Alan D. Mighell

Crystal Structure of Benzene II at 25 Kilobars

Crystals of a high-pressure form of benzene (benzene 11) were grown in the diamond-anvil pressure cell at elevated temperature and pressure from the transition of solid I to solid II. X-ray precession data were obtained from a single-crystal in the high-pressure cell. At 21�C and about 25 kilobars, benzene II crystallizes in the monoclinic system with a = 5.417 � 0.005 angstroms (S.D.), b = 5.376 � 0.019 angstroms, c = 7.532 � 0.007 angstroms, β = 110.00� � 0.08�, space group P21/ c, Pc= 1.26 grams per cubic centimeter. The crystal structure was solved by generating all possible molecular packing configurations and calculating structure factors, reliability factors, and packing energies for each configuration. This procedure produced a unique solution for the molecular packing of benzene II.

JCPDS-ICDD Research Associateship (cooperative program with NBS/NIST)

The Research Associateship program of the Joint Committee on Powder Diffraction-International Centre for Diffraction Data (JCPDS-ICDD, now known as the ICDD) at NBS/NIST was a long standing (over 35 years) successful industry-government cooperation. The main mission of the Associateship was to publish high quality x-ray reference patterns to be included in the Powder Diffraction File (PDF). The PDF is a continuing compilation of patterns gathered from many sources, compiled and published by the ICDD. As a result of this collaboration, more than 1500 high quality powder diffraction patterns, which have had a significant impact on the scientific community, were reported. In addition, various research collaborations with NBS/NIST also led to the development of several standard reference materials (SRMs) for instrument calibration and quantitative analyses, and computer software for data collection, calibration, reduction, for the editorial process of powder pattern publication, analysis of powder data, and for quantitative analyses. This article summarizes information concerning the JCPDS-ICDD organization, the Powder Diffraction File (PDF), history and accomplishments of the JCPDS-ICDD Research Associateship.

Platinum thiotungsten compounds. Crystal and molecular structure of bis(triethylphosphine)platinum tetrathiotungsten

Abstract Reaction of (Ph3P)2PtCl2 with Ph3P and (Ph3PCH3)2WO2S2 produced (Ph3P)2PtWS4 and (Ph3P)2PtWOS3. [(C2H5)3P]2PtWS4 was similarly synthesized from [(C2H5)3P]2PtCl2 and its structure determined by X-ray diffraction (R = 0.026). Crystal data: P21/n, a = 9.081, b = 14.516, c = 16.819 A, β = 82.33°, Z = 4. The molecule consist of an approximately tetrahedral WS4 unit bridged on one edged by [(C2H5)3P]2Pt.

Conventional cells: monoclinic I- and C-centered cells.

In the selection of a centered cell in the monoclinic system, it is recommended that the experimentalist select an I-centered cell for those cases in which it is the conventional cell - cases in which a and c are coincident with the shortest two translations in the net perpendicular to b (b-axis unique). The common practice of selecting a non-conventional C-centered cell in such cases should be discontinued.

Lattice Symmetry and Identification— The Fundamental Role of Reduced Cells in Materials Characterization

In theory, physical crystals can be represented by idealized mathematical lattices. Under appropriate conditions, these representations can be used for a variety of purposes such as identifying, classifying, and understanding the physical properties of materials. Critical to these applications is the ability to construct a unique representation of the lattice. The vital link that enabled this theory to be realized in practice was provided by the 1970 paper on the determination of reduced cells. This seminal paper led to a mathematical approach to lattice analysis initially based on systematic reduction procedures and the use of standard cells. Subsequently, the process evolved to a matrix approach based on group theory and linear algebra that offered a more abstract and powerful way to look at lattices and their properties. Application of the reduced cell to both database work and laboratory research at NIST was immediately successful. Currently, this cell and/or procedures based on reduction are widely and routinely used by the general scientific community: (i) for calculating standard cells for the reporting of crystalline materials, (ii) for classifying materials, (iii) in crystallographic database work (iv) in routine x-ray and neutron diffractometry, and (v) in general crystallographic research. Especially important is its use in symmetry determination and in identification. The focus herein is on the role of the reduced cell in lattice symmetry determination.

