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David Harker

Bio: David Harker is an academic researcher from Johns Hopkins University. The author has contributed to research in topics: Crystal structure & Decaborane. The author has an hindex of 12, co-authored 23 publications receiving 1617 citations.

Papers
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Journal ArticleDOI
TL;DR: In this article, the Patterson method was used to determine the positions of the heavy atoms in proustite, Ag3AsS3, by using the covalent atomic radii and assumptions concerning bond directions.
Abstract: It is shown that the three‐dimensional Patterson method can be so simplified by the use of the symmetry properties of the crystal under consideration that its use in determining the positions of atoms in crystals is practicable. This method is then used to determine the positions of the heavy atoms in proustite, Ag3AsS3. The positions of the sulfur atoms are found by the use of the covalent atomic radii and assumptions concerning bond directions. The structure so derived is found to be compatible with the observed data. Proustite is found to contain pyramidal AsS3′′′ groups and silver atoms forming two bonds to sulfur in almost opposed directions. Pyrargyrite, Ag3SbS3, is found to have almost the same structure as proustite. The analytical statement of the structures is as follows:— The space group, C3v6 — R3c, is common to both proustite and pyrargyrite. The special positions of C3v6 are (in hexagonal axes) (2a)0,0,Z;0,0,12+Z;(6b)X,Y,Z;Y¯,X−Y,Z;Y−X,X¯,Z;Y¯,X¯,12+Z;X,X−Y,12+Z;Y−X,Y,12+Z; and positions der...

136 citations

Journal ArticleDOI
TL;DR: In this article, it was shown that the application of Schwarz's and Cauchy's inequalities to the formulas for calculating Fhkl from, respectively, the density of scattering matter and the atomic positions in a crystal leads to relations between the magnitudes of some fhkl's and the signs or phases of others.
Abstract: It is shown that the application of Schwarz's and Cauchy's Inequalities to the formulas for calculating Fhkl from, respectively, the density of scattering matter and the atomic positions in a crystal leads to relations between the magnitudes of some Fhkl's and the signs or phases of others. These relations are in the form of inequalities, which vary with the symmetry of the crystal under consideration. A table of the simplest inequalities applicable to crystals possessing each of the simple symmetry elements is included. Examples of the inequalities arising from the presence of combinations of symmetry elements are presented.

113 citations

Journal ArticleDOI
TL;DR: In this paper, the authors describe a cubic close-packed structure of nickel molybdenum alloy, which can be described analytically as follows: space group: C4h5-I4/m; dimensions of unit cell: a0=5.720A, c0=3.564A,c/a=0.6231; atomic positions: 2 Mo in 2(a), 8 Ni in 8(h), x=0.,0.200.
Abstract: A nickel‐molybdenum alloy containing about 20 atom percent of molybdenum exists above 900°C as a cubic close‐packed structure. If this alloy is quenched and then held at temperatures below 840°C, an ordering reaction occurs which results in a slight tetragonal distortion of the cubic structure. This ordered structure requires for its description a tetragonal cell with 2.5 times the volume of the distorted face‐centered cubic cell. The completely ordered structure can be described analytically as follows: space group: C4h5—I4/m; dimensions of unit cell: a0=5.720A, c0=3.564A, c/a=0.6231; atomic positions: 2 Mo in 2(a), 8 Ni in 8(h), x=0.200. y=0.400.

91 citations


Cited by
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Journal ArticleDOI
TL;DR: The General Utility Lattice Program (GULP) as discussed by the authors has been extended to include the ability to simulate polymers and surfaces, as well as adding many other new features, and the current status of the program is fully documented.
Abstract: The General Utility Lattice Program (GULP) has been extended to include the ability to simulate polymers and surfaces, as well as adding many other new features, and the current status of the program is fully documented. Both the background theory is described, as well as providing a concise review of some of the previous applications in order to demonstrate the range of its use. Examples are presented of work performed using the new compatibilities of the software, including the calculation of Born effective charges, mechanical properties as a function of applied pressure, calculation of frequency-dependent dielectric data, surface reconstructions of calcite and the performance of a linear-scaling algorithm for bond-order potentials.

1,987 citations

Journal ArticleDOI
TL;DR: The currently proposed shape-guiding mechanisms are presented and the important pioneering studies on the assembly of shape-controlled nanocrystals into ordered superlattices and the fabrication of prototype advanced nanodevices are discussed.
Abstract: Inorganic nanocrystals with tailored geometries exhibit unique shape-dependent phenomena and subsequent utilization of them as building blocks for the fabrication of nanodevices is of significant interest. Herein, we review the recent developments in the shape control of colloidal nanocrystals with a focus on the scientifically and technologically important semiconductor and metal oxide nanocrystals obtained by nonhydrolytic synthetic methods. Many structurally unprecedented motifs have been discovered including polyhedrons, rods and wires, plates and prisms, and other advanced shapes such as branched rods, stars, inorganic dendrites, and dumbbells. The currently proposed shape-guiding mechanisms are presented and the important pioneering studies on the assembly of shape-controlled nanocrystals into ordered superlattices and the fabrication of prototype advanced nanodevices are discussed.

1,101 citations

Journal ArticleDOI
TL;DR: In this article, the growth mechanism and growth habit of oxide crystals are investigated from a kinetics viewpoint, starting from the hypothesis of growth unit, and a new rule concerning the growth habit is deduced considering the relation between the growth rate and the orientation of the coordination polyhedron at the interface.

1,080 citations

Book ChapterDOI
TL;DR: Quantitative measurements of the rate of hydrogen exchange in a given protein, under specified experimental conditions, provide a multiparameter characterization of the protein conformation (or distribution of conformations) present under these conditions.
Abstract: Publisher Summary The chapter reviews the results obtained from measurements of hydrogen exchange (H–H, H–D, and H–T) in proteins and related compounds. The slowness of the rate of hydrogen exchange in a protein, relative to the exchange rate observed with simple peptides under the same experimental conditions is closely related to the conformation of the protein molecule in aqueous solution. Quantitative measurements of the rate of hydrogen exchange in a given protein, under specified experimental conditions, provide a multiparameter characterization of the protein conformation (or distribution of conformations) present under these conditions. Moreover, the chapter discusses the experimental techniques that have been used to measure quantitatively the rate of hydrogen exchange. The chapter also discusses arguments that support case, as the mechanism of exchange of the slowly exchanging hydrogen atoms.

1,054 citations

Journal ArticleDOI
TL;DR: In this paper, high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) peak broadening (Scherer) analysis were used to study coarsening and morphology development of nanocrystals.

940 citations