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Neutron diffraction and electrical transport measurements have been carried out on the heavy rare-earth metal terbium at high pressures and low temperatures in order to elucidate the onset of ferromagnetic (FM) order as a function of pressure. The electrical resistance measurements show a change in slope as the temperature is lowered through the FM Curie temperature. The temperature of this FM transition decreases at a rate of−16.7 K/GPa up to a pressure of 3.6 GPa, at which point the onset of FM order is suppressed. The neutron diffraction measurements as a function of pressure at temperatures ranging from 90 to 290 K confirm that the change of slope in the resistance is associated with the FM ordering, since this occurs at pressures similar to those determined from the resistance results at these temperatures. A disappearance of FM ordering was observed as the pressure is increased above 3.6 GPa and is correlated with the phase transition from the ambient hexagonal close packed structure to an α-Sm-type...

Pressure-induced structural phase transformations in a trivalent rare-earth metal praseodymium (Pr) were studied at room temperature in a diamond anvil cell to 103 GPa by energy dispersive x-ray diffraction using a synchrotron source. Our x-ray diffraction studies document the following crystal structure sequence: $\mathrm{dhc}\stackrel{\ensuremath{\rightarrow}}{p}\mathrm{fc}\stackrel{\ensuremath{\rightarrow}}{c}\mathrm{distorted}\mathrm{}\mathrm{fcc}(hR24\mathrm{type})\ensuremath{\rightarrow}\mathrm{monoclinic}(C2/m)\ensuremath{\rightarrow}\ensuremath{\alpha}\ensuremath{-}$uranium with increasing pressure. We measure a 16.7% volume collapse at the transition to the \ensuremath{\alpha}-uranium phase at 20 GPa. The high-pressure \ensuremath{\alpha}-uranium phase in Pr was found to be stable to the highest pressure of 103 GPa, which corresponds to a volume compression ${V/V}_{0}=0.407.$

The light rare-earth metal, neodymium, has been studied up to 155 GPa in a diamond-anvil cell using energy dispersive x-ray diffraction with a synchrotron source. The pressures were calibrated using copper as an internal x-ray pressure standard. A phase transformation from a monoclinic phase $(C2/m,$ $4\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{l})$ to an orthorhombic \ensuremath{\alpha}-U phase $(Cmcm,4\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{l})$ was observed at 113\ifmmode\pm\else\textpm\fi{}6 GPa without any observable volume collapse. The observation of \ensuremath{\alpha}-U phase in Nd and, previously, in cerium and praseodymium clearly establishes this phase in light rare-earth metals. Our equation of state measurements suggest that delocalization of the f shell in Nd occurs without any volume collapse unlike Ce and Pr.

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.

Ytterbium (Yb), a member of the rare earth group metals with a filled $f$ shell and a divalent electronic character, has been studied under static high pressures in a diamond anvil cell to 202 GPa. We report a new transformation from a face-centered-cubic structure to a hexagonal phase with 3 atoms/cell (space group ${P3}_{1}21$ referred to as $\mathrm{hP}3$) at $98\ifmmode\pm\else\textpm\fi{}5\mathrm{GPa}$. This $\mathrm{hP}3$ phase is similar to that observed in trivalent neodymium and samarium at pressures near 40 GPa. Our high pressure study thus clearly demonstrates that Yb is no longer an ``irregular'' member of the rare earth metals, and divalent to trivalent electronic transition is completed at megabar pressures.

We have designed a new diamond-anvil cell capable of radial x-ray diffraction to pressures of a few hundred GPa. The diffraction geometry allows access to multiple angles of Ψ, which is the angle between each reciprocal lattice vector g(hkl) and the compression axis of the cell. At the 'magic angle', Ψ≈54.7°, the effects of deviatoric stresses on the interplanar spacings, d(hkl), are significantly reduced. Because the systematic errors, which are different for each d(hkl), are significantly reduced, the crystal structures and the derived equations of state can be determined reliably. At other values of Ψ, the effects of deviatoric stresses on the diffraction pattern could eventually be used to determine elastic constants.

Static high pressure diamond anvil cell experiments were performed on three polycrystalline Zr samples having varying interstitial impurity concentrations. Systematic increase in transition pressure with the increase in the amount of interstitial impurities is observed for the martensitic α →ω structural phase transition in Zr. Significant room temperature crystal grain growth is also observed for the two highest purity samples at the α →ω transition. In the case of the lowest purity sample interstitial impurities obstruct the α →ω transition, while possibly helping impede grain growth-even as the sample is heated to 1279 K.

Ti‐6Al‐4V was examined under static‐high pressure conditions using a diamond anvil cell. The angle‐dispersive x‐ray diffraction experiments were performed at the Advanced Photon Source, Argonne National Laboratory. Radial and axial geometry were used to examine multiple samples. The purpose of the experiment was to generate pressure‐volume data at room temperature (which is non‐existent in literature) and to examine deviatoric stress effects on such a hard alloy.

