Quantifying the partial ionization effect of gold in the transition region between condensed matter and warm dense matter.

It is now well known that solids under ultra-high-pressure shock compression will enter the warm dense matter (WDM) regime which connects condensed matter and hot plasma. How condensed matter turns into the WDM, however, remains largely unexplored due to the lack of data in the transition pressure range. In this letter, by employing the unique high-Z three-stage gas gun launcher technique developed recently, we compress gold into TPa shock pressure to fill the gap inaccessible by the two-stage gas gun and laser shock experiments. With the aid of high-precision Hugoniot data obtained experimentally, we observe a clear softening behavior beyond ~560 GPa. The state-of-the-art ab-initio molecular dynamics calculations reveal that the softening is caused by the ionization of 5d electrons in gold. This work quantifies the partial ionization effect of electrons under extreme conditions, which is critical to model the transition region between condensed matter and WDM.

Development of a three-stage gas gun launcher for ultrahigh-pressure Hugoniot measurements.

A three-stage gas gun, composed of a two-stage gas gun and the add-on part, has been developed to launch high-Z (tantalum, for example) flyer plates up to 10 km/s for ultrahigh-pressure Hugoniot measurements. Great care has been taken to optimize the add-on part in which a specially designed graded density impactor is employed to quasi-isentropically accelerate the high-Z flyer plate for maximizing its impact velocity. The shock wave in the target generated by the flyer plate is characterized with the flatness of the shock-front better than 1 ns in the concerned area and the uncertainty of the shock-wave velocity less than 2%, thus satisfying the requirements for high-precision Hugoniot measurements. As a demonstration, we measured the ultrahigh-pressure Hugoniot equation of state of tantalum ranging from 0.45 TPa to 0.85 TPa with a symmetric impacting geometry in which the shock-wave velocity and the particle velocity are simultaneously determined. The results obtained are well consistent with data available in the literature, indicating the extended capability of the gas-gun launcher technique.

Pressure-induced structural phase transition and equation of state of LiTaO3.

Using in situ high-pressure x-ray diffraction and ab initio techniques, a high-pressure structure of LiTaO3 has been determined to be an orthorhombic phase with the space group Pnma. At ambient temperature, the transition pressure from the R3c phase (the ordinary phase at ambient pressure and temperature) to the Pnma phase is about 33.0 GPa and the phase transition is reversible. This phase transition can be reproduced qualitatively by ab initio calculations, but with a lower transition pressure of 19.9 GPa. The equation of state of LiTaO3 is also reported.

Development of a simultaneous Hugoniot and temperature measurement for preheated-metal shock experiments: melting temperatures of Ta at pressures of 100 GPa.

Equations of state of metals are important issues in earth science and planetary science. A major limitation of them is the lack of experimental data for determining pressure-volume and temperature of shocked metal simultaneously. By measuring them in a single experiment, a major source of systematic error is eliminated in determining from which shock pressure release pressure originates. Hence, a non-contact fast optical method was developed and demonstrated to simultaneously measure a Hugoniot pressure-volume (P(H)-V(H)) point and interfacial temperature T(R) on the release of Hugoniot pressure (P(R)) for preheated metals up to 1000 K. Experimental details in our investigation are (i) a Ni-Cr resistance coil field placed around the metal specimen to generate a controllable and stable heating source, (ii) a fiber-optic probe with an optical lens coupling system and optical pyrometer with ns time resolution to carry out non-contact fast optical measurements for determining P(H)-V(H) and T(R). The shock response of preheated tantalum (Ta) at 773 K was investigated in our work. Measured data for shock velocity versus particle velocity at an initial state of room temperature was in agreement with previous shock compression results, while the measured shock data between 248 and 307 GPa initially heated to 773 K were below the Hugoniot evaluation from its off-Hugoniot states. Obtained interfacial temperatures on release of Hugoniot pressures (100-170 GPa) were in agreement with shock-melting points at initial ambient condition and ab initio calculations of melting curve. It indicates a good consistency for shock melting data of Ta at different initial temperatures. Our combined diagnostics for Hugoniot and temperature provides an important approach for studying EOS and the temperature effect of shocked metals. In particular, our measured melting temperatures of Ta address the current controversy about the difference by more than a factor of 2 between the melting temperatures measured under shock and those measured in a laser-heated diamond anvil cell at ∼100 GPa.