Phase Transition Lowering in Dynamically Compressed Silicon

Silicon, being one of the most abundant elements in nature, attracts wide-ranging scientific and technological interest. Specifically, in its elemental form, crystals of remarkable purity can be produced. One may assume that this would lead to silicon being well understood, and indeed, this is the case for many ambient properties, as well as for higher-pressure behaviour under quasi-static loading. However, despite many decades of study, a detailed understanding of the response of silicon to rapid compression—such as that experienced under shock impact—remains elusive. Here, we combine a novel free-electron laser-based X-ray diffraction geometry with laser-driven compression to elucidate the importance of shear generated during shock compression on the occurrence of phase transitions. We observe lowering of the hydrostatic phase boundary in elemental silicon, an ideal model system for investigating high-strength materials, analogous to planetary constituents. Moreover, we unambiguously determine the onset of melting above 14 GPa, previously ascribed to a solid–solid phase transition, undetectable in the now conventional shocked diffraction geometry; transitions to the liquid state are expected to be ubiquitous in all systems at sufficiently high pressures and temperatures

Domain structure of 001-Poled PZN-4.5%PT using neutron diffraction.

The domain structure of PZN-4.5%PT was studied as a function of temperature in an electric field of 500 V/cm applied along 001. The measurements provides direct information on the orientational relationships between the cubic, tetragonal and rhombohedral domains during the field cooling process, and on the volume fraction of domains in each set of orientations. The domains in the tetragonal phase are all related by 90° twins mainly on the (101) and (011) twin planes. The domain structure of the rhombohedral phase agrees with an earlier room temperature and zero field study

Nitrile groups as hydrogen-bond acceptors in a donor-rich hydrogen-bonding network

A complex hydrogen-bonding network involving the guanidinium cation and the dinitrile anion C(CN)(2)(CONH2)(-) has been studied using Laue neutron diffraction. The role of nitrile groups as hydrogen bond acceptors is highlighted in this donor-rich system combined with a survey of the CSD exploring R-2(1)(6) synthons containing nitrile groups. © 2012, Royal Society of Chemistr

Crystal structures of fluorinated aryl biscarbonates and a biscarbamate: a counterpoise between weak intermolecular interactions and molecular symmetry.

Conformational features and supramolecular structural organization in three aryl biscarbonates and an aryl biscarbamate with rigid acetylenic unit providing variable spacer lengths have been probed to gain insights into the packing features associated with molecular symmetry and the intermolecular interactions involving ‘organic’ fluorine. Four structures but-2-yne-1,4-diyl bis(2,3,4,5,6-pentafluorophenylcarbonate), 1; but-2-yne-1,4-diyl bis(4-fluorophenylcarbonate), 2; but-2-yne-1,4-diyl bis(2,3,4,5,6-pentafluorophenylcarbamate), 3 and hexa-2,4-diyne-1,6-diyl bis(2,3,4,5,6-pentafluorophenylcarbonate), 4 have been analyzed in this context. Compound 1 adopts a non-centrosymmetric “twisted” (syn) conformation, whereas 2, 3 and 4 acquire a centrosymmetric “extended” (anti) conformation. Weak intermolecular interactions and in particular those involving fluorine are found to dictate this conformational variation in the crystal structure of 1. A single-crystal neutron diffraction study at 90 K was performed on 1 to obtain further insights into these interactions involving ‘organic’ fluorine. © 2011, Royal Society of Chemistr

Neutron diffuse scattering in deuterated para-terphenyl, C18D14

Neutron diffuse scattering is used to explore the short-range order (SRO) in deuterated para-terphenyl, C18D14. The crystal shows SRO because the central of the three phenyl groups of each molecule can twist positively or negatively and these twists are correlated over the local scale. The presence of incipient Bragg peaks at (1/2 1/2 0) at 200 K shows that these flips are negatively correlated along the a direction (nearest neighbour correlation coefficient of similar to-0.3) and b direction (nearest neighbour correlation coefficient of similar to-0.87) and appear essentially uncorrelated along c. Diffuse peak anisotropy indicates that the range of the correlations along b is found to be similar to 3 times that along a. These correlations persist, although weaker, at room temperature. A Monte Carlo simulation was used to impose a correlation structure on the population of central ring twists that was deduced from Bragg scattering. By then allowing displacive relaxation of the structure, the observed diffuse scattering was well reproduced. Modelling the displacive motions of molecules showed that the positions of nearest ab-plane neighbour molecules are strongly positively correlated, particularly for motions approximately parallel to a, while the displacive correlations are weaker between molecules stacked along c. The apparent contradiction that the displacements are most strongly correlated along a while the occupancies are most strongly correlated along b is explained in terms of the connectivity of molecular interactions. © 2009, Institute of Physic

Structure of BiRe2O6 re-investigated using single-crystal neutron Laue diffraction

Single crystals of BiRe 2 O 6 of typical volume 0.03 mm 3 were grown by chemical vapor transport and characterized using room-temperature single-crystal neutron diffraction in monoclinic C 2/ c symmetry with cell parameters a = 16.1178(11), b = 4.9235(3), c = 5.5278(3) Ã… and β = 92.475(5) ° ( R all = 11.39 wR all = 7.97). The structure contains ordered layers of corner sharing units of Re 2 O 10 with Re-Re distances of 2.519(1) Ã…. The unit cell is doubled along c in comparison to previous studies based on X-ray diffraction data where the layer stacking was described as disorderd. © 2010, Insitute of Physics

Reassessment of Large Dipole Moment Enhancements in Crystals: A Detailed Experimental and Theoretical Charge Density Analysis of 2-Methyl-4-nitroaniline

The molecular dipole moment of MNA in the crystal has been critically reexamined, to test the conclusion from an earlier experimental charge density analysis that it was substantially enhanced due to a combination of strong intermolecular interactions and crystal field effects. X-ray and neutron diffraction data have been carefully measured at 100 K and supplemented with 'ab initio' crystal Hartree−Fock calculations. Considerable care taken in the measurement and reduction of the experimental data excluded most systematic errors, and sources of error and their effects on the experimental electron density have been carefully investigated. The electron density derived from a fit to theoretical structure factors assisted in the determination of the scale and thermal motion model. The dipole moment enhancement for MNA in the crystal is much less than that reported previously and only on the order of 30−40% (~2.5 D). In addition to the dipole moment, experimental deformation electron density maps, bond critical point data, electric field gradients at hydrogen nuclei, and atomic and group charges all agree well with theoretical results and trends. Anisotropic modeling of the motion of hydrogen atoms, integral use of periodic 'ab initio' calculations, and improved data quality are all aspects of this study that represent a considerable advance over previous work