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In contrast to polyvalent metals, transition metals have low melting slopes(dT/dP) that are due to partially filled d-bands that allow for a lowering of liquid phase energy through s-d electron transfer and the formation of local structures. In the case of bcc transition metals we show the apparent discrepancy of DAC melting measurements with shock melting of Mo can be understood by reexamining the shock data for V and Ta and introducing the presence of an icosahedral short range order (ISRO) melt phase.

The stability of methane (CH4), ethane (C2H6), octane (C8H18), decane (C10H22), octadecane (C18H38), and nonadecane (C19H40) were studied in a CO2-laser heated diamond anvil cell at pressures up to 25 GPa and temperatures up to about 7300 K. Methane and ethane were found to decompose to form hydrogen and diamond. Substantially greater yields of diamond were obtained from the longer chain alkanes.

For the better general understanding of melting behavior at high pressure, we investigated the influence of both crystallographic and electronic structure, and compressibility on melting temperatures for a large class of materials. In particular, we have established a large data base for melting of transition metals to megabar pressures. In general, bcc metals ( e.g. W, Ta, Mo, V, Cr) have very flat melting curves, and the initially steeper melting curves of fcc metals (Fe, Co, Ni) flatten significantly at high pressure. We also observed this trend for the more complicated alkaline earth, and rare earth metals. The flattening of the melting curves is due to the similarity of the solid and liquid structures and volumes. For iron there may exist an additional complication which may explain the reported results on both melting temperatures and structures. Due to the similarity in the free energies of its high pressure structures, these may coexist over a large pressure range. This phenomenon has been recentl...

Melting curves for Pr, Nd, Sm, Gd, and Y were measured in a diamond-anvil-cell to nearly 100 GPa and 4000 K. $f$-electron volume collapses are observed as triple points for Pr (24 GPa and 1400 K) and Gd (65 GPa and 3100 K). These pressures coincide with the volume collapses observed at room temperature. For Nd and Sm, the $f$-electron volume collapse has not been observed at room temperature but appears at $\ensuremath{\sim}2000--2500\mathrm{K}$ as a broad minimum in the melting curve, similar to that of Ce, near 50 GPa (Nd) and 70 GPa (Sm). The melting curve of Y goes smoothly along the entire rare earth sequence.

In the present paper we report theoretical calculations for the Hugoniot of shock-compressed rubidium. We use an all-electron full-potential method based on density-functional theory to obtain the total energy, pressure and electronic density of states. Thermal contributions to the equation of state (EOS) were introduced through temperature dependent occupation of the electronic states and a Gruneisen model was used for determining the nuclear thermal motion. The calculated Hugoniot is in very good agreement with the shock experiments. A temperature of about 10 000 K is reached at the pressure of about 300 kbar, the highest reached experimentally. Rubidium was chosen for this study because it undergoes a sequence of unusual pressure-induced structural transitions which have been attributed to s-d electron transfer. Since most studies of s-d transfer have been carried out at temperatures below 500 K, little is known of how it effects the equation of state at very high temperature. We found that the predicted Rb Hugoniot is very sensitive to the thermal s-d electron transfer, which leads to a considerable lowering of the predicted Hugoniot pressure and temperature.

This report summarizes research on shock-compressed liquids: argon, xenon, nitrogen, and hydrogen. The purpose is to illustrate the common thread that connects all of these studies with the recent work on deuterium.

An investigation on the density dependence of the dynamic structure factor, S(Q,ω), for liquid rubidium was performed by inelastic neutron scattering experiments for a variety of conditions along the vapor pressure curve. Down to densities of about three times the critical density we found that the dynamics are still controlled by the metallic properties of the system. At higher temperatures and pressures and corresponding lower densities a deviation from these properties is found: at about twice the critical density S(Q,ω) is affected by molecular excitations. The effects of the excitations continue to even higher temperatures and lower densities. The interpretation of molecular excitations is qualitatively in accord with a simple model in which the total energy of Rb-lattices at 0 K is calculated from first principles. The agreement between theoretical and experimental results provides evidence for the appearance of molecules or clusters in expanded liquid alkali metals.

A model has been developed for the equation of state of deuterium that builds in the correct limiting behavior for the molecular fluid at low pressure and extends smoothly through dissociation to the very high-density monatomic-metallic fluid. The key assumption is that the Helmholtz free energy of the dissociating mixture is a function that can be approximated by the composition average of the free energy of the pure molecular and metallic hydrogen equations of state. The composition is determined by minimizing the free energy. In comparison to earlier studies this model leads to an enhancement of molecular dissociation and a lowering of shock temperatures and pressures. Calculations for shock-compressed liquid deuterium are in agreement with experiments to a pressure of 2.1 Mbar. At about 1 Mbar and 20 000 K liquid deuterium is 90% dissociated and is a nearly degenerate metal. The model predicts that molecular dissociation will lead to negative values of $(\ensuremath{\partial}P/\ensuremath{\partial}{T)}_{V}$ in the range 4000 to 10 000 K and volumes below 7 cc/mol. This feature suggests the formation of covalently bonded species in the partially dissociated mixture.

The melting curve of aluminum was measured in a laser-heated diamond cell up to a pressure of 0.8 Mbar in order to test the agreement between this technique and shock wave measurements, which has been lacking in the case for iron. At this pressure, which is over an order of magnitude higher than in previous experiments [1, 2], the melting temperature is 3800 K, comparable to that measured for iron at 2 Mbar [3]. The present results for aluminum extrapolate smoothly to the previous melting measurements in a multi-anvil apparatus to 60 kbar and to the calculated shock melting point of 4750 K at 1.25 Mbar. They are also in excellent agreement with theoretical calculations. A review of the shock data reported for Al, Ta and Mo, close-packed metals, in which a break in the sound velocity-pressure curve is used to determine the melting pressure, shows that the change in velocity at melting is about 10% for all three metals. In the case of iron, the sound velocity data have been used to infer two transitions: a solid-solid transition at 2.0 Mbar and melting at 2.4 Mbar, each of these transitions having about a 5% change in sound velocity. It is unlikely that a phase transition between close-packed cubic structures will have a 5% velocity change, the same as is found in the melting transition. We therefore suggest that for iron there exists only a single transition, starting at 2.0 Mbar, a region of incomplete shock melting between 2 and 2.4 Mbar, and a total change in sound velocity of about 10%, which is closer to the value of the other metals studied. This interpretation introduces a very good agreement between the shock melting results of Brown and McQueen [4] and diamond cell measurements for iron [3] which has up to now been lacking.