TeO2 glass has been studied by Raman spectroscopy up to the record pressure of 70 GPa. The boson peak frequency ωb exhibits a decrease of the ∂ωb/∂P slope at 5–6 GPa and saturates above 30 GPa with a practically constant value up to 70 GPa. Experiment and theory indicate that pressures up to 20 GPa induce the transformation of single Te–O–Te bridges to double Te–O2–Te bridges, leading to a more compact structure, while Raman activity developing at higher pressures around 580 cm–1 signals the increase of Te coordination from 4- to 6-fold. Natural bond orbital analysis shows that double Te–O2–Te bridges favor the s → d transition and promote the increase of Te coordination through d2sp3 hybridization. This transition leads to the formation of TeO6 octahedra, in strict difference with crystalline TeO2 at the same pressure range, and to the development of a 3D network that freezes the medium range order.
A. G. Papadopoulos, N. S. Tagiara, E. Stavrou*, F. Li, G. Yang, and E. I. Kamitsos*: “Raman study of pure TeO2 glass under pressure: interplay between glass network transformations and electronic transitions”, J. Phys. Chem. Lett. 14, 387 (2023)
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
E. Stavrou, A. A. Maryewski, S. S. Lobanov, A. R. Oganov, Z. Konôpková, V. B. Prakapenka, and A. F. Goncharov: "Ethane and methane at high pressures: Structure and stability" J. Chem. Phys. 155, 184503 (2021)
Equation of State for Natural Almandine, Spessartine, Pyrope Garnet: Implications for Quartz-In-Garnet Elastic Geobarometry
The equation of state (EoS) of a natural 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.
S. R. Mulligan, E. Stavrou, S. Chariton, O. Tschauner, A. Salamat, M.L. Wells, A. G. Smith, T. D. Hoisch, V. Prakapenka: "Equation of State for Natural Almandine, Spessartine, Pyrope Garnet: Implications for Quartz-In-Garnet Elastic Geobarometry", Minerals 11., 458 2021
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.
R. Leversee, K. Rode, E. Greenberg, V. B Prakapenka, J. Smith, M. Kunz, C. J. Pickard and E. Stavrou*: “High pressure chemical reactivity and structural study of the Na-P and Li-P systems”, J. Mater. Chem. A 8, 21797 (2020)
High-pressure structural study of α-Mn: Experiments and calculations
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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