A new, tabular, SESAME equation of state, along with new diamond‐anvil, Hugoniot, and thermal‐expansion data, is presented for Ti‐Beta‐21S (TIMETAL® 21S), a high‐strength, high‐temperature, beta‐stabilized alloy of Ti, Mo, Nb, Al and Si. The new equation of state combines an empirical, Vinet description of the cold curve with the Johnson ionic model and the Thomas‐Fermi‐Dirac model for the thermal electronic contributions. Two coexisting phases, HCP (α) and BCC (β), are observed at room temperature and pressure, with the α phase disappearing above 67 GPa. Predictions for the room‐temperature isotherm, principal Hugoniot, thermal expansion, and heat capacity are compared to new and existing experimental results.

The heaviest member of the rare-earth-metal series lutetium has been studied up to a pressure of 163 GPa in a diamond-anvil cell using energy-dispersive x-ray diffraction with a synchrotron source. The pressures were carefully calibrated using copper as an internal x-ray standard. A phase transformation at 88{plus_minus}5GPa is reported from a double-hexagonal-close-packed structure to a distorted fcc structure (hR24) with a volume change of 4.9{percent}. Such a first-order transformation is unusual in an element with a filled f shell and may indicate increased hybridization between the localized f shell and the spd conduction band at megabar pressures. We present the equation of state data to 163 GPa and attribute high compressibility to an electronic transfer from s{r_arrow}d band under pressure. {copyright} {ital 1998} {ital The American Physical Society}

Cerium is a fascinating f‐electron metal with a complex phase diagram. Using the diamond anvil cell and synchrotron x‐ray sources we have been able to study cerium to a pressure of 330 kilobars. Radial x‐ray diffraction techniques have been used to ensure a more accurate pressure‐volume curve at room temperature. Two different purities of cerium (99.9% and 99.99%) have been examined with noticeable differences. This work has produced high quality data that can be used to generate an accurate multi‐phase EOS for cerium. The techniques developed in these experiments can further be utilized to obtain accurate EOS data for other materials of interest.

Cerium, a member of the rare earth metals, has been studied up to 300 kilobars in a diamond‐anvil cell using energy‐ and angular‐dispersive x‐ray diffraction with a synchrotron source. The purpose of this experiment was to examine the electronic and structural behavior of cerium and to examine the effects of deviatoric stress due to non‐hydrostatic conditions within the sample environment. Using the standard sample orientation and data from various orientations, the effects of deviatoric stress are shown.

The structural and electrical properties of alkaline-earth metal beryllium were studied to 66 GPa using designer diamond anvils. The ambient pressure hexagonal close-packed phase was found to be stable to the highest pressure of 66 GPa and the measured static equation of state differs substantially from the one derived from the existing shock wave data. The electrical resistance data at high pressure shows a gradual decrease in resistance to pressure up to 25 GPa followed by a slight increase to 53 GPa. The axial ratio $(c/a)$ remains strongly nonideal with $c/a=1.562$ at ${V/V}_{0}=0.708,$ indicating that deviations from nearly free electron behavior are still dominant at high compressions.

A high-pressure phase of gadolinium is completely described for the first time. This phase, HR24, is found to exist at 46 GPa. The structure is examined for evidence of delocalization of the f shell, and the evolution of the HR24 phase with increasing pressure is discussed.

Neutron diffraction and electrical transport measurements have been made on the heavy rare earth metal holmium at high pressures and low temperatures in order to elucidate its transition from a paramagnetic (PM) to a helical antiferromagnetic (AFM) ordered phase as a function of pressure. The electrical resistance measurements show a change in the resistance slope as the temperature is lowered through the antiferromagnetic Néel temperature. The temperature of this antiferromagnetic transition decreases from approximately 122 K at ambient pressure at a rate of -4.9 K GPa(-1) up to a pressure of 9 GPa, whereupon the PM-to-AFM transition vanishes for higher pressures. Neutron diffraction measurements as a function of pressure at 89 and 110 K confirm the incommensurate nature of the phase transition associated with the antiferromagnetic ordering of the magnetic moments in a helical arrangement and that the ordering occurs at similar pressures as determined from the resistance results for these temperatures.

Structural stability of Ti alloy, stabilized as a mixture of bcc (β) and hcp (α) phases, was investigated up to 71GPa. Over this pressure, β phase remains stable, while for α phase, a decrease in c∕a ratio from 1.601 at ambient pressure to 1.568 at 36GPa is observed, followed by an increase to 1.613 at 44GPa. A steady value is then observed up to 67GPa. Above 58GPa, the authors also observe that α phase peak intensity is decreasing. At 67GPa, the authors only observe β phase peaks, which indicates that α→β structural phase transition is complete.