J.C. Barry

Crystal morphology, composition, and dissolution behavior of carbonated apatites prepared at controlled pH and temperature

A range of carbonated apatites was prepared by aqueous precipitation at 37, 60, and 85°C and at controlled pH values varying from 6.00–9.50 in 0.25 increments. The products were analyzed for Ca, P, Sr, Mg, Na, F, and carbonate. Their initial dissolution rates were measured in a pH 4.5, 0.01 mol · liter−1 acetate buffer. Information about crystal morphologies and crystal defects was obtained by x-ray diffraction and high-resolution transmission electron microscopy (TEM). The CaP molar ratios of the products, together with their Sr and Mg contents, increased with increasing pH. Initial dissolution rates of the products, when adjusted for carbonate content, were in the order 37 > 60 > 85°C whereas apparent particle sizes determined by TEM and x-ray diffraction were ordered 37 < 60 < 85°C. Carbonated apatites precipitated at pHs of 7.0 or less were observed to have planar defects parallel to (100) that were identified as unit-cell-thick intergrowths of octacalcium phosphate. Carbonated apatites precipitated at higher pHs and noncarbonated apatites did not have these defects. A crystal growth mechanism is proposed to account for the presence of the (100) defects.

The structure of (100) defects in carbonated apatite crystallites: A high resolution electron microscope study

Planar defects parallel to (100) with an approximate [1/400] displacement vector have been identified by high resolution transmission electron microscopy and by micro-electron diffraction in the center of synthetic carbonated apatite crystallites. Similar intergrowths, 0.8-1.5 nm in width, have been observed in dental enamel, dentin and bone apatite crystallites. Four possible structural models of the defect core are proposed to explain these experimental features, and computer-simulated lattice images of the models are compared with the experimental images. Typical defects were consistent with a two-dimensional octacalcium phosphate inclusion, one unit cell thick, embedded in an apatite matrix.

Microstructure and superconductivity in a new superconductor series: (Tl,Pb)1Sr2Can−1CunO2n+3−δ

The synthesis procedure and superconducting properties of the first two members of the new series of superconductors of a general formula: (Tl, Pb)1 Sr2 Can−1 Cun O2n+3−δ , are reported. The detailed atomic level microstructure of the compounds have been determined and computer simulated with structural models. Tc increases with n, the number of Cu‐O2 layers per unit cell, as in the single Tl‐layer Tl‐Ba‐Ca‐Cu‐O compounds, but the new materials have higher Tc ’s for the same value of n.

LEAD-DOPED BI2SR2CUO6 : A SUPERCONDUCTOR FREE OF MODULATION AND SUPERSTRUCTURE

Abstract Crystals of lead-containing 2201 superconductor, (Bi 2− x Pb x )Sr 2 CuO 6 , were synthesized and characterized by electron microscopy XANES, X-ray diffraction, and electronic band structure calculations. Lead is present as Pb 2+ and the maximum value of x is approximately 0.25 ( T c ≈ 20 K). The crystal structure does not exhibit the modulations and superstructures seen in the lead-free Bi 2 Sr 2 CuO 6 superconductor. Bi/Pb-O “rock-salt” double layers consist of Bi/Pb-O chains containing 3-coordinated Bi/Pb atoms. Formation of these chains causes the CuO 6 octahedra of the CuO 2 sheets to buckle. It appears that substitution of Pb 2+ for Bi 3+ hinders the insertion of additional oxygen atoms into the double Bi-O layers such that lead-doped Bi 2 Sr 2 CuO 6 does not exhibit structural modulation.