Crystal data space‐group tables

Crystal Data Space‐Group Tables lists over 17,000 materials whose space groups and symmetry have been determined mainly by x‐ray diffraction. These tables comprise a companion publication to Crystal Data Determinative Tables. The space groups are listed in the same order and orientation as in International Tables for x‐ray Crystallography. Within each space group, the materials are arranged in increasing order of the ratios of the cell parameters. The space‐group tables enable the user to find crystals of any specified symmetry, to locate isostructural molecules, and to compare the population frequencies of the various space groups.

NIST Crystallographic Databases for Research and Analysis

The NIST Crystal and Electron Diffraction Data Center builds a comprehensive database with evaluated chemical, physical, and crystallographic information on all types of well-characterized substances. The data are evaluated and standardized by specially designed computer programs as well as by experts in the field. From its master database, the Data Center produces NIST Crystal Data and an Electron Diffraction Database with over 220 000 and 81 000 entries, respectively. These distribution databases are made available to the scientific community via CD-ROM, scientific instruments and online systems. In addition, the Data Center has developed theory and software that can be used for establishing all types of lattice relationships, for the determination of symmetry, for the identification of unknowns using lattice matching techniques, and for data evaluation.

Conventional Cells-The Last Step Toward General Acceptance of Standard Conventional Cells for the Reporting of Crystallographic Data.

In 1969, a seminal section on reduced forms and conventional cells was published in the International Tables for X-Ray Crystallography. The section contains a table that gives a metric classification of the 44 reduced forms. In 2001, this table with appropriate revisions was republished in the Journal of Research of the National Institute of Standards and Technology. An especially valuable feature of the table is that it defines and allows the user to determine a standard conventional cell. Since 1969, there has been an evolution toward acceptance and widespread use of such conventional cells. An inspection of the articles in key crystallographic journals reveals that most cells follow the conventions. However, one major exception remains—the centered monoclinic lattices. In approximately one-third of these cases, non-conventional C-centered cells are used, apparently to avoid the use of I-centered cells. It is recommended that the crystallographic community routinely use the I-centered conventional cell in such cases.

Neutron powder diffraction study of the crystal structure of the oxycarbonate phase YBa2Cu2.85(CO3)0.15O6.73

Abstract The oxycarbonate phase YBa2Cu2.85(CO3)0.15O6.73 has been analyzed by neutron powder diffraction. The phase crystallizes with the symmetry of space group P4/mmm and with unit cell parameters a=3.8717(3), c=11.607(1) A. Rietveld refinements show that the basic structure of the oxycarbonate is the same as that of the 123 superconductor YBa2Cu3O6+w. The CO32− ions are located on the basal plane of the unit cell, with the carbon atoms replacing an equal number of “chain” copper atoms. The presence of the CO32− defects explains why the oxygen stoichiometry of the oxycarbonate can be larger than seven atoms per formula unit.

Ambiguities in Powder Indexing: Conjunction of a Ternary and Binary Lattice Metric Singularity in the Cubic System

A lattice metric singularity occurs when unit cells defining two (or more) lattices yield the identical set of unique calculated d-spacings. The existence of such singularities, therefore, has a practical and theoretical impact on the indexing of powder patterns. For example, in experimental practice an indexing program may find only the lower symmetry member of a singularity. Obviously, it is important to recognize such cases and know how to proceed. Recently, we described: a binary singularity involving a monoclinic and a rhombohedral lattice in a subcell-supercell relationship and a second type of singularity—a ternary singularity–in which two of the three lattices are in a derivative composite relationship. In this work, we describe a ternary lattice metric singularity involving a cubic P, a tetragonal P, and an orthorhombic C lattice. Furthermore, there is a binary singularity, involving a hexagonal P and orthorhombic P lattice, which is characterized by a set of unique d-spacings very close to that of the ternary singularity. The existence of such singularities is more common than once thought and requires a paradigm shift in experimental practice. In addition singularities provide opportunities in material design as they point to highly specialized lattices that may be associated with unusual physical properties.