Carbon dioxide is a prototypical molecular system, with strong covalent bonds within the O=C=O molecules and rather weak quadrupolar interactions between them. At high pressures and temperatures, CO 2 transforms to a series of solid polymorphs with differing intermolecular interactions, chemical bonding and crystal structures (Fig. 1).Phase V, in particular, consists of a network of corner sharing CO 4 tetrahedra, structurally similar to SiO 2 tridymite. Thus, CO 2 -V is a fundamentally new material that exhibits extremely low compressibility 5 and strong optically nonlinearity 3 . The large disparity in chemical bonding between the extended network and molecular CO 2 results in a broad metastability domain for phase V, to room temperature and almost to ambient pressure. In this letter, we report the discovery of fully extended stishovite-like CO 2 phase VI, formed at pressures below 100GPa ( Fig. 1). Based on a large number of resistive-and laser-heating experiments using membrane-diamond anvil cells (mDACs), we propose the relationship between the molecular and extended phases in the pressure-temperature domain shown in Fig. 1.CO 2 -VI is obtained by isothermal compression of phase II to pressures above 50GPa at temperatures 530-650 K. As shown in Fig. 2, the most notable Raman feature of CO 2 -VI is the emergence of a strong band around 1010cm -1 at 50GPa. This mode frequency is substantially higher than that of four-fold coordinated carbon in CO 2 -V (~800cm -1 ) 3 , indicating it likely originates from six-fold coordinated carbons in octahedral sites -similar to the A 1g mode of stishovite. 16 Following this assignment, we further associate the peaks at 300cm -1 to B 1g , 680cm -1 for E g , and a weak, but measurable band at 1100cm -1 to B 2g , thus accounting for all four Raman-active modes reported in stishovite. 16 Importantly, the frequencies of these modes scale very well to those observed in SiO 2 polymorphs (see Table I). In addition, in Figure 3 we compare the observed Raman spectra of CO 2 -VI with those of other Group IV dioxides in rutile structures. We find that the frequencies of all four Raman-active modes scale linearly with the reduced mass, strongly supporting the present assignment of phase VI as stishovite-like. The data for SiO 2 , GeO 2 , and SnO 2 are from previous studies.17 Figure 4 summarizes the pressure dependence of the Raman modes of the new material. In addition to the four modes assigned to the stishovite-like structure, we observe a number of broad Raman features in CO 2 -VI, centered at ~2000, 950, and 700cm -1 at 65GPa, which we assign to disorder in the stishovite structure. In fact, the 950 The II-to-VI transition is strongly affected by kinetics, requiring slow compression over several hours in a wide pressure range. While the conversion to CO 2 -VI initiates at ~50GPa (530K), residual CO 2 -II is observable to 60-65GPa. In this pressure range, the A 1g mode of phase VI gradually increases while the E g mode of phase II gradually decreases in intensity and eventually dis...

Vanadium has been reported to undergo phase transition upon compression from bcc to rhombohedral structure around 62 GPa. In this paper we confirm the bcc to rhombohedral phase transition at 61.5 GPa under quasi hydrostatic compression in Ne pressure medium. Under nonhydrostatic condition we find the phase transition occurring at 30 GPa at ambient temperature, and 37 GPa at 425 K. We find the transition under hydrostatic condition is hindered and it can occur at much lower pressure under non-hydrostatic condition.

Static compression experiments over 4 Mbar are rare, yet critical for developing accurate fundamental physics and chemistry models, relevant to a range of topics including modeling planetary interiors. Here we show that focused ion beam crafted toroidal single-crystal diamond anvils with ~9.0 μm culets are capable of producing pressures over 5 Mbar. The toroidal surface prevents gasket outflow and provides a means to stabilize the central culet. We have reached a maximum pressure of ~6.15 Mbar using Re as in situ pressure marker, a pressure regime typically accessed only by double-stage diamond anvils and dynamic compression platforms. Optimizing single-crystal diamond anvil design is key for extending the pressure range over which studies can be performed in the diamond anvil cell.

A modular, multi-diagnostic, automated shock tube for gas-phase chemistry Review of Scientific Instruments 90, 064104 (2019);