William A. Bassett

Miniature diamond anvil pressure cell for single crystal x‐ray diffraction studies

A new miniature gasketed diamond anvil high pressure cell has been constructed to perform optical and x‐ray diffraction studies on single crystals under hydrostatic pressure. For x‐ray studies the cell is mounted on a standard goniometer head which may be attached to either a standard precession camera or single crystal orienter taking advantage of counting methods. The pressure cell has been used successfully in the study of the two high pressure phases of calcium carbonate, CaCO3(II) and CaCO3(III).

Elastic moduli of NaCl by Brillouin scattering at high pressure in a diamond anvil cell

Instrumentation has been developed to conduct Brillouin scattering measurements on small (0.15 mm) transparent single crystals at hydrostatic pressures up to 35 kilobars in a gasketed diamond anvil cell. Phonon velocities obtained this way can be used to calculate elastic moduli as a function of pressure. Results of measurements made on NaCl yield zero‐pressure values of c11=4.82×1011 dynu2009cm−2, c12=1.28×1011 dynu2009cm−2, and c44=1.27×1011 dynu2009cm−2. The plots of the elastic moduli versus pressure are fitted with straight lines with the following slopes: ∂c11/∂P=11.62, ∂c12/∂P=3.05, and ∂c44/∂P=0.759.

X‐Ray Diffraction and Optical Observations on Crystalline Solids up to 300 kbar

Diamond anvils driven by a simple piston and screw device have been used to subject solid samples to pressures up to 300 kbar at ambient room temperature. Samples can be examined optically and by x‐ray diffraction while under pressure. Pressure is measured by means of the lattice parameter of sodium chloride mixed with the sample. Reflectivity and volume data on iron, as well as crystallographic data on PbS, PbSe, and PbTe, are presented.

Pressure‐Induced Phase Transformation in NaCl

A pressure‐induced phase transformation in NaCl which occurs rapidly and reversibly at approx 300 kbar and room temperature has been observed in a diamond‐anvil high‐pressure cell. X‐ray diffraction data indicate that the high‐pressure polymorph has the cesium chloride (B2) structure. The lattice parameters of the low‐ (B1) and high‐ (B2) pressure phases at the transformation pressure are, respectively, 4.872±0.004 A and 2.997±0.004 A, and the volume change for the transformation is − 1.00±0.05 cm3 mole−1. The entropy change for the phase transformation has been calculated from the volume change and from the high‐temperature‐pressure data obtained by the shock experiments of Fritz et al. and found to be 1.5±0.3 cal deg−1 mole−1. Comparison with other alkali chlorides indicates that a linear relationship exists between the entropy change and the volume change for the B1‐B2 phase transformation. A thermodynamic equation accounting for this relationship has been derived under the assumption that the Gruneise...

Effect of Pressure on Crystal Structure and Lattice Parameters of Iron up to 300 kbar

Lattice parameters of the bcc and hcp phases of iron have been determined as a function of pressure up to 300 kbar at 23°±3°C by means of x‐ray diffraction techniques. The c/a ratio for hcp iron has been determined to be 1.603±0.001 at pressures between 80 and 300 kbar and is independent of pressure. Based upon the extrapolation of the high‐pressure data using an exponential form of the Murnaghan equation of state, the volume of hcp iron at zero pressure is 6.72±0.06 cm3/mole. The volume change for the bcc‐hcp transformation at 130 kbar is −0.34±0.01 cm3/mole. This value satisfies the triple point conditions for the bcc‐hcp‐fcc triple point when published values for the other phase transformations are used.

High-Pressure Polymorph of Iron

An x-ray diffraction study of iron under pressure has shown that alpha-iron (body-centered cubic) transforms to hexagonal-close-packing (designated epsilon-iron) at 130 kb, room temperature. The volume change for the transformation is -0.20 � 0.03 cm2/ mole. The slope for the gamma-epsilon phase boundary has been calculated to be 2 � 1�C/kb. The molar volume of iron decreases from 7.10 cm3/mole (density = 7.85 g/cm3) at room pressure to 6.10 � 0.08 cm3/mole (density = 9.1 � 0.1 g/cm3) at 200 kb and room temperature. The extrapolation of the gamma-epsilon phase boundary, consistent with shock wave data, suggests that it may have an influence on the properties of the earths core

