High-resolution Synchrotron X-ray Diffraction Research Articles

X-ray diffraction from a coherently illuminated Si(001) grating surface.

High-resolution synchrotron x-ray diffraction from a Si(001) grating surface reveals resolution-limited grating interference peaks around each Bragg reflection. The peaks can be explained by kinematic scattering theory using the concept of a grating form factor. The positions and the intensities of the satellite peaks yield structural information such as the period, the width, and the height of the gratings, as well as its shape and its orientation and registry with respect to substrate lattice, and possible crystal strains.

A synchrotron X-ray diffraction study of the structural phase behaviour of multilayer xenon on single-crystal graphite

We present a high-resolution synchrotron X-ray diffraction study of xenon adsorbed on single-crystal graphite for coverages of two, three and six monolayers. We observe the evolution of the lattice mismatch between the xenon overlayer and a square root 3* square root 3R30 degrees structure commensurate with the underlying substrate. In the case of the bilayer we see a first-order incommensurate-commensurate (IC-C) phase transition at 62.0 K+or-1.2 K, which we demonstrate is consistent with the layer closest to the substrate having locked into the commensurate sites. The role of substrate quality is discussed and we conclude that previous studies on relatively poorer substrates may have favoured equilibrium structures strongly influenced by the surface imperfections. The risk of an insufficiently equilibrated adsorbate is discussed in the context of the very slow kinetics we observe for multilayer xenon on high-quality substrates. For the three- and six-layer adsorbates we observe a systematic increase in lattice mismatch with film thickness and no indication of an incommensurate-commensurate transition. We conclude that the trilayer system consists of many distinct domains approximately 500 AA across, each of which has a random close packed structure.

Superconductivity and the low-temperature orthorhombic to tetragonal phase transition in La2-xBaxCuO4.

The relationship between the low temperature orthorhombic-tetragonal (OT) phase transition in ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ba}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ and the suppression of the superconducting state in samples with x=0.125 has been investigated. Using high resolution synchrotron x-ray diffraction we show that the phase composition of a superconducting x=0.15 sample varies with temperature identically to an x=0.125 sample. This shows that the tetragonal structure does support bulk superconductivity in ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ba}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ and the electronic transition which suppresses superconductivity at x=0.125 is distinct from the OT structural transition.

A conventional and high-resolution synchrotron X-ray diffraction study of phase separations in Pr 2NiO 4+δ

Several Pr 2NiO 4+δ samples with varying oxygen content have been examined by conventional and high-resolution synchrotron X-ray powder diffraction techniques. Three different phase separation regions have been identified with synchrotron diffraction, and the room temperature lattice parameters of each phase are reported, as well as conventional X-ray powder diffraction data on four single-phase regions of the phase diagram. The low temperature behavior of a sample with an average excess oxygen content of 0.020 was also studied, revealing phase coexistence and a tetragonal to orthorhombic phase transition on heating at 118 K. A semiquantitative oxygen concentration phase diagram is proposed on the basis of the amount of excess oxygen in the individual phases, as estimated from iodometric titrations and relative phase volumes. In addition, the overall stability field of Pr 2NiO 4+δ, showing the oxygen fugacity at the reduction and oxidation boundaries as a function of temperature, is reported.

Phase separation in Nd2-χCeχCuO4

Abstract A series of samples with bulk compositions Nd 2-χ Ce χ CuO 4 , 0≤χ≤0.25, have been examined by pulsed neutron and high-resolution synchrotron X-ray diffraction. The diffraction data show that single phase material is formed only for the undoped compound, Nd 2 CuO 4 , and (possibly) for the optimum superconducting composition, χ≈0.165. Other samples appear to be a mixture of this superconducting composition and non-superconducting material of other compositions. The phase separation cannot be explained in the context of an equilibrium phase diagram and, thus, must result from non-equilibrium behavior during synthesis.

Commensurate-incommensurate and rotational transitions of monolayer xenon on single-crystal graphite.

We have studied the structures and transitions of high-density xenon monolayers on single-crystal graphite using high-resolution synchrotron x-ray diffraction techniques. In contrast to the previous x-ray experiment, we find that at low temperatures, the xenon has a commensurate $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R30\ifmmode^\circ\else\textdegree\fi{}$ structure for a broad range of coverages. With decreasing temperature the novel reentrant sequence: aligned\ensuremath{\rightarrow}rotated\ensuremath{\rightarrow}aligned, occurs; this sequence is followed by a first-order incommensurate-commensurate transition.

Pressure-Driven Commensurate-Incommensurate Transition in Low-Temperature Submonolayer Krypton on Graphite

By using ${\mathrm{D}}_{2}$ gas as a source of two-dimensional spreading pressure, we have studied the commensurate-incommensurate (C-I) transition in submonolayer Kr on $\mathrm{ZYX}$ graphite at temperatures near 40 K. High-resolution synchrotron x-ray diffraction results show both hysteresis and C-I phase coexistence, clear signatures of a first-order transition. This agrees with theoretical predictions, but apparently gives different behavior from earlier studies at higher temperatures and coverages.