E. R. Carpenter

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.

Variable temperature pressure cell for polycrystalline x‐ray studies down to 2 K—application to Bi

A variable pressure diamond anvil cell is described for operation at temperatures continuously variable from 300 down to 2 K and controllable within 10 mK. Polycrystalline x‐ray data are collected from the pressure cavity by means of either (1) standard photographic techniques, (2) diffractometer measurements, or (3) energy dispersive diffractometry. The facility has been used to examine parts of the phase diagram of Bi. Results from this study indicate that the Bi‐I and Bi‐III structures are retained to low temperatures; however, there is no evidence to support a structural transition from Bi‐V to Bi‐VIII.

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-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.

X-ray characterization of thin films using a Read camera ― an improvement

An improvement to the modified Debye–Scherrer camera is described which allows for the accurate determination of the lattice parameter for thin films. The x‐ray collimator assembly has been altered to allow the thin film sample to cut the x‐ray beam in half at zero angle of incidence resulting in a ‘‘D’’ spot on the x‐ray film. The lattice parameter of an aluminum film was measured using this modified camera and compared with that determined by an x‐ray diffractometer scan and the published value. The results agree within 3 mA. An additional method using difference angles is also described which can be employed when the origin has not been determined.

High pressure-high temperature synthesis of distorted perovskite-type Cu-based oxides

Abstract We have synthesized the rhombohedrally distorted perovskite phase of LaCuO3 by reacting mixtures of La2O3 and CuO in an oxygen rich atmosphere at 1500°C and 6.5 GPa. We find this phase to be metastable; at 410°C and ambient pressure, it undergoes an irreversible transition to a tetragonal structure. By selective replacement of some or all of the La or Cu with one or more of the following elements: Ba, Ca, Cr, Ni, Pb, Sc, Sr, Ti, Y, Zn, and Zr, over 150 different alloys have been formed. Magnetic susceptibility measurements have failed to reveal the presence of superconductivity in any of these new polymorphs.

Novel techniques for the prepartion of synthetic diamond

Abstract The objectives of this work are two fold: (1) to study the effect of using oxygen-acetylene flame grown synthetic diamond as seed crystals for the high pressure-high temperature conversion of graphite into diamond and (2) to demonstrate the ability to produce small crystallites of diamond by a simple, electron beam evaporation technique. In each case, the production of diamond from graphite was confirmed.