Margaret M. Elcombe

Novel rare gas interstitial fullerenes of C60 with Ar, Kr and Xe

Abstract In this paper we report the formation and characterisation of rare gas C 60 interstitial compounds of Ar, Kr and Xe. The materials were produced by hot isostatically pressing (HIP) the fullerene solid at temperatures between 200 and 550 °C and under rare gas pressures in the range 170–200 MPa. With this method, we have been able to make rare gas fullerene compounds with stoichiometries of Ar 1 C 60 , Kr 0.9 C 60 and Xe 0.66 C 60 . Thermal gravimetric analysis (TGA) showed that the HIPed materials contained rare gas after treatment, and gave a method for determining the stoichiometry. TGA also enabled the thermal stabilities of these materials with respect to rare gas loss to be investigated. The structure of the rare gas fullerenes was characterised by both X-ray and neutron powder diffraction. By Rietveld analysis of the diffraction data it has been shown that only the octahedral interstices of the fullerene face centred cubic (FCC) lattice were occupied by the rare gas, and the site occupancy of this site gave a stoichiometry agreeing within 5% of that obtained from TGA. The face centred to primitive cubic orientational ordering phase transition for these rare gas fullerenes was studied using neutron diffraction. The transition temperature was found to decrease as the size of the rare gas increases. This lowering is a result of the net weakening of the c 60 c 60 interaction potential, as the rare gas pushes the c 60 molecules slightly apart; a consequence of not only their size but also a result of their thermal motion (internal pressure) within the interstitial site. Differential scanning calorimetry (DSC) confirmed the transition temperatures obtained from neutron diffraction. In addition, transmission electron microscopy (TEM) and 13 C NMR studies have been performed on these materials and the results are discussed.

A neutron scattering investigation of cubic stabilised zirconia (CSZ). I: The average structure of Y-CSZ

Abstract Single crystal neutron diffraction has been used to study the relaxations of the O-atom from its ideal fluorite position in yttria stabilised cubic zirconia (Y-CSZ). The samples used in this study cover a compositional range of 9.5–24 mol% Y 2 O 3 ZrO 2 . We find that the O-atom is predominantly displaced in the 〈100〉 direction and to a lesser extent in the 〈111〉, while appreciable amounts remain at the ideal fluorite site. Further the average ZrO distances decrease with Y 2 O 3 content to values similar to that of tetragonal zirconia for compositions close to the cubic-tetragonal zirconia phase boundary. The controversy in the literature over the direction of relaxation of the O-atom is resolved with the aid of probability density functions. We demonstrate that there is more than one way to describe the distribution of the O-atom about its ideal site. A probability maximum is always produced at the ideal site, while probability density in the 〈100〉 directions is strong.

The crystal chemistry of the alkaline-earth apatites A 10 (PO 4 ) 6 Cu x O y (H) z (A = Ca, Sr and Ba)†

The crystal chemistry of the cuprate apatites A(I)(4)A(II)(6)(PO(4))(6)Cu(x)O(y)(H)(z) (A = Ca, Sr and Ba) was investigated by powder X-ray (PXRD) and neutron diffraction (PND) and X-ray photoelectron spectroscopy (XPS). The refined crystal structures confirmed earlier X-ray diffraction studies that showed copper resides in the apatite channels and additionally, located hydrogen. For all materials copper is primarily divalent (Cu(2+)) but in the calcium and strontium analogues co-exists with minor Cu(3+). This is in contrast with a previous work where Cu(1+) and Cu(2+) were reported.

Development of Carbon Nanotube Reinforced Hydroxyapatite Bioceramics

This paper reports development of a production method to produce a composite material that is biocompatible, with high mechanical strength and resilience. The chemical precipitation conditions necessary for the production of synthetic hydroxyapatite (HAp) were determined and include pH, temperature and rate of reaction. A gas phase purification method was optimised to remove the soot impurity from the nanotubes, with transmission electron microscopy showing the preservation of the carbon nanotubes. Subsequent development of chemical and physical reinforcement techniques to produce a HAp + carbon nanotube composite material have been trialled. Hot isostatically pressed samples showed excellent densification and strength.

