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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]

Both the vibrational and structural properties of coronene have been investigated upon compression up to 30.5 GPa at room temperature by a combination of Raman scattering and synchrotron x-ray diffraction measurements. The spectroscopic and crystallographic results demonstrate that two pressure-induced structural phase transitions take place at 1.5 GPa and 12.2 GPa where the high-pressure phases are identified as monoclinic and orthorhombic crystal structures with space groups of P2/m and Pmmm, respectively. A kink in the slope of the cell parameters as a function of pressure is associated with the disappearance of several internal Raman modes, which suggests the existence of structural distortions or reorganizations at approximately 6.0 GPa. Above 17.1 GPa, almost no evidence of crystallinity can be observed, indicating a possible transformation of coronene into an amorphous phase. [ABSTRACT FROM AUTHOR]

The structural and vibrational properties of phenanthrene are measured at high pressures up to 30.2 GPa by Raman spectroscopy and synchrotron X-ray diffraction techniques. Two phase transitions are observed in the Raman spectra at pressures of 2.3 GPa and 5.4 GPa which correspond to significant changes of intermolecular and intramolecular vibrational modes. Above 10.2 GPa, all the Raman peaks are lost within the fluorescence background; however, upon further compression above 20.0 GPa, three broad peaks are observed at 1600, 2993, and 3181 cm-1, indicating that phenanthrene has transformed into amorphous phase. Using X-ray diffraction, the structures of corresponding phases observed from Raman spectra are indexed with space groups of P21 for phase I (0-2.2 GPa), P2/m for phase II (2.2-5.6 GPa), P2/m+Pmmm for phase III (5.6-11.4 GPa) which has a coexistence of structures, and above 11.4 GPa the structure is indexed with space group of Pmmm. Although phenanthrene has transformed to a hydrogenated amorphous carbon structure above 20.0 GPa, these amorphous clusters still show characteristic crystalline behavior based on our X-ray diffraction patterns. Our results suggest that the long-range periodicity and the local disorder state coexist in phenanthrene at high pressures. [ABSTRACT FROM AUTHOR]

Abstract: Single crystals of Y7Co6Sn23 and RE5Co6Sn18 (RE=Sc, Ho) have been obtained by solid-state reactions of the corresponding pure elements in welded tantalum tubes at high temperature. Their crystal structures have been established by single-crystal X-ray diffraction studies. Y7Co6Sn23 belongs to the Ho7Co6Sn23 type and crystallizes in the trigonal space group P-3m1 (No. 164) whereas RE5Co6Sn18 (RE=Sc, Ho) adopt the Tb5Rh6Sn18 type structure and crystallizes in the tetragonal space group I41/acd (No. 142). Their structures all feature a three-dimensional (3D) anionic framework based on [CoSn6] trigonal prisms. The structure of Y7Co6Sn23 is made from the ordinal stacking one type of 2D slab composed of the same [CoSn6] trigonal prisms via corner-sharing, forming two types of narrow tunnels along the c-axis, which are occupied by Y or residual Sn atoms. The 3D structure of RE5Co6Sn18 is built up from alternately stacking two types of 2D slabs, which are composed of different [CoSn6] trigonal prisms via corner-sharing and Sn–Sn bonds, respectively. The RE and residual Sn atoms are located in the spacers of the 3D framework. Band structure calculations indicate that Y7Co6Sn23 and Sc5Co6Sn18 are metallic. [Copyright &y& Elsevier]

The vibrational and structural properties of a hydrogen-rich group IVa hydride, Sn(CH3)4, have been investigated by combining Raman spectroscopy and synchrotron x-ray diffraction measurements at room temperature and at pressures up to 49.9 GPa. Both techniques allow the obtaining of complementary information on the high-pressure behaviors and yield consistent phase transitions at 0.9 GPa for the liquid to solid and 2.8, 10.4, 20.4, and 32.6 GPa for the solid to solid. The foregoing solid phases are identified to have the orthorhombic, tetragonal, monoclinic crystal structures with space groups of Pmmm for phase I, P4/mmm for phase II, P2/m for phase III, respectively. The phases IV and V coexist with phase III, resulting in complex analysis on the possible structures. These transitions suggest the variation in the inter- and intra-molecular bonding of this compound. [ABSTRACT FROM AUTHOR]

Abstract: The title compounds have been obtained by solid state reactions of the corresponding pure elements at high temperature, and structurally characterized by single-crystal X-ray diffraction studies. Yb5Ni4Sn10 adopts the Sc5Co4Si10 structure type and crystallizes in the tetragonal space group P4/mbm (No. 127) with cell parameters of a=13.785(4)Å, c=4.492 (2)Å, V=853.7(5)Å3, and Z=2. Yb7Ni4Sn13 is isostructural with Yb7Co4InGe12 and crystallizes in the tetragonal space group P4/m (No. 83) with cell parameters of a=11.1429(6)Å, c=4.5318(4)Å, V=562.69(7)Å3, and Z=1. Both structures feature three-dimensional (3D) frameworks based on three different types of one-dimensional (1D) channels, which are occupied by the Yb atoms. Electronic structure calculations based on density functional theory (DFT) indicate that both compounds are metallic. These results are in agreement with those from temperature-dependent resistivity and magnetic susceptibility measurements. [Copyright &y& Elsevier]