Anthony V. Powell

Recent developments in nanostructured materials for high-performance thermoelectrics

This highlight discusses recent trends in the search for new high-efficiency thermoelectric materials. Thermoelectric materials offer considerable attractions in the pursuit of a more efficient use of existing energy resources, as they may be used to construct power-generation devices that allow useful electrical power to be extracted from otherwise waste heat. Here, we focus on the significant enhancements in thermoelectric performance that have been achieved through nanostructuring. The principal factor behind the improved performance appears to be increased phonon scattering at interfaces. This results in a substantial reduction in the lattice contribution to thermal conductivity, a low value of which is a key requirement for improved thermoelectric performance.

The magnetic properties of iridium in mixed-metal oxides

The magnetic susceptibilities of La2Mg1−xZnxIrO6 (0< x <1), La2NiIrO6, La2CuIrO6, La2LiIrO6, BaLaMgIrO6, BaLaFeIrO6, BaLaCoIrO6, BaLaNiIrO6 and Sr4IrO6 have been measured as a function of temperature and applied magnetic field. Sr4IrO6, BaLaNiIrO6 and BaLaCoIrO6 order antiferromagnetically at 12, 87 and 56 K respectively. BaLaFeIrO6 shows a spin-glass transition at 65 K. La2NiIrO6 and BaLaCoIrO6 order as weak ferromagnets at 80 K and 70 K respectively. The solid solution La2Mg1−xZnxIrO6 shows a gradual change from antiferromagnetism (TN = 11.7 K) when x = 0 to ferromagnetism (Tc = 9.5 K, μsat = 0.4 μB) when x = 1. The Ir(V) oxides La2LiIrO6 and BaLaMgIrO6 are paramagnetic in the temperature range 6 K < T < 300 K.

Powder neutron diffraction study of the mixed uranium–vanadium oxides Cs2(UO2)2(V2O8) and UVO5

The crystal structures of UVO5[space group Pbcm, a= 4.1231(1)A, b= 12.3641 (1)A, c= 7.2071 (1)A, Z= 4] and Cs2(UO2)2V2O8[space group P21/a,a= 10.521(2)A, b= 8.4369(9)A, c= 7.308(3)A, β=106.08(1)°, Z= 2], previously obtained by single-crystal X-ray diffraction, were refined using powder neutron diffraction and Rietveld analysis adapted to time-of-flight data. The main structural features of the two compounds were confirmed but more accurate oxygen positions are reported here.

A copper-containing oxytelluride as a promising thermoelectric material for waste heat recovery

The new thermoelectric material BiOCuTe exhibits an electrical conductivity of 224 S cm−1 and a Seebeck coefficient of +186 μV K−1 at 373 K, together with an extremely low lattice thermal conductivity of ∼0.5 W m−1 K−1. This results in a ZT of 0.42 at 373 K, which increases to 0.66 at the maximum temperature investigated, 673 K.

Synthesis and characterisation of transition-metal-substituted gallium phosphates with the laumontite structure

Three novel metal-substituted gallium phosphates, (C3N2H5)8[Me8Ga16P24O96] (Me = Co, Fe, Mn), have been synthesised under solvothermal conditions and their structures determined using single-crystal X-ray diffraction. They are isostructural, with monoclinic symmetry, space group C 2/c and unit-cell dimensions ca. 15 × 13 × 15 A, β ca. 111°. The materials all have frameworks with the laumontite topology (structure type code LAU), constructed from alternating PO4 and MO4 tetrahedra (M = Me and Ga) with imidazole cations occupying sites within the channels of the framework. Magnetic susceptibility measurements show that the materials exhibit Curie-Weiss behaviour over wide ranges of temperature and that the transition metals are present as divalent cations.

The magnetic properties of BaLaCoIrO6 and Ba3CoIrxRu2−xO9

Abstract The magnetic structure of the antiferromagnetic phase of the pseudo-cubic perovskite BaLaCoIrO 6 has been determined from time-of-flight powder neutron diffraction data collected at a temperature of 4 K. The Co(II) cations adopt a Type II collinear spin structure with an ordered moment of 2.8(2) μ B per Co(II); the Ir(V) cations do not have a detectable ordered magnetic moment. Magnetic susceptibility measurements on the compounds Ba 3 CoIr x Ru 2− x O 9 (0 x ⩽2) resulted in the observation of weak ferromagnetism; the saturation magnetization decreased with increasing Ru content. Comparisons are made between BaLaCoIrO 6 and the spin glass BaLaCoRuO 6 , and also between Ba 3 CoIr 2 O 9 and antiferromagnetic Ba 3 CoRu 2 O 9 . The behaviour of the nominally non-magnetic Ir(V) cations is discussed and contrasted with that of diamagnetic Sb(V) in Ba 3 CoSb 2 O 9 and Ba 3 CoIrSbO 9 .