Low temperature phase transition in KOH and KOD

Abstract Dielectric constant and differential scanning calorimetry measurements have shown a transition to a new phase in both KOH (at 233 K) and KOD (at 257 K); the shape of the dielectric anomaly suggests electrical ordering at low temperature. Structural parameters obtained from high resolution neutron powder diffraction data demonstrate the ordering to be antiferroelectric. A preliminary account is given of the structures at 293 K and 77 K.

Coexistence of ferroelectricity and magnetism in transition-metal-doped n = 3 Aurivillius phases

Magnetic-cation-doped three-layer Aurivillius phases Bi2−xSr2+x(Nb/Ta)2+xM1−xO12, x≈ 0.5 and M = Ru4+, Ir4+ or Mn4+, are shown to have the same orthorhombic space group symmetry and similar dielectric and ferroelectric properties as their (non-magnetic) ferroelectric parent compounds Bi2−xSr2+xNb2+xTi1−xO12, x = 0, 0.5. The magnetic-cation-doped phases also show evidence for short-range ferromagnetic (M = Mn) and antiferromagnetic (M = Ru and Ir) exchange, demonstrating the potential of these naturally layered phases as templates for multiferroic (magnetoelectric) materials.

The crystal structure of Zr2NiD4

Abstract The crystal structure of Zr 2 NiD 4 at room temperature has been investigated by high resolution neutron powder diffraction. The diffraction pattern has been successfully fitted on the basis of a monoclinic structure in space group I 2/ c with cell parameters a = 0.6789 nm, b = 0.6758 nm, c = 0.5675 nm, β = 91.04° relative to the neutron wavelength of 0.1893(1) nm. The site occupancies of the deuterium atoms are compared with model calculations and results from previous X-ray and neutron data from Zr 2 Ni hydrides and deuterides of similar compositions based on the similar tetragonal CuAl 2 -type structure of Zr 2 Ni.

A neutron scattering investigation of Cubic Stabilised Zirconia (CSZ)—II. Lattice dynamics of Y- and Ca-CSZ

Abstract We have measured the acoustic phonon dispersion curves of three Cubic Stabilised Zirconias (CDZ) (9.5 and 24mol% Y2O3− and 12.5 mol% CaO−ZrO2) at room temperature using a triple axis spectrometer. Phonons become broader and decrease in intensity as the Brillouin zone boundary is approached. The behaviour of the [ζζ0]TA1 branch is anomalous in that it broadens to the extent that it cannot be observed beyond 2π a (0.4, 0.4, 0) . To a lesser extent the same behaviour is observed for the other LA and TA branches. Despite an intensive search, we have not been able to measure any optic phonons. Previous workers have attributed these effects to the static disorder of the oxygen sublattice. In this paper, we present lattice dynamical calculations which model the effect of oxygen vacancies and relaxations which qualitatively explain the experimental phonon observations.

Low temperature phase transition in CsOH and CsOD

Abstract Dielectric constant and differential thermal analysis measurements have confirmed the phase transition in both CsOH (at 232 K) and CsOD (at 262 K); the shape of the dielectric anomaly suggests electrical ordering at low temperature. Structural parameters obtained from high resolution neutron powder diffraction data demonstrate that the ordering is antiferroelectric. A preliminary account is given of the structures at 293 K and 77 K.

Microstrain in hydroxyapatite carbon nanotube composites

Synchrotron radiation diffraction data were collected from hydroxyapatite-carbon nanotube bioceramic composites to determine the crystallite size and to measure changes in non-uniform strain. Estimates of crystallite size and strain were determined by line-profile fitting of discrete peaks and these were compared with a Rietveld whole-pattern analysis. Overall the two analysis methods produced very similar numbers. In the commercial hydroxyapatite material, one reflection in particular, (0 2 3), has higher crystallite size and lower strain values in comparison with laboratory-synthesized material. This could indicate preferential crystal growth in the [0 2 3] direction in the commercial material. From the measured strains in the pure material and the composite, there was a degree of bonding between the matrix and strengthening fibres. However, increasing the amount of carbon nanotubes in the composite has increased the strain in the material, which is undesirable for biomedical implant applications.