Large thermopower in metallic misfit cobalt oxides : improvement by cationic substitutions

ABSTRACTThe thermoelectric properties of misfit cobaltites have been investigated. Their crystallographic structure consists in the stacking of layers of tilted CoO6 edge-shared octahedra, separated by rock-salt type layers. The Tl based family was first investigated : it is shown that by increasing the Tl content, the resistivity ρ can be reduced while keeping a large thermopower S. Another way to improve the figure of merit Z=S2/(ρκ) is to partially substitute Bi for Tl to increase S and keep a small ρ. A new family of Pb-based misfits has also been investigated. Two different techniques have been attempted to decrease ρ and/or increase S in this family : the partial substitution of Cu for Co, and the partial substitution of Ca for Sr.

Rhombohedral Twin Structure in Hematite (α-Fe2O3)

Electron optical imaging of the structure of haematite ( α-Fe2O3) in two projections has allowed the microscopic twinning elements of rhombohedral twins to be determined. A systematic analysis of alternative structural models, using computer-simulation and image-matching considerations, as well as crystallochemical arguments, is presented. The twin has a screw operation consisting of a pure rotation of 180� about [0111], the twinning direction, combined with a parallel translation of (1/22) [0111]. The twinning plane (0112) forms a coherent interface, consisting of face-and edge-shared octahedra , with no change in the normal nearest-neighbour coordination.

Crystal Structure of LiVO3

Lithium metavanadate is monoclinic with a = 10.158(2), b = 8.4175(9), c = 5.8853(9) Å, β = 110.48(2)° and probable space group C2/c. The structure, refined by least-squares to a ωR = 0.033 using 851 reflections, is isotypic with LiAlSi2O6 (spodumene) and LiFeSi2O6. Bands of Li—O edge-shared octahedra run parallel to c and are linked by chains of VO4 tetrahedra which also run parallel to c. The mean Li(1)—O and Li(2)—O distances are significantly different, 2.153(3) and 2.284(3) Å, and are related to the degree of distortion of individual distances from the mean. The V—O distances of 1.821, 1.795, 1.662, and 1.628 Å with a mean value of 1.725 Å are consistent with bond strength sums around the oxygen atoms and empirical bond length – bond strength curves.