Vicky Lynn Karen

The crystal structure of magnesium dicarbide

Abstract The crystal structure of MgC 2 is solved from powder X-ray and neutron diffraction data. Magnesium dicarbide is synthesized from Mg powder and ethyne, with yields of up to 40 wt.% (balance: unreacted Mg and MgO). The X-ray pattern is indexed on a tetragonal unit cell, with space group P 4 2 / mnm suggested from systematic extinctions. The unit cell refined from the powder neutron diffraction data has a =3.9342(7) and c =5.021(1) A and Z =2. The crystal structure contains C 2 groups, with a triple-bond length of 1.215(6) A, aligned in …MgCCMgCCMg… chains having Mg–C bond lengths of 2.174(4) A and a weak interaction [2.510(1) A] between Mg and the triple bond of the crossing chains above and below.

Neutron powder diffraction study of nuclear and magnetic structures of oxidized and reduced YBa2Fe3O8+w

Abstract YBa2Fe3O8+w has been investigated by neutron powder diffraction as function of temperature and oxygen nonstoichiometry close to the limits of the homogeneity range, −0.24 0) in the structural layers of Y, or by creating oxygen vacancies (w (1 1 0/110/002 ), having orthorhombic symmetry when the nuclear structure is tetragonal and monoclinic symmetry when the nuclear structure is orthorhombic. The iron moments are coupled antiferromagnetically in all three directions, the Neel temperature is almost constant as a function of w (T N ≈660 K ) , and so is also the low-temperature saturation moment μAF≈4.0μB.

Framework-Type Determination for Zeolite Structures in the Inorganic Crystal Structure Database

In this work a structural characterization of zeolite crystals is performed by identifying the framework type to which each zeolite belongs. The framework type is assigned for 1433 zeolitedatabase entries in the FIZ/NIST Inorganic CrystalStructureDatabase (ICSD) populating 95 framework types. These entries correspond to both natural and synthetic zeolites. Each ICSD entry is based on published work containing crystallographic information of the zeolite crystalline structure and some physical and chemical data. Today, the Structure Commission of the International Zeolite Association recognizes crystalline materials as belonging to the “zeolite” family only if they possess one of the approved framework types by the organization. Such information is of fundamental importance for identifying zeolites, for reference, for zeolite standards, for supporting the discovery of new zeolites, and for crystalline substance selection based on application. Unfortunately, framework-type information is not contained in the ICSD records. The long term goal of this work is filling such gap. Although the ICSD contains an extensive collection of zeolites, inclusion of zeolites belonging to the 191 accepted framework types could substantially expand such collection. The structural determination was achieved via several structuralanalysis methods based on numerical-computer implementations.

Substitution of Co3+ in YBa2Fe3O8

Abstract The accommodation of Co in the oxygen-saturated solid-solution phase YBa2(Fe1−zCoz)3O8+w has been investigated by powder X-ray and neutron diffraction techniques, as well as by 57 Fe Mossbauer spectroscopy. Of the nominal composition range 0.00⩽z⩽1.00 tested, the solid-solution limit under syntheses at 950°C in 1 bar O 2 is z=0.47(5). No symmetry change in the nuclear and magnetic structures is seen as a consequence of the Co substitution, and the Co atoms are distributed evenly over the two sites that are square-pyramidally and octahedrally coordinated for w=0. The oxygen-saturated samples maintain their oxygen content roughly constant throughout the homogeneity range, showing that Co3+ replaces Fe3+. Despite the nearly constant value of w, Mossbauer spectroscopy shows that the amount of tetravalent Fe slightly increases with increasing z, and this allows Co to adopt valence close to 3.00 to a good approximation. The magnitude of the antiferromagnetic moment (located in the a,b plane) decreases with z in accordance with the high-spin states of the majority Fe3+ and Co3+ ions. Bond-valence analyses are performed to illustrate how the structural network becomes increasingly frustrated as a result of the substitution of Fe3+ by the smaller Co3+ ion. A contrast is pointed out with the substitution of cobalt in YBa2Cu3O7 where it is a larger Co2+ ion that replaces smaller Cu2+.

Report of the working group on crystal phase identifiers.

The proposed crystalline phase identifier consists of a number of components (layers) describing enough properties of the phase to allow a unique identification. These layers consist of the chemical formula, a flag indicating the state of matter, the space-group number and the Wyckoff sequence. They are defined in such a way that they can be incorporated into the IUPAC International Chemical Identifier (InChI) proposed by the International Union of Pure and Applied Chemistry (IUPAC).

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.