X-ray Diffraction to 302 Gigapascals: High-Pressure Crystal Structure of Cesium Iodide

X-ray diffraction measurements have been carried out on cesium iodide (CsI) to 302 gigapascals with a platinum pressure standard. The results indicate that above 200 gigapascals CsI at 300 K has a hexagonal close-packed crystal structure with the ideal c/a ratio of 1.63 +/- 0.01. The crystal structure and pressure-volume relations converge at high pressure with those of solid xenon, which is isoelectronic with CsI. The results indicate a significant loss of ionic bonding in the hexagonal close-packed metallic phase of CsI at ultrahigh pressure.

Laser Techniques in High-Pressure Geophysics

Laser techniques in conjunction with the diamond-anvil cell can be used to study high-pressure properties of materials important to a wide range of problems in earth and planetary science. Spontaneous Raman scattering of crystalline and amorphous solids at high pressure demonstrates that dramatic changes in structure and bonding occur on compression. High-pressure Brillouin scattering is sensitive to the pressure variations of single-crystal elastic moduli and acoustic velocities. Laser heating techniques with the diamond-anvil cell can be used to study phase transitions, including melting, under deep-earth conditions. Finally, laser-induced ruby fluorescence has been essential for the development of techniques for generating the maximum pressures now possible with the diamond-anvil cell, and currently provides a calibrated in situ measure of pressure well above 100 gigapascals.

High-Pressure Ruby and Diamond Fluorescence: Observations at 0.21 to 0.55 Terapascal

A diamond-anvil, high-pressure apparatus was used to extend the upper pressure limit of static laboratory experiments. Shifts of the R(1) strong fluorescent line of ruby were observed that correspond to static pressures of 0.21 to 0.55 terapascal (2.1 to 5.5 megabars) at 25 degrees C. Sensitive spectroscopic techniques in the pressure range 0.15 to 0.28 terapascal were used to observe ruby and diamond fluorescence separately; these two fluorescent emissions overlap strongly at high pressures. At pressures greater than approximately 0.28 terapascal, the diamond fluorescence diminished and the ruby fluorescence reappeared strongly. Pressure was determined by extrapolation of the calibrated shift of the ruby R(1) line.

Ultrahigh Pressure: Beyond 2 Megabars and the Ruby Fluorescence Scale

A new design of the diamond-window, high-pressure cell has permitted static pressure of 2.8 megabars to be generated for the first time. The design is unusually stable mechanically, and thus it should be possible to use the new cell to study most materials, including hydrogen, in the unexplored pressure region above 1 megabar.

B1-B2 Transition in Calcium Oxide from Shock-Wave and Diamond-Cell Experiments

Volume and structural data obtained by shock-wave and diamond-cell techniques demonstrate that calcium oxide transforms from the B1 (sodium chloride type) to the B2 (cesium chloride type) structure at 60 to 70 gigapascals (0.6 to 0.7 megabar) with a volume decrease of 11 percent. The agreement between the shockwave and diamond-cell results independently confirms the ruby-fluorescence pressure scale to about 65 gigapascals. The shock-wave data agree closely with ultrasonic measurements on the B1 phase and also agree satisfactorily with equations of state derived from ab initio calculations. The discovery of this B1-B2 transition is significant in that it allows considerable enrichment of calcium components in the earth's lower mantle, which is consistent with inhomogeneous accretion theories.

Absolute pressure measurements and analysis of diamonds subjected to maximum static pressures of 1.3-1.7 Mbar

Force per unit area measurements made in the megabar pressure cell, independently of other pressure calibration systems, are consistent with the ruby R1 scale of Mao, Bell, Shaner, and Steinberg and its extrapolation to 1.4 Mbar. Physical analysis of diamond anvils removed after experiments to maximum pressures of 1.3-1.7 Mbar suggests that the nitrogen platelet concentration may be related to the strength of the diamonds. The pressure face of one of the diamonds from the 1.7-Mbar experiment was deformed plastically by a macroscopic amount.

Observations of Hydrogen at Room Temperature (25°C) and High Pressure (to 500 Kilobars)

Hydrogen becomes a solid at 25 degrees C when subjected to a pressure of 57 kilobars. The high-pressure phase appears as a transparent crystalline mass. The refractive index of the high-pressure phase increases sharply with pressure, indicating a density increase of similar magnitude. At 360 kilobars the calculated density of the high-pressure phase is 0.6 to 0.7 grams per cubic centimeter.

High-Pressure Physics: Sustained Static Generation of 1.36 to 1.72 Megabars

The pressure in experiments with the diamond-window pressure cell exceeded 1.7 megabars (at 25 degrees C). This is the highest sustained pressure ever generated under static conditions where the pressure in the sample itself was measured. At 1.72 megabars, macroscopic flow of one of the diamond pressure faces was observed.

Electrical Conductivity and the Red Shift of Absorption in Olivine and Spinel at High Pressure

Above 100 kilobars the apparent absorption edges (approximately 3 electron volts) of single-crystal and polycrystalline samples of the metastable olivine and stable spinel forms of Fe(2)SiO(4) shift rapidly with pressure from the near-ultraviolet into the lower-energy infrared region. Simultaneously, an exponential increase in electrical conductivity occurs. These effects are reversible as pressure is reduced or reapplied and are not accompanied by a first-order phase change in olivine or spinel. These observations relate to fundamental concepts of electrical conductivity and photon absorption in complex transition-metal silicates in that they cannot be readily interpreted in terms of an intrinsic band-gap model. The intensity and energy changes are too great and the effect occurs at too low a pressure to be explained by processes such as spin-pairing and other crystal-field effects. The results suggest that a new mechanism of conduction, perhaps symbiotic and employing an efficient charge-transfer process, is induced at high pressure.

A High-Pressure Polymorph of Troilite, FeS

A new polymorph of FeS was produced in a diamond-anvil cell and observed at high pressure both optically and by x-ray diffraction. Fourteen x-ray reflections of the high-pressure FeS were recorded; however, the crystal structure is unknown. This form of FeS is stable at 25 degrees C only at pressures above approximately 55 kilobars. The transition to the lower pressure polymorph, troilite, is rapid and reversible.