Arthur W. Sleight

ACu3Ti4O12 and ACu3Ru4O12 perovskites: high dielectric constants and valence degeneracy

Abstract ACu3M4O12 phases crystallize in perovskite-related structure (space group: Im3). Dielectric constants for ACu3Ti4O12 phases are found to be much higher than expected based on the dielectric polarizabilities of the constituent atoms, and this property seems not to be related to either underlying ferroelectric or relaxor behavior. New data on these compounds is presented. Valence degeneracy between Cu and Ru is shown to exist for ACu3Ru4O12 phases where A may be a +1, +2, or +3 cation. Structural refinements based on single crystal X-ray diffraction data are given for CaCu3Ti4O12, CaCu3Ru4O12 and NaCu3Ru4O12.

Transparent p-type conducting CuScO2+x films

Transparent films of CuScO2+x have been prepared which show p-type electrical conductivity. The temperature dependence of the conductivity indicates semiconducting behavior with an apparent room temperature activation energy of 0.11 eV. The highest room temperature conductivity observed was 30 S cm−1. Films 110 nm thick show 40% transparency in most of the visible spectrum and become much more transparent in the infrared spectrum. The p-type behavior was confirmed by the Seebeck effect.Transparent films of CuScO2+x have been prepared which show p-type electrical conductivity. The temperature dependence of the conductivity indicates semiconducting behavior with an apparent room temperature activation energy of 0.11 eV. The highest room temperature conductivity observed was 30 S cm−1. Films 110 nm thick show 40% transparency in most of the visible spectrum and become much more transparent in the infrared spectrum. The p-type behavior was confirmed by the Seebeck effect.

Negative thermal expansion materials

Substances showing negative thermal expansion (NTE) over small temperature ranges have long been kno…

Colossal Magnetoresistance Without Mn3+/Mn4+ Double Exchange in the Stoichiometric Pyrochlore Tl2Mn2O7

Structural analysis from powder neutron and single-crystal x-ray diffraction data for a sample of the Tl2Mn2O7 pyrochlore, which exhibits colossal magnetoresistance (CMR), shows no deviations from ideal stoichiometry. This analysis gives an Mn-O distance of 1.90 angstroms, which is significantly shorter than the Mn-O distances (1.94 to 2.00 angstroms) observed in phases based on LaMnO3 perovskites that exhibit CMR. Both results in Tl2Mn2O7 indicate oxidation states very close to Tl3+2Mn4+2O7. Thus, Tl2Mn2O7 has neither mixed valence for a double-exchange magnetic interaction nor a Jahn-Teller cation such as Mn3+, both of which were thought to have an important function in CMR materials. An alternate mechanism for CMR in Tl2Mn2O7 based on magnetic ordering driven by superexchange and strong spin-fluctuation scattering above the Curie temperature is proposed here.

p-Type oxides for use in transparent diodes

Several p-type oxides of the delafossite structure have been investigated in the hope that the conductivity and transparency will be high enough to render them useful in the manufacture of transparent p–n junction diodes and other transparent devices. The highest conductivity achieved to date has been 220 S/cm in CuCr1−xMgxO2 thin films. Oxygen intercalation in CuSc1−xMgxO2+y films improves the conductivity at the expense of optical transparency. We have improved the conductivity of CuGaO2-based films from 0.02 to 1 S/cm by substitution of Fe for Ga. p-Type conductivity has been demonstrated in an Ag-based delafossite film. A sputter-deposited AgCoO2 film has a conductivity of 0.2 S/cm, a Seebeck coefficient of 230 μV/K and a band gap of 4.1 eV at room temperature. CuNi2/3Sb1/3O2 films have been produced that are p-type conductors when doped with Sn.

Structure refinement of triclinic tungsten trioxide

Abstract The thermodynamically stable form of WO 3 at room temperature is triclinic. The structure of this form of WO 3 was for the first time refined from neutron diffraction data. The data were obtained on a new high resolution powder diffractometer. The structure obtained appears to be somewhat more accurate than that obtained from single crystal X-ray diffraction data. Rietveld approach was used to refine the neutron powder diffraction data. Results using the software packages GSAS, Rietan and PROFIL are compared. Using GSAS, an R wp of 7.97 and an R p of 5.87 were obtained. Neutron diffraction data were also collected and analysed on a sample which was a mixture of the monoclinic and triclinic forms of WO 3 ; Rietveld refinements of this data were stable and yielded reasonable results.

Highly conducting transparent thin films based on zinc oxide

Doped zinc oxide thin films were prepared by rf magnetron sputtering using the dopants Al, Ga, In, and Ge. The best results were obtained with Al and Ga doping where room temperature conductivities were as high as 1600 and 1800 ohm -1 cm -1 , respectively. Hall measurements were performed at 77 K and 298 K. The Hall mobility as in the range of 9 to 22 cm 2 /Vs, and there was generally very little temperature dependence of the mobility or conductivity. Cation doping levels were as high as 10 at. %, but the conductivities did not increase beyond 3 at. % doping level. For films with high conductivity, electron carrier concentrations from Hall measurements were significantly lower than the concentrations of dopants. Optical measurements on the films showed that the average transmittance though the visible range is higher than 85%. The measurements also indicated a blueshift of the absorption edge with doping.

Negative thermal expansion in Y2W3O12

Abstract Neutron diffraction data were collected on polycrystalline Y 2 W 3 O 12 at seven temperatures from 15 to 1373 K. All three cell edges of orthorhombic Y 2 W 3 O 12 decrease with increasing temperature, giving an average linear thermal expansion coefficient of −7.0×10 −6 K −1 . Rietveld refinements of the neutron diffraction data show an apparent decrease in the average W–O distance of 0.05 A from 15 to 1373 K. This apparent decrease causes the decrease in the cell edges, but it is not an actual decrease of average W–O distances. The apparent shrinkage is instead considered to be caused by the transverse thermal motion of oxygen in the Y–O–W linkages.

Order-disorder in A2M3+M5+O6 perovskites

Using x-ray and neutron diffraction data, the degree of order of the octahedral site cations has been determined for the perovskites Sr 2 AlNbO 6 and Sr 2 AlTaO 6 , which have been prepared by several different methods and annealed at temperatures up to 1690 °C. The degree of order generally increases with increasing synthesis temperature. The amount of cation ordering is, therefore, primarily controlled by kinetic processes and not by thermodynamic equilibrium considerations. Increased order obtained with increased heating time confirms this general kinetic limitation on the degree of order. However, annealing Sr 2 AlNbO 6 in the highest temperature region resulted in some decrease in order, presumably due to thermodynamic considerations. The cubic edge of both compounds decreases significantly with increasing order. Ordered domains are separated by antiphase boundaries which occur in high concentrations. The cubic cell edge within the ordered domains is significantly smaller than the overall cell edge when the concentration of antiphase boundaries is high. The antiphase boundaries cause significant lattice strain which generally decreases as the concentration of antiphase boundaries decreases. Results on other A 2 M 3+ M 5+ O 6 systems are briefly presented.

The role of rigid unit modes in negative thermal expansion

Abstract The rigid unit mode (RUM) model provides a valuable computational method to investigate correlations of transverse thermal motions of atoms important in negative thermal expansion (NTE) materials. We report here detailed RUM calculations of ten framework oxide structures that have been studied for their negative (or ultra low) thermal expansion properties. The results negate any simple and direct correlation between presence or absence of RUMs in a structure and its NTE property. All the structures considered can be viewed as networks of polyhedral connected by corners only. All evidence supports the importance the transverse motion of the atoms at the corners, but NTE does not correlate well with the presence RUMs for the polyhedra.