Publications

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The high-pressure dynamic response of titanium dioxide (TiO 2) is not only of interest because of its numerous industrial applications but also because of its structural similarities to silica (SiO 2). We performed plate impact experiments in a two-stage light gas gun, at peak stresses from 64 to 221 GPa to determine the TiO 2 response along the Hugoniot. The lower stress experiment at 64 GPa shows an elastic behavior followed by an elastic-plastic transition, whereas the high stress experiments above 64 GPa show a single wave structure. Previous shock studies have shown the presence of high-pressure phases (HPP) I (26 GPa) and HPP II (100 GPa); however, our data suggest that the HPP I phase is stable up to 150 GPa. Using a combination of data from our current study and our previous Z-data, we determine that TiO 2 likely melts on the Hugoniot at 157 GPa. Furthermore, our data confirm that TiO 2 is not highly incompressible as shown by a previous study.

Pulsed-power generators using the magnetic loading technique are able to produce well-controlled continuous ramp compression of condensed matter for high-pressure equation-of-state studies. X-ray diffraction (XRD) data from dynamically compressed samples provide direct measurements of the elastic compression of the crystal lattice, onset of plastic flow, strength-strain rate dependence, structural phase transitions, and density of crystal defects such as dislocations. Here, we present a cost effective, compact X-ray source for XRD measurements on pulsed-power-driven ramp-loaded samples. This combination of magnetically-driven ramp compression of materials with single, short-pulse XRD diagnostic will be a powerful capability for the dynamic materials community. The success in fielding this new XRD diagnostic dramatically improves our predictive capability and understanding of rate-dependent behavior at or near phase transition. As Sandia plans the next-generation pulse-power driver platform, a key element needed to deliver new state-of-the-art experiments will be having the necessary diagnostic tools to probe new regimes and phenomena. These diagnostics need to be as versatile, compact, and portable as they are powerful. The development of a platform-independent XRD diagnostic gives Sandia researchers a new window to study the microstructure and phase dynamics of materials under load. This project has paved the way for phase transition research in a variety of materials with mission interest.

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In this work, we have studied the pressure-induced structural and electronic phase transitions in WO₃ to 60 GPa using micro-Raman spectroscopy, synchrotron X-ray diffraction, and electrical resistivity measurements. The results indicate that WO₃ undergoes a series of phase transitions with increasing pressure: triclinic WO₃-I initially transforms to monoclinic WO₃-II (P2₁/c) at 1 GPa, involving a tetrahedral distortion in a corner-shared octahedral framework, and then to a mixed corner and edge-shared seven-coordinated WO₃-III (P2₁/c) at 27 GPa with a large volume change of ~6% and further to WO₃-IV (Pc) above 37 GPa. These structural phase transitions also accompany a significant drop in resistivity from insulating WO₃-I to semiconducting WO₃-II, and poor metallic WO₃-III and IV, arising from the Jahn–Teller distortion in WO6 and the hybridization between O 2p and W 5d orbitals in WO7, respectively. Unlike its molecular analogue of MoO₃, the transitions in WO₃ show little effect on the use of different pressure transmitting media.

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The high-pressure response of titanium dioxide (TiO2) is of interest because of its numerous industrial applications and its structural similarities to silica (SiO2). We used three platforms - Sandia's Z machine, Omega Laser Facility, and density-functional theory-based quantum molecular dynamics (QMD) simulations - to study the equation of state (EOS) of TiO2 at extreme conditions. We used magnetically accelerated flyer plates at Sandia to measure Hugoniot of TiO2 up to pressures of 855 GPa. We used a laser-driven shock wave at Omega to measure the shock temperature in TiO2. Our Z data show that rutile TiO2 reaches 2.2-fold compression at a pressure of 855 GPa and Omega data show that TiO2 is a reflecting liquid above 230 GPa. The QMD simulations are in excellent agreement with the experimental Hugoniot in both pressure and temperature. A melt curve for TiO2 is also proposed based on the QMD simulations. The combined experimental results show TiO2 is in a liquid at these explored pressure ranges and is not highly incompressible as suggested by a previous study.

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We have studied the compression behavior of H₂-He mixtures in comparison with pure H₂ and He using powder synchrotron x-ray diffraction and present the pressure-volume (PV) compression data of H₂-He mixtures to 160 GPa. The results indicate that both H₂ and He in H₂-He mixtures remain in hcp to the maximum pressure studied, yet develop a substantial level of lattice distortion in the (100) plane, most profound in He-rich solids and below 66 GPa. The measured PV data also indicate softening of He (or H₂)-rich lattice upon increasing the level of the guest H₂ (or He) concentration. We suggest that the observed softening and lattice distortion are due to a substitutional incorporation of H2 (guest) molecules into the basal plane of hcp-He (host) lattice and, thereby, reflect the miscibility between H₂ and He in H₂-He mixtures. Interestingly, solid He exhibits a lesser degree of preferred orientation in H2-He mixtures than in pure He, likely due to the presence of solid H₂ disturbing the crystalline ordering of He-rich solids. Finally, the present PV compression data of H₂-rich and He-rich solids to 160 GPa deviate from those of pure H₂ and pure He above ~70 and 45 GPa respectively, providing new constraints for development of the EOS for H₂-He mixtures for planetary models.

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