enable us to investigate the properties and crystal structures of the Earth's lower mantle constituents directly at the pressure-temperature conditions of the deep mantle. Using numerical simulations, I confirmed that the diamond anvil cell data improves the accuracy of high order thermodynamic parameters determined from pressure-volume-temperature (P-V-T) data significantly. It is also found that the temperature gradients in a diamond cell could be a serious systematic error source if not treated properly. I conducted in situ X-ray studies on the stability, crystal structure, phase relations, and equation of state of silicate perovskites, MgSiO3 and CaSiO3, directly at lower mantle conditions. The first in situ laser-heated diamond cell experiment on the post-spinel phase boundary in Mg2SiO 4 confirms that this boundary is consistent with the properties of the 660-km seismic discontinuity. It is also found that the lower pressure boundary reported by recent in situ multianvil studies may be due to the inaccuracy in the gold pressure scale as well as other possible sources. The stability of (Mg,Fe)SiO3 perovskite is determined along a mantle geotherm to 2300-km depth conditions. Unlike earlier studies, no evidence of dissociation or orthorhombic-to-cubic phase transformation was found. However, a new feature observed above 88 GPa suggests that a phase transformation to another perovskite structure is still a viable explanation for the recently observed seismic structure in the mid-mantle. The stability, crystal structure, and equation of state of CaSiO3 perovskite have been measured to 2200-km depth conditions. It is shown that CaSiO3 perovskite is stable at lower mantle conditions with a cubic crystal structure. Use of the lattice strain theory enables us to characterize the stress conditions of the sample and to obtain a quasi-hydrostatic equation of state. The P-V-T equation of state directly measured at lower mantle conditions shows that the thermoelastic properties of CaSiO3 perovskite are close to those of the bulk lower mantle.
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