Search

The reactivity between NaH and H2 has been investigated through a series of high-temperature experiments up to pressures of 78 GPa in diamond anvil cells combined with first principles calculations. Powder X-ray diffraction measurements show that heating NaH in an excess of H2 to temperatures around 2000 K above 27 GPa yields sodium trihydride (NaH3), which adopts an orthorhombic structure (space group Cmcm). Raman spectroscopy measurements indicate that NaH3 hosts quasi-molecular hydrogen (H2-) within a NaH lattice, with the H2- stretching mode downshifted compared to pure H2 (Av 120 cm-1 at 50 GPa). NaH3 is stable under room temperature compression to at least 78 GPa, and exhibits remarkable P-T stability, decomposing at pressures below 18 GPa. Contrary to previous experimental and theoretical studies, heating NaH (or NaH3) in excess H2 between 27 and 75 GPa does not promote further hydrogenation to form sodium polyhydrides other than NaH3. [ABSTRACT FROM AUTHOR]

Based on the density functional theory, a theoretical determination method is applied to investigate structural and electronic properties of anthracene up to 27 GPa. The lattice parametersa, b, c, decrease by 1.53(−18.2%), 0.76(−12.8%), and 0.93(−8.4%), respectively, while the monoclinic angle β increased by 4.65° in this pressure region. At the highest pressure of 27 GPa the unit cell volume is decreased by 58.9%. These findings are shown to be in agreement with experimental results and hint towards the evolution of intermolecular interaction with pressure. The calculated electronic band splitting and the band gap reduce smoothly to some extent with the pressure increasing. Moreover, a pressure-induced decrease of the band gap is observed. [ABSTRACT FROM PUBLISHER]

The crystal structures of actinium dihydride (AcH 2 ) in a wide pressure range of 0–200 GPa have been investigated by using an unbiased structure searching method coupling with first-principles density functional calculations. A series of pressure-induced phase transitions are predicted for AcH 2 , as Fm -3 m → P 4 2 / mmc → Imma → P 6/ mmm , and the calculated pressures of the phase transitions are 12, 27, and 68 GPa. Under pressure, the coordination number of H atom increases and the nature of chemical bonds changes. The electronic band structures show that all of the competitive phases are metallic. The pressure-induced charge transfer drives the phase transition. Phonon calculations show the competitive phases are thermodynamically stable within their favored pressure range. Our results elucidate the phase transition mechanisms of AcH 2 under pressure, and have major implications for the high-pressure behaviors of actinide and rare-earth metal dihydrides. [ABSTRACT FROM AUTHOR]

Highlights: [•] High-pressure behavior of α-UH3 has been studied using first-principles calculations. [•] The α-UH3 is stable up to 20GPa from elastic constants. [•] Under pressure, the high-frequency vibration modes shift upward. [•] The entropy and heat capacity increase with pressure increasing. [Copyright &y& Elsevier]