Synthesis, structure and magnetic characterisation of a new layered ammonium manganese(II) diphosphate hydrate, (NH4)2[Mn3(P2O7)2(H2O)2]

A new layered ammonium manganese(II) diphosphate, (NH4)2[Mn3(P2O7)2(H2O)2], has been synthesised under solvothermal conditions at 433 K in ethylene glycol and the structure determined at 293 K using single-crystal X-ray diffraction data (Mr = 584.82, monoclinic, space group P21/a, a = 9.4610(8), b = 8.3565(7), c = 9.477(1) A, β = 99.908(9)°, V = 738.07 A3, Z = 2, R = 0.0351 and Rw = 0.0411 for 1262 observed data (I > 3(σ(I))). The structure consists of chains of cis- and trans-edge sharing MnO6 octahedra linked via P2O7 units to form layers of formula [Mn3P4O14(H2O)2]2− in the ab plane. Ammonium ions lie between the manganese-diphosphate layers. A network of interlayer and ammonium-layer based hydrogen bonding holds the structure together. Magnetic measurements indicate Curie–Weiss behaviour above 30 K with μeff = 5.74(1) μB and θ = −23(1) K, consistent with the presence of high-spin Mn2+ ions and antiferromagnetic interactions. However, the magnetic data reveal a spontaneous magnetisation at 5 K, indicating a canting of Mn2+ moments in the antiferromagnetic ground state. On heating (NH4)2[Mn3(P2O7)2(H2O)2] in water at 433 K under hydrothermal conditions, Mn5(HPO4)2(PO4)2·4H2O, synthetic hureaulite, is formed.

Powder neutron diffraction study of potassium uranate(V), KUO3

Powder neutron diffraction data have been recorded for KUO3. Rietveld refinement confirms that the compound adopts a regular perovskite structure [space group Pm3m, a= 4.29601(2)A] with regular UO6 octahedra.

Ball milling as an effective route for the preparation of doped bornite: synthesis, stability and thermoelectric properties

Bornite, Cu5FeS4, is a naturally-occurring mineral with an ultralow thermal conductivity and potential for thermoelectric power generation. We describe here a new, easy and scalable route for synthesising bornite, together with the thermoelectric behaviour of manganese-substituted derivatives, Cu5Fe1−xMnxS4 (0 ≤ x ≤ 0.10). The electrical and thermal transport properties of Cu5Fe1−xMnxS4 (0 ≤ x ≤ 0.10), which are p-type semiconductors, were measured from room temperature to 573 K. The stability of bornite was investigated by thermogravimetric analysis under inert and oxidising atmospheres. Repeated measurements of the electrical transport properties confirm that bornite is stable up to 580 K under an inert atmosphere, while heating to 890 K results in rapid degradation. Ball milling leads to a substantial improvement in the thermoelectric figure of merit of unsubstituted bornite (ZT = 0.55 at 543 K), when compared to bornite prepared by conventional high-temperature synthesis (ZT < 0.3 at 543 K). Manganese-substituted samples have a ZT comparable to that of unsubstituted bornite.

A time-of-flight powder neutron diffraction study of non-stoichiometry in barium iridate BaIrO3−δ

Abstract Rietveld refinement of room temperature time-of-flight powder neutron diffraction data for a sample of BaIrO3−δ, prepared by slow cooling of polycrystalline single-phase material from 1223 K, was carried out in the space group C2 m ; a = 9.9992(3) A , b = 5.7490(1) A , c = 15.1707(2) A , β = 103.27(1)° . Refinement of oxygen site occupancy factors indicated that the material was almost ideally stoichiometric with a composition BaIrO2.94. Analogous refinement of data for a sample produced by rapidly quenching from 1223 K resulted in similar parameters; space group C2 m , a = 10.0010(4) A , b = 5.7521(1) A , c = 15.1747(3) A , β = 103.26(1)° . Refinement of oxygen site occupancy factors indicated significant numbers of defects on the oxide sublattice corresponding to a composition BaIrO2.74. The distribution of these defects and their effect on the detailed structure of the phases are discussed.