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Covalently bonded extended phases of molecular solids made of first- and second-row elements at high pressures are a new class of materials with advanced optical, mechanical and energetic properties. The existence of such extended solids has recently been demonstrated using diamond anvil cells in several systems, including nitrogen, carbon dioxide and carbon monoxide. However, the microscopic quantities produced at the formidable high-pressure/temperature conditions have limited the characterization of their predicted novel properties, including high-energy content. In this paper, we present experimental evidence that these extended low-Z solids are indeed high-energy-density materials, by milligram-scale high-pressure synthesis, recovery and characterization of polymeric CO (p-CO). Our spectroscopic data reveal that p-CO is a random polymer made of lactonic entities and conjugated C=C with an energy content rivalling or exceeding that of HMX (cyclo-tetramethylene tetranitramine, a commonly used conventional high explosive). Solid p-CO explosively decomposes to CO(2) and glassy carbon, and thus might be used as an advanced energetic material.

X-ray diffraction measurements of cerium in the vicinity of the isostructural gamma-alpha transition have been performed with high precision and accuracy from room temperature to almost 800 K. The disputed location of the critical point has been found to occur at 1.5+/-0.1 GPa and 480+/-10 K. The data are well fit by the Kondo volume collapse model plus a quasiharmonic representation of the phonons. The resultant free energy is validated against data for the thermodynamic Grüneisen parameter and, beyond the dominant spin-fluctuation contribution, indicates a dramatic change in the lattice Grüneisen parameter across the transition.

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We have studied pressure-induced chemical reactions in carbon monoxide using both a diamond-anvil cell and a modified large volume press. Our spectroscopic data reveal that carbon monoxide disproportionates into molecular CO 2 and a solid lactone-type polymer; photochemically above 3.2 GPa, thermochemically above 5 GPa at 300K, or at 3 GPa and ~2000K as achieved by laser heating. The solid product can be recovered at ambient conditions with a high degree of conversion, measured to be up to 95% of the original CO. Its fundamental chemical structure includes β-lactone and conjugated C=C, which can be considered a severely modified polymeric carbon suboxide with open ladders and smaller five-membered rings. The polymer is metastable at ambient conditions, spontaneously liberating CO 2 gases exothermically. We find that the recovered polymer has a high energy density, 1-8 KJ/g, and is very combustible. We estimate the density of recovered CO polymer to be at least 1.65 g/cm 3 .2

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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...

We present evidence for an isostructural, first-order Mott transition in MnO at 105+/-5 GPa, based on high-resolution x-ray emission spectroscopy and angle-resolved x-ray diffraction data. The pressure-induced structural and spectral changes provide a coherent picture of MnO phase transitions from paramagnetic B1 to antiferromagnetic distorted B1 at 30 GPa, to paramagnetic B8 at 90 GPa, and to diamagnetic B8 at 105+/-5 GPa. The last is the Mott transition, accompanied by a significant loss of magnetic moment, an approximately 6.6% volume collapse and the insulator-metal transition as demonstrated by recent resistance measurements.