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Register a new userThe Structural Analysis Possibilities of Reflection High Energy Electron Diffraction
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The epitaxial growth of complex oxide thin films provide three avenues to generate unique properties: the ability to influence the 3-dimensional structure of the film, the presence of a surface, and the generation of an interface. In all three cases, a clear understanding of the resulting atomic structure is desirable. However, determining the full structure of an epitaxial thin film (lattice parameters, space group, atomic positions, surface reconstructions) on a routine basis is a serious challenge. In this paper we highlight the remarkable information that can be extracted from both the Bragg scattering and inelastic multiple scattering events that occur during Reflection High Energy Electron Diffraction. We review some methods to extract structural information and show how mature techniques used in other fields can be directly applied to the {em in-situ} and real-time diffraction images of a growing film. These collection of techniques give access to both the epitaxially influenced 3 dimensional bulk structure of the film, and any reconstructions that may happen at the surface.
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We have investigated the atomic structure of superconducting Ca-intercalated bilayer graphene on a SiC(0001) substrate using total-reflection high-energy positron diffraction. By comparing the experimental rocking-curves with ones calculated for various structural models using a full-dynamical theory, we have found that Ca atoms are intercalated in the graphene-buffer interlayer, rather than between the two graphene layers. From transport measurements, the superconducting transition was observed to be at Tc_onset = 4K for this structure. This study is the first to clearly identify the relation between the atomic arrangement and superconductivity in Ca-intercalated bilayer graphene.
We report on the crystallographic structure of the layered compound Pb3Mn7O15. Previous analysis based on laboratory X-ray data at room temperature gave contradictory results in terms of the description of the unit cell. Motivated by recent magnetic bulk measurements of this system, we re-investigated the chemical structure with high-resolution synchrotron powder diffraction at temperatures between 15 K and 295 K. Our results show that the crystal structure of stoichiometric Pb3Mn7O15 has a pronounced 2-dimensional character and can be described in the orthorhombic space group Pnma.
We performed high-pressure angle dispersive x-ray diffraction measurements on Fe5Si3 and Ni2Si up to 75 GPa. Both materials were synthesized in bulk quantities via a solid-state reaction. In the pressure range covered by the experiments, no evidence of the occurrence of phase transitions was observed. On top of that, Fe5Si3 was found to compress isotropically, whereas an anisotropic compression was observed in Ni2Si. The linear incompressibility of Ni2Si along the c-axis is similar in magnitude to the linear incompressibility of diamond. This fact is related to the higher valence-electron charge density of Ni2Si along the c-axis. The observed anisotropic compression of Ni2Si is also related to the layered structure of Ni2Si where hexagonal layers of Ni2+ cations alternate with graphite-like layers formed by (NiSi)2- entities. The experimental results are supported by ab initio total-energy calculations carried out using density functional theory and the pseudopotential method. For Fe5Si3, the calculations also predicted a phase transition at 283 GPa from the hexagonal P63/mcm phase to the cubic structure adopted by Fe and Si in the garnet Fe5Si3O12. The room-temperature equations of state for Fe5Si3 and Ni2Si are also reported and a possible correlation between the bulk modulus of iron silicides and the coordination number of their minority element is discussed. Finally, we report novel descriptions of these structures, in particular of the predicted high-pressure phase of Fe5Si3 (the cation subarray in the garnet Fe5Si3O12), which can be derived from spinel Fe2SiO4 (Fe6Si3O12).
SrMoO4 was studied under compression up to 25 GPa by angle-dispersive x-ray diffraction. A phase transition was observed from the scheelite-structured ambient phase to a monoclinic fergusonite phase at 12.2(9) GPa with cell parameters a = 5.265(9) A, b = 11.191(9) A, c = 5.195 (5) A, and beta = 90.9, Z = 4 at 13.1 GPa. There is no significant volume collapse at the phase transition. No additional phase transitions were observed and on release of pressure the initial phase is recovered, implying that the observed structural modifications are reversible. The reported transition appeared to be a ferroelastic second-order transformation producing a structure that is a monoclinic distortion of the low-pressure phase and was previously observed in compounds isostructural to SrMoO4. A possible mechanism for the transition is proposed and its character is discussed in terms of the present data and the Landau theory. Finally, the EOS is reported and the anisotropic compressibility of the studied crystal is discussed in terms of the compression of the Sr-O and Mo-O bonds.
We present low-energy electron diffraction (LEED) as elastic electron-atom scattering (EEAS) operating in a target crystal waveguide where a Coulombic carrier wave is wavenumber modulated by exchange-correlation (XC) interaction. Carrier potential is designed using a KKR (Korringa-Kohn-Rostoker) muffin-tin model built on overlapping free atoms. XC potential is constructed using Serneliuss many-particle theory on electron self-energy. EEAS phase shifts are derived from Diracs differential equations, and four recent LEED investigations are recalculated: Cu(111)+$( 3!surd3times!surd3 ) mathrm{R30^circ}$-TMB, Ag(111)+$(4!times !4 )$-O, Ag(111)+$( 7!times!surd3 ) mathrm{rect}$-$mathrm{SO}_4$, Ru(0001)+$( surd3!times!surd3 ) mathrm{R30^circ}$-C. TMB stands for 1,3,5-tris(4-mercaptophenyl)-benzene with chemical formula C$_{24}$H$_{15}$S$_3$. We are able to report substantially improved reliability factors.
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