Jagannadham Akella

Phase Diagram of Iron by in Situ X-ray Diffraction: Implications for Earth's Core

The phase diagram of iron has been studied to 130 gigapascals (1 gigapascal = 104 atmospheres) and 3500 kelvin by a combined laser-heated diamond-anvil cell and x-ray diffraction technique that provides direct identification of the solid phases. Iron in the hexagonal close-packed (hcp) phase (ϵ-Fe) is stable from 50 to at least 110 gigapascals at high temperatures. The wide stability field of ϵ-Fe indicates that this polymorph should currently be considered the most relevant solid phase for Earths core. The triple point between the γ, ϵ, and liquid phases is located at 2500 ± 200 kelvin and 50 ± 10 gigapascals. There is evidence for a phase with a double hcp structure below 40 gigapascals and for another transition above 110 gigapascals and 3000 kelvin.

Epitaxial diamond encapsulation of metal microprobes for high pressure experiments

Diamond anvils with diamond encapsulated thin-film microcircuits have been fabricated for ultrahigh pressure electrical conductivity experiments. The diamond films were homoepitaxially deposited onto the diamond anvil substrates with microwave plasma chemical vapor deposition using a 2% methane in hydrogen gas mixture and a diamond substrate temperature of 1300 °C. The diamond embedded thin-film microprobes remain functional to megabar pressures. We have applied this technology to the study of the pressure-induced metallization of KI under pressures up to 1.8 Mbar. This technology has the potential of greatly advancing the pressure range of a number of existing high-pressure diagnostic techniques, and for expanding the capabilities of diamond anvil cells into new directions.

High-pressure phase transformation studies in gadolinium to 106 GPa

Abstract We investigated the high-pressure structural transformations in gadolinium at room temperature. Up to a pressure of 106 GPa, gadolinium metal undergoes four structural transformations: h.c.p.→ Sm-type → d.h.c.p. → f.c.c. → t.h.c.p. The corresponding transformation pressures are: 1.5 ± 0.2 GPa, 6.5 ± 0.5 GPa; onset at 24.0 GPa and completion at 29.0 GPa; and between 44.0 and 55.0 GPa, respectively. We found a much larger stability range for f.c.c.-gadolinium phase than that reported by others.

Ultrapressure equation of state of cerium metal to 208 GPa

We report static pressure compression of cerium metal to 208 GPa (volume compression V/V0=0.37) in a diamond anvil cell at room temperature. Cerium is unique in the 4f elements because of proximity of the f shell to the Fermi energy and related phase transformations induced by pressure. The energy-dispersive x-ray diffraction studies were carried out on cerium metal to 208 GPa using a synchrotron x-ray source and an internal copper pressure standard. A collapsed body centered tetragonal phase is found to be stable to the highest pressure with axial ratio remarkably constant at 1.680±0.006 in the 90–208 GPa pressure range in excellent agreement with theory. Cerium is thus isostructural and isoelectronic with 5f-band metal thorium at ultrapressures. We present equation of state parameters, which describe the compression of cerium to ultrapressures.

On the possibility of Pr III having a thcp structure

Abstract Crystallographic calculations have been carried out for a triple hexagonal close-packed (thcp) structure using data for Pr III, a high-pressure form shown by praseodymium metal. McMahan and Young proposed thcp as an additional high-pressure phase in the rare earth metals structural sequence. As applied to Pr III data at 14.4 GPa pressure, this structure type accounts for most but not all of the diffraction lines. Thus, we are unable to establish conclusively that Pr III has a thcp structure. However, it is shown that the thcp structure fits the Pr III data as reasonably as other structural forms proposed by other authors.

Static EOS of uranium to 100 GPa pressure

Abstract The crystal structure and compressibility of uranium has been determined by energy dispersive X-ray measurements in a diamond-cell apparatus up to pressures of 100 GPa. The alpha phase of uranium remains stable up to the highest pressures as suggested by earlier shock-Hugoniot data. An equation-of-state for alpha-uranium derived from both types of data implies that this phase also remains stable up to 2500 K at Hugoniot pressures of 100 GPa.

Static high-pressure structural studies on Dy to 119 GPa

Structural phase transitions in the rare-earth metal dysprosium have been studied in a diamond anvil cell to 119 GPa by x-ray diffraction. Four transformations following the sequence hcp→Sm-type→dhcp→hR24 (hexagonal)→bcm (monoclinic) are observed at 6, 15, 43, and 73 GPa, respectively. The hexagonal to monoclinic transformation is accompanied by a 6% reduction in volume, which is attributed to delocalization of the 4f electrons, similar to that seen in Ce, Pr, and Gd.

Diamond-anvil cell high pressure X-ray studies on thorium to 100 GPa

Abstract High pressure crystal structural changes and volume compression for thorium were investigated to 100 GPa in a diamond-anvil cell apparatus using the energy-dispersive X-ray diffraction technique. No structural change was observed in this pressure range. However, a slope change in the P-V curve was observed between 20 and 30 GPa. We surmise that this break in the slope may be due to an electronic transformation.

High pressure phase transformations in neodymium studied in a diamond anvil cell using diamond-coated rhenium gaskets

Diamond anvil cells are used to generate high static pressures up to several megabars (hundreds of GPa) in very small volumes of material. We have explored a technique which employs a microwave plasma chemical vapour deposited diamond layer on one side of the rhenium gasket. The high yield strength of the diamond layer prevents excessive thickness reduction of the sample in the gasket hole. As a test case, we show energy dispersive x-ray diffraction data on rare earth metal neodymium to high pressures of 153 GPa using a synchrotron source. The increased sample thickness results in an unambiguous crystal structure determination of a monoclinic phase in neodymium above 75 GPa.

Structural stability in uranium

Diamond-anvil cell experiments and first-principles theory have been used to investigate the structural stability of uranium up to 1 Mbar in pressure. Experiments and theory agree; there is no phase transition in uranium below 1 Mbar. Previous speculations about a crystallographic phase transition in uranium below this pressure are thus shown to be incorrect. In this regard, uranium is exceptional in the series of light actinides, where pressure-induced phase transitions typically occur at pressure below 1 Mbar. The ground-state crystal structure of uranium is orthorhombic with three structural parameters: the axial ratios b/a and c/a, and an internal parameter y measuring the displacement, along the b-axis, of alternate planes. The experimental and theoretical results reported here indicate that one of these parameters, c/a, is substantially more sensitive to pressure than the other two, changing by as much as 5%, while b/a and y are constant within 1% within the pressure range studied. This flexibility in the structure facilitates this structure over a wide pressure range. Theory suggests that electrostatic contributions to the total energy drive the variation in the c/a ratio as a function of pressure, and a simple model is utilized to show this.