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The lanthanide metal Pr has been studied by x-ray powder diffraction from 5 to 23 GPa with the aim of resolving the long-standing disagreements about the identity of the high-pressure phases, transitions, and structures of this element. We find that the distorted-fcc (d-fcc) phase observed above 7.4 GPa has the rhombohedral hR24 structure reported previously by Hamaya et al. and can definitely rule out the monoclinic structure proposed by other authors. At 13.7 GPa, the d-fcc phase starts to transform to another phase, Pr-VII, the structure of which is body-centered orthorhombic or a very slight distortion thereof. Pr-VII transforms to the alpha-uranium phase at 20.5(5) GPa. In addition to identifying the phase transitions and structures, we have also resolved many of the disagreements between previous diffraction studies and show that almost all previous work is in agreement with the present study. The likely existence of the Pr-VII phase in a second lanthanide, Nd, suggests that it might be an additional member of the sequence of phases observed across the lanthanide elements.

The crystal structures of the high-pressure delta and delta* phases of nitrogen have been investigated using single-crystal x-ray diffraction. The structure of the delta phase is very similar to isostructural gamma-O-2 and comprises spherically disordered molecules, with a preference for avoiding pointing along the cubic < 100 > directions, and disklike molecules with a uniform distribution of orientations. The structure of the delta* phase is tetragonal and the space group is identified unambiguously as P4(2)/ncm with unit cell parameters of a=8.603(5) angstrom and c=5.685(5) angstrom at 14.5 GPa. The orientations of the partially disordered molecules have been experimentally determined for the first time and are similar to those predicted on the basis of molecular dynamics simulations. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3204074]

Synchrotron single-crystal diffraction data show that sodium transforms from the oP8 structure to an incommensurate host-guest composite structure above 125 GPa in which the guest component is nearly ``melted.'' At 147 GPa, the correlation length between the guest chains is only $28(3)\phantom{\rule{0.3em}{0ex}}\text{\AA{}}$, or approximately six times the chain spacing. This configuration shows a wide range of stability from above 125 GPa to at least 155 GPa and 550 K. The transition to this phase is accompanied by a marked decrease in optical reflectivity. Electronic band-structure calculations are used to interpret this change.

Longitudinal-acoustic (LA) phonons have been studied by inelastic x-ray scattering in the high-pressure incommensurate host-guest system Rb-IV in the pressure range of 16.3 to 18.4 GPa. Two LA-like phonon branches are observed along the direction of the incommensurate wave vector, which are attributed to separate lattice vibrations in the host and guest subsystems. The derived sound velocities for the host and the guest, nu(h) and nu(g), respectively, are similar in magnitude [nu(h)=nu(g)=3840(100) m/s at 18 GPa], but our results indicate rather different pressure dependences of dv(h)/dP=140(60) m/s GPa(-1) and d nu(g)/dP=280(80) m/s GPa(-1). The observations for the one-dimensional Rb guest chains are reproduced quantitatively on the basis of the monatomic linear chain model and the measured compressibility of the chains.

The equation of state for γ-Fe 2 SiO 4 was determined by single-crystal X-ray diffraction up to 10.2 GPa at room temperature. The pressure–volume data, measured at 10 different pressures, do not show any evidence of a phase transformation and were fit by a third-order Birch–Murnaghan equation of state. The following coefficients were refined simultaneously: V 0 = 559.44(6) A 3 , K T0 = 187.3(1.7) GPa, and K ′ = 5.5(4). This result implies that the Mg/Fe substitution along the γ-Fo–Fa join does not significantly affect the bulk modulus. The sample shows higher K ′ than other compositions along the Fo–Fa join, which generally have K ′ ∼ 4. Based on our results we can calculate the sound bulk velocity of γ-Fe 2 SiO 4 up to the pressures of the Transition Zone. Due to the larger K ′ of γ-Fe 2 SiO 4 , the difference in bulk sound velocity between the two end-members decreases with increasing pressure.