Crystal structures at high pressures and temperatures.

摘要

The diamond anvil cell (DAC) is a unique instrument that can generate pressures equivalent to those inside planetary interiors (pressures on the order of 1 million atmospheres) under sustained conditions. When combined with a bright source of collimated x-rays, the DAC can be used to probe the structure of materials in-situ at ultra-high pressures. An understanding of the high-pressure structure of materials is important in determining what types of processes may take place in the Earth at great depths.; Motivated by previous studies showing that xenon becomes metallic at pressures above ∼1 megabar (100 GPa), we examined the stable structures and reactivity of xenon at pressures approaching that of the core-mantle boundary in the Earth. Our findings indicate the transformation of xenon from face-centered cubic (fcc) to hexagonal close-packed (hcp) structures is kinetically hindered at room temperature, with the equilibrium fcc–hcp phase boundary at 21 (±3) gigapascals, a pressure lower than was previously thought. Additionally, we find no tendency on the part of xenon to form a metal alloy with iron or platinum to at least 100 to 150 gigapascals, making it unlikely that the Earth's core serves as a reservoir for primordial xenon.; Measurements of the compressibility of natural (Mg_.75,Fe _.25)₂SiO₄ γ-spinel at pressures of the Earth's transition zone yield a pressure derivative of the bulk modulus K₀ ′ = 6.3 (±0.3). As γ-spinel is considered to be a dominant mineral phase of the transition-zone of the Earth's mantle (400–670 km depth), the relatively high value of K₀′ for γ-spinel may help explain the rapid increase with depth of seismic velocities through the transition zone.; The thermodynamics, mechanisms and kinetics of pressure-induced amorphization are not well understood. We report here new studies indicating little or no entropy difference between the crystalline and glassy states of Ca(OH) ₂ (portlandite). Additional work on the pressure-induced amorphization of AlPO₄ (berlinite) shows that this material, which is a close analog to quartz, shows a rich behavior that is dependent upon the pressure, temperature, stress-state and time-scales of the experimental conditions.