Coherent electron microdiffraction from small metal particles

Abstract Coherent electron microdiffraction patterns have been calculated for small Au clusters and, in particular, for a FCC cuboctahedral cluster of 55 atoms. The incident electron probe has energy 100 keV. It is found that the intensity distributions of microdiffraction patterns are affected by various parameters, such as the beam position of the small probe, the defocus and spherical aberration of the probe-forming (objective) lens, the particle size and the probe size. Any phase change of the small electron probe will affect the intensity distributions as a result of coherent interference effect. In general the sensitivity of intensity distributions to these parameters is inversely proportional to the size of the incident electron probe. Some problems associated with the experimental observations of microdiffraction patterns from small metal particles are discussed.

Synthesis, properties and microstructure of the ∼ 110 K Bi(Pb)-Sr-Ca-Cu oxide superconductor

Abstract The three copper layer ∼ 110 K Bi 2 Sr 2 Ca 2 Cu 3 O 10+ δ phase has been prepared as a nearly pure compound by alloying with Pb under various oxygen pressures, and by long-term annealing. Studies of the superconducting properties and the atomic level microstructure of these samples using high resolution electron microscope imaging, are reported. The role of the substituent atoms in producing the 110 K phase is briefly discussed.

Voidites in diamond — do they contain nitrogen?

Abstract Octahedral void-like objects (voidites) with diameters of up to 7 nm are observed in some type Ia diamonds. The voidites appear to be associated with the decomposition of platelets. The platelets in diamond contain nitrogen and it is very likely that this nitrogen is going into the voidites when the platelet collapses. The gas pressure in the voidites is expected to be of the order of 2GPa. Experimental images of diamond in the [001] and [011] orientations show fringes within the voidites. Most of the voidites have fringes in them. The fringe spacings are found to be consistent with solid NH3 at 2.8 ± 0.8 GPa pressure.

Synthesis microstructure and superconducting properties of Ce-Doped Ti-Sr-Ca-Cu-O

Abstract A new Tl 1− x Ce x Sr 2 CaCu 2 O 7 compound with x =0.25, which forms a primitive tetragonal structure with a=b=3.816 A and c=12.112 A , has been synthesized in nearly pure form. The structure has been confirmed for the heavy atoms at the microstructural level by high resolution lattice imaging techniques. Selected area electron diffraction did not indicate the presence of structural modulations. The presence of Ce has been confirmed by analytical electron microscopic methods and by X-ray photoemission spectroscopy; the latter also indicates that cerium is present as Ce +4 ions. SQUID magnetometry, AC susceptibility and four-probe resistance measurements indicate superconductivity with a T c between 62 and 68 K. Meissner fractions greater than 10% of ideal diamagnetism signal the presence of bulk superconductivity in the samples. The nature of the substitution process, and the reasons for a 20 K drop in T c from a value of near 85 K for the isostructural Tl 0.5 Pb 0.5 Sr 2 CaCu 2 O 7 superconductor, are discussed.

Structure of twinned {113} defects in high-dose oxygen implanted silicon-on-insulator material

Conventional and high resolution electron microscopy (HREM) were used to study the structure of {113} defects in high-dose oxygen implanted silicon. The defects are created with a density of 10 cm below the buried oxide layer in the substrate region. The HREM images of the {113} defects are similar to the ribbon-like defects in bulk silicon. It is proposed that there is a third possible structure of the defects, in addition to coesite and/or hexagonal structures. Portions of some defects exhibit the original cubic diamond structure which is twinned across {115} planes. The atomic model shows that the {115} interface is a coherent interface with alternating five and seven-membered rings and no dangling bonds.

Electron-induced decomposition of rubidium chromium fluoride

Abstract The decomposition of Rb x CrF 3 hexagonal tungsten bronzes (HTB) with x =0.20 and 0.30 have been followed by high resolution electron microscopy (HREM). In the electron beam of the microscope Rb is depleted from the tunnels of the bronze structure leading to the formation CrF 3 which crystallizes in a distorted ReO 3 -type structure. An amorphous phase is frequently intermediate to this transformation. A qualitative comparison of the rate of reaction, depending upon the energy and direction of the electrons, of the compounds of different composition is made. These observations suggest a mechanism for the reaction.