A. W. Webb

Cryostat for generating pressures to 100 kilobar and temperatures to 0.03 K

An apparatus is described which allows studies to pressures of 100 kilobar and to temperatures of 0.03 K. A low temperature hydraulic press is incorporated with a dilution refrigerator to permit the continuous variation in pressure while at low temperature; thus, the entire pressure study of a material can be accomplished in one low temperature run. The sample is sensed magnetically using a SQUID magnetometer to detect changes in its magnetic moment. Operation of the system is described for measurements of the transition temperature of aluminum as a function of temperature and pressure.

Energy-dispersive x-ray diffraction with synchrotron radiation at cryogenic temperatures

A facility is described which has been developed for the rapid acquisition of structural information through the use of heterochromatic synchrotron radiation from a sample pressurized in a diamond-anvil cell and simultaneously cooled to cryogenic temperature. The system employs a closed-cycle He refrigerator, which can be continuously operated, independent of any liquid cryogens, from a remote station. The compressive contact force between the diamonds, and hence the sample pressure, is also externally controlled, thereby providing remote control capabilities for both the pressure and temperature. NaCl has been used as an internal pressure calibrant and existing empirical equation-of-state calculations for NaCl have been extended to reduced temperatures for this purpose. Preliminary data on the lower pressure critical point associated with the isomorphic phase transition in Ce0.8Th0.1La0.1 are presented.

STATIC P-T-V MEASUREMENTS ON MgO: COMPARISON WITH SHOCK WAVE DATA

Simultaneous P-V-T measurements of MgO have been carried out at pressures to 20 GPa and simultaneously to 680 K in a high pressure-temperature diamond-anvil cell using the x-ray diffraction technique and the radiation source at Stanford Synchrotron Research Laboratory. The sample temperature was measured with a precalibrated Pt-Pt% Rh thermocouple and the pressure was determined from an internal Au standard by using the deduced shock wave data of Jamieson et al _. [1982]. The experimental P-V isotherms are approximately parallel, indicating the invariance of KT with temperature at a constant volume, as observed in alkali metals and alkali halides. At high temperatures, however, the present isothermal data yield consistently ˜0.5% higher compression (V/VO) values as compared to the deduced shock wave data, resulting in higher pressure values (˜1 GPa) from the gold standard than from MgO. Until this discrepancy is resolved, the suitability of MgO as a pressure calibrant at high temperatures cannot be established.

A novel instrument for high-pressure research at ultra-high temperatures

Abstract A resistively heated diamond-anvil cell has been used to achieve pressures of 110 kbar at temperatures exceeding 1200°C for periods long enough to collect X-ray powder diffraction data with energy-dispersive techniques using “white” X-radiation produced at the Stanford Synchrotron Radiation Laboratory.

High temperature-high pressure synthesis of a new phase of LaCuO3

Abstract We report evidence of a modified procedure for the synthesis of LaCuO3 at elevated pressures and temperatures. LaCuO3 crystallizes in a rhombohedral lattice which represents a small distortion of the perovskite cubic cell; the rhombohedral lattice parameters are: a = 5.426 A and α = 60.89°. We find that upon heating the material to temperatures above 408°C, it undergoes an irreversible phase transition to a new crystal structure. X-ray data from the new polymorph can be indexed to a tetragonal unit cell having lattice parameters of a = 5.431 ± 0.007 A and c = 7.836 ± 0.013 A . This new cell can also be viewed as a distorted perovskite and, assuming 4 formula units in the cell, there is fractional density decrease of 0.3% in transforming from the rhombohedral to the tetragonal cell. Based on the similarly between these structures and the recently discovered high Tc Cu-oxides, tests have been run for evidence of superconductivity. So far the results have been negative.

Effects of pressure on Cd1−xZnxTe alloys (0≤x<0.5)

Electrical resistance measurements have been performed on samples of Cd1−xZnxTe (0≤x<0.50) as a function of pressure up to 7.0 GPa. Based on a multidecade decrease in the resistance, the transition pressure of the B3 to B1 phase change is observed to increase from 3.3 to 6.7 GPa as x increases from 0 to 0.49. An anomalous drop in the resistance is observed to precede this transition; the magnitude of this drop increases with increasing x up to x=0.45, where it takes the form of a local minimum in the resistivity. The origin of this anomaly is not understood, but it is presumed to be similar to the local resistance minimum previously reported in ZnTe.

High pressure structural phase transition in AgGaTe2

From in situ diffraction measurements of synchrotron produced x radiation, we have found that AgGaTe2 transforms from the tetragonal, chalcopyrite structure to a face‐centered cubic structure at 4.0±0.5 GPa. The resistivity shows a minimum near this pressure, but the material retains its semiconducting nature.

High pressure-high temperature synthesis of new Cu-based oxides

Abstract LaCuO 3 cannot be made at ambient pressure. We have successfully synthesized this compound and over 100 related polymorphs by reacting appropriate oxide mixtures at elevated pressures and temperatures. These new compounds have been formed by systematic substitution of some or all of the La and/or Cu with one or more of the following elements: Ba, Ca, Cr, Ni, Pb, Sc, Sr, Ti, Y, Zn, and Zr. Magnetic susceptibility tests have failed to show evidence of superconductivity.

Pressure induced resistivity changes in diluted magnetic semiconductors of (Hg, Fe)Se and (Hg, Mn)Te

Abstract Electrical resistance measurements have been performed on samples of Hg1-xFexSe and Hg1-xMnxTe for × ≤ 0.30 as a function of pressure to 7.0 GPa. Both materials crystallize in the B3 structure at atmospheric pressure and room temperature and belong to the class of materials known as diluted magnetic semiconductors. At elevated pressures, HgTe undergoes a phase change from the B3 to B9 phase which is accompanied by an increase in the electrical resistivity of several orders of magnitude. The results reported here are that increasing × in both materials serves to increase the B3-B9 transition pressure. The implication being that partial replacement of the Hg-bonds by Fe-Se and Mn-Te, respectively, increases the stability range of the B3 phase

Effects of laser-driven shocks on silicon single crystals

Damage of single‐crystal Si induced by 25‐ns pulses from a ruby laser (694 nm) has been studied after retrieval of the samples. Macroscopic evaluation was made using optical microscopy and measurement of the surface profiles. Microscopic study techniques included Laue and topographic x‐ray methods as well as x‐ray rocking curves. Cleavage affected both spallation and front surface damage. Some surface fracturing takes place after the environment has cooled below the melting point of Si, while other instances seem to be controlled by the shock geometry rather than cleavage planes. The impact zone remains single crystal with narrowly oriented mosaic structure noted occasionally. Material around the impact site is polycrystalline, evidently redeposited from the plasma.