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Ethane and methane at high pressures: Structure and stability


 We have performed a combined experimental and theoretical study of ethane and methane at high pressures of up to 120 GPa at 300 K using x-ray diffraction and Raman spectroscopies and the USPEX ab initio evolutionary structural search algorithm, respectively. For ethane, we have determined the crystallization point, for room temperature, at 2.7 GPa and also the low pressure crystal structure (phase A). This crystal structure is orientationally disordered (plastic phase) and deviates from the known crystal structures for ethane at low temperatures. Moreover, a pressure induced phase transition has been identified, for the first time, at 13.6 GPa to a monoclinic phase B, the structure of which is solved based on good agreement with the experimental results and theoretical predictions. For methane, our x-ray diffraction measurements are in agreement with the previously reported high-pressure structures and equation of state (EOS). We have determined the EOSs of ethane and methane, which provides a solid basis for the discussion of their relative stability at high pressures

Structural study of TATB under detonation-induced shock conditions


We explore the response of the insensitive high explosive (IHE) 1,3,5-Triamino-2,4,6-trinitrobenzene (TATB) under detonation-induced shock conditions using in−situ synchrotron X-ray diffraction in the100 ns time-scales, using either a conventional or a colliding detonation drive. In all of the detonation experiments on various sizes and morphologies of TATB, we observe an extended stability of the TATB crystal structure. As the detonation front passes through TATB, X-ray diffraction indicates a portion of the TATB exhibits a compression up to 30+ GPa, followed subsequently by a pressure release and continued decomposition over a few hundred nanoseconds. Likewise, for colliding detonation-driven shock compression of single crystals of TATB, a significant portion of crystalline TATB appears to be stable up to 60+ GPa. Conversely, in similar detonations of an LLM-105 polymer-bonded explosive, X-ray diffraction is simply indicative of fast decomposition without the apparent compression and slow decomposition seen in TATB. The results indicate the surprising resilience of TATB under these high-pressure, temperature and shock conditions, providing a baseline for understanding the insensitivity of TATB. The results also provide intriguing information for the extended reaction zone in TATB, and the hot-spot mechanisms for initiating and propagating detonation in this uniquely insensitive explosive.

Equation of State for Natural Almandine, Spessartine, Pyrope Garnet: Implications for Quartz-In-Garnet Elastic Geobarometry


The equation of state (EoS) of a natural almandine74spessartine13pyrope10grossular3 garnet of a typical composition found in metamorphic rocks in Earth’s crust was obtained using single crystal synchrotron X-ray diffraction under isothermal room temperature compression. A third-order Birch-Murnaghan EoS was fitted to P-V data and the results are compared with published EoS for iron, manganese, magnesium, and calcium garnet compositional end-members. This comparison reveals that ideal solid solution mixing can reproduce the EoS for this intermediate composition of garnet. Additionally, this new EoS was used to calculate geobarometry on a garnet sample from the same rock, which was collected from the Albion Mountains of southern Idaho. Quartz-in-garnet elastic geobarometry was used to calculate pressures of quartz inclusion entrapment using alternative methods of garnet mixing and both the hydrostatic and Grüneisen tensor approaches. QuiG barometry pressures overlap within uncertainty when calculated using EoS for pure end-member almandine, the weighted averages of end-member EoS, and the EoS presented in this study. Grüneisen tensors produce apparent higher pressures relative to the hydrostatic method, but with large uncertainties.

High pressure chemical reactivity and structural study of the Na–P and Li–P systems


The Na–P and Li–P chemical systems were studied under pressure using synchrotron X-ray diffraction in a diamond anvil cell up to 20 GPa, combined with the AIRSS ab initio random structure searching technique. The results reveal an enhanced reactivity of both alkali metals with phosphorous at slightly elevated pressures. This enables the synthesis of Li3P and Na3P at room temperature (RT) starting from element precursors, bypassing the established chemical synthesis methods. Both compounds undergo a pressure-induced phase transition from the hexagonal Na3As-type structure (stable at ambient conditions) towards a Fm3m (FCC) structure that remains stable up to 20 GPa. Attempts to synthesize compounds with higher alkali metal content (such as Li5P) using high-temperature and -pressure conditions (up to 2000+ K and 30 GPa), inspired by recent theoretical predictions, were not successful.

High-pressure structural study of α-Mn: Experiments and calculations

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Manganese, in the α-Mn structure, has been studied using synchrotron powder x-ray diffraction in a diamond anvil cell up to 220 GPa at room temperature combined with density functional calculations (DFT). The experiment reveals an extended pressure stability of the α-Mn phase up to the highest pressure of this study, in contrast with previous experimental and theoretical studies. On the other hand, calculations reveal that the previously predicted hcp-Mn phase becomes lower in enthalpy than the α-Mn phase above 160 GPa. The apparent discrepancy is explained due to a substantial electron transfer between Mn ions, which stabilizes the α-Mn phase through the formation of ionic bonding between monatomic ions under pressure.

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