Laser heating in the diamond anvil press up to 2000°C sustained and 3000°C pulsed at pressures up to 260 kilobars

Samples held at pressures up to 260 kilobar in a diamond anvil pressure cell have been heated to very high temperatures by means of a laser beam introduced through one of the transparent diamond anvils. A 7 J pulsed ruby laser is able to produce temperatures as high as 3000°C in a high pressure sample. A 60 W cw YAG laser is able to produce sustained temperatures up to 2000°C. An optical pyrometer has been used in the latter case to measure the brightness temperature with an accuracy of ±50°C. The ruby laser has been successful in directly converting graphite to diamond; the YAG laser has made it possible to show that (Fe,Mg)2SiO4 disproportionates to (Fe,Mg)O plus SiO2 (stishovite) at pressures in excess of 200 kilobars and temperatures above 1000°C.

Compression of Ag and phase transformation of NaCl

The isothermal compression of silver up to 309 kbar has been measured at 23±3°C by high‐pressure x‐ray diffraction employing a diamond‐anvil cell. The B1 phase of NaCl mixed with the silver was used both as a pressure transmitting medium and as an internal pressure calibrant. The data fitted to two forms of the Birch equation yield K0=1180±50 kbar, K0′=3.8±0.5, and K0=1105±95 kbar, K0′=4.6±0.8. When the pressure exceeded 300 kbar, the NaCl underwent a phase transformation to the B2 phase. Increasing the load did not result in the complete disappearance of the B1 phase up to a pressure of 311 and 334 kbar based on the extrapolation of the Ag and Re compression data, respectively. The average volume change measured for the B1‐B2 transformation is −0.83±0.11 cm3/mole, which is 17% smaller than the value reported by Bassett et al. This smaller volume change improves the agreement of NaCl with the linear relationship between volume change and entropy change proposed by Bassett et al. for the B1‐B2 transformati...

Strength of MgO and NaCl polycrystals to confining pressures of 250 kbar at 25 °C

The strengths of MgO and NaCl polycrystals under confining pressures to 250 kbar have been determined in a diamond‐anvil high‐pressure cell. The stresses in the samples are inferred from the strains determined by x‐ray diffraction. The strength of MgO polycrystals is found to rise from a 1‐bar confining pressure value of about 4 kbar to a maximum of 30±10 kbar at a pressure of 50±20 kbar and to remain constant at that value to a pressure of 250 kbar. The strength of NaCl polycrystals is found to rise from a 1‐bar confining pressure value of about 0.3 kbar to a maximum of 4.0±1.5 kbar at a confining pressure of 250 kbar. The interpretation of the form of strength versus pressure for these two compounds is discussed in terms of a brittle‐ductile transition. Preliminary transmission electron microscope data are presented.

Disproportionation of Fe2SiO4 to 2FeO+SiO2 at pressures up to 250kbar and temperatures up to 3000 °C

Abstract A sample of Fe 2 SiO 4 (spinel) at ≈250 kbar in a diamond anvil press has been heated to ≈3000 °C for a duration of some ms using a focused light beam from a pulsed ruby laser. After quenching and unloading, X-ray diffraction patterns indicated that the portion of the sample that had been heated contained wustite and stishovite. A sample of Fe 2 SiO 4 (fayalite) at ≈200 kbar in a diamond anvil press has been heated to 800 °C for 20 min in a furnace. After quenching and unloading, the sample was found to have concentric zones with the fayalite phase at the lowest pressure, the spinel phase at the intermediate pressure, and a dark region at the center where the pressure was highest. An X-ray diffraction pattern of the dark central region indicated the presence of wustite. A microprobe analysis of a sample produced by a similar procedure ( Mao and Bell , 1971) has indicated that SiO 2 is apparently evolved along with wustite but is not sufficiently crystalline to be detected by X-ray diffraction. Several runs have been made by the laser technique at various pressures. Although the pressure determinations are probably not very reliable due to transient increases in pressure during heating, the results seem to be consistent with the phase relationship predicted on the basis of thermodynamic calculations. The results from the laser and static heating taken together provide strong evidence in favor of separate oxides being the stable phase assemblage in the system Fe 2 SiO 4 at pressures above 200 kbar.