Peter R. Slater

Synthesis and electrical characterisation of doped perovskite titanates as potential anode materials for solid oxide fuel cells

This work reports the synthesis and electrical characterisation over a range of oxygen partial pressures (10–20–1 atm) of the A-site deficient perovskites Sr1–3x/2LaxTiO3–δ, with a view to establishing their potential as anode materials for solid oxide fuel cells. Single phase samples were observed for synthesis in air for 0≤x≤0.6, and the materials remained phase pure for both high and low oxygen partial pressures at the measurement temperature of 930 °C. Good electrical conductivity, which increased with increasing La content, was observed on reduction in low oxygen partial pressures, with values as high as 7 S cm–1 [ P(O2)= 10–20 atm], similar to values observed for the related system, Sr1–x/2Ti1–xNbxO3–δ, examined previously. The conductivity of the fully reduced samples showed metallic character from 100 to 930 °C. As the oxygen partial pressure was raised, the conductivity dropped, showing an approximate [P(O2)]–1/6 dependence for porous samples. New samples, Sr1–y/2–3x/2LaxTi1–yNbyO3–δ, with both La and Nb substitutions, were also studied, and these phases showed similar electrical behaviour. Further results for the Sr1–x/2Ti1–xNbxO3–δsystem are presented and compared with the La doped systems.

A combined single crystal neutron/X-ray diffraction and solid-state nuclear magnetic resonance study of the hybrid perovskites CH3NH3PbX3 (X = I, Br and Cl)

The 1H and 13C NMR spectra in methylammonium lead halide perovskites, CH3NH3PbX3 (X = I, Br and Cl) show that the CH3NH3+ units undergo dynamic reorientation, as the organic component tumbles in the perovskite cage. In addition, the differences in the anomalously long relaxation times of the protons associated with the CH3 and not the NH3 groups indicate that only the amine end of the CH3NH3+ group is interacting with the inorganic network. Using this information, we have refined some single crystal X-ray and neutron diffraction data to probe their unusual structures in more detail. Furthermore, impedance spectroscopy has been used to monitor the high-temperature phase transition of CH3NH3PbI3, which confirms a significant increase in conductivity, when it is in its high temperature and higher symmetry structural regime. The optical band-gaps of each halide perovskite were determined using UV-visible spectroscopy and are consistent with previous reports.

Synthesis and structural characterisation of YSr2Cu3−xMxO7±y (M = Fe, Ti, Al, Co, Ga, Pb; 0.5 < x < 1), and the non-existence of the parent phase YSr2Cu3O7

Abstract The possibility of synthesising phases of general composition YSr 2 Cu 3− x M x O 7± y has been examined for a wide variety of metals M. Single phase tetragonal products (typically a = 3.83 A , c = 11.5 A ) can be synthesised provided that the concentration of M is sufficiently large ( x > 0.5), and M has a preference for the Cu(1) sites (the four-coordinate sites) of the YBa 2 Cu 3 O 7 , structure. Neutron diffraction has demonstrated that a high degree of disorder exists in the (001) planes containing the M cations and, in particular, significant oxygen displacements are found. The non-existence of the unsubstituted variant YSr 2 Cu 3 O 7 is explained in terms of the high degree of compressive stress on the Cu(1) sites which could exist in such a material.

Copper oxide superconductors containing sulphate and phosphate groups

Abstract Following observations that carbonate anions can be substituted into perovskite structures containing Cu to yield superconducting phases, the possibility that other oxy-anions may behave similarly has been explored. Here we report the incorporation of sulphate and phosphate groups into the YBa 2 Cu 3 O 7−δ structure, to form materials such as YSr 2 Cu 2.79 (SO 4 ) 0.21 O 6.16 . The partial replacement of Y 3+ ions by Sr 2+ or Ca 2+ ions injects holes into the CuO 2 layers and results in a change from antiferromagnetic to superconducting behaviour with T c (onset) up to ≈60 K. These materials contain not only new charge reservoir layers but also novel superconducting CuO 2 sheets with both 4-coordinate Cu (as in Nd 2 CuO 4 ) and 5-coordinate Cu (as in YBa 2 Cu 3 O 7 ).

Superconductivity up to 64 K in the copper oxyfluorides Sr2−xAxCuO2F2+δ(A = Ca, Ba) prepared using NH4F as a fluorinating reagent

Abstract We report here a simple and efficient synthetic route to high-temperature superconducting oxyfluorides. Solid phase fluorination of Sr 2 CuO 3 with NH 4 F at relatively low temperatures produces superconducting Sr 2 CuO 2 F 2+δ ; this simple preparation route eliminates the need for F 2 gas. Importantly, as-synthesised samples from the NH 4 F route appear to have the optimally doped value of the superconducting transition temperature, T c , of 46 K. Moreover, Ba doping for Sr results in an increase in T c , for both F 2 gas and NH 4 F routes, to a maximum value of 64 K for Sr 1.4 Ba 0.6 CuO 2 F 2+δ ; this represents the highest ever recorded value for a material with the confirmed La 2 CuO 4 structure.

An improved route to the synthesis of superconducting copper oxyfluorides Sr2−xAxCuO2F2+δ (ACa, Ba) using transition metal difluorides as fluorinating reagents

Abstract We report here a simple and efficient synthetic route to high temperature superconducting oxyfluorides. Solid phase fluorination of Sr 2− x A x CuO 3 (ACa, 0 ⩽ x ⩽ 2.0; ABa, 0 ⩽ x ⩽ 0.6) with the transition metal difluorides, CuF 2 , ZnF 2 , AgF 2 , NiF 2 , at relatively low temperatures (225–250°C) produces superconducting Sr 2− x A x CuO 2 F 2+δ with a maximum T c of 64 K (for ABa, x = 0.6). This simple preparation route eliminates the need for F 2 gas, and at the same time produces negligible (Sr/A)F 2 impurity, unlike the corresponding fluorination with NH 4 F. Using this route the synthesis of the oxyfluoride, Ba 2 CuO 2 F 2+δ , is also reported for the first time. These studies and the examination of the fluorination of other cuprate systems have shown that this is a powerful and versatile fluorination route.

Oxyanion doping strategies to enhance the ionic conductivity in Ba2In2O5

In this paper we report the successful incorporation of phosphate and sulfate groups into the ionic conductor, Ba2In2O5, with the samples analysed through a combination of X-ray diffraction, NMR, TGA, Raman spectroscopy and conductivity measurements. The results show that such oxyanion incorporation leads to a conversion from an ordered brownmillerite-type structure to a disordered perovskite-type, and hence increases the conductivity at temperatures <800 °C. In wet atmospheres, there is evidence for a significant enhancement of the conductivity through a protonic contribution.

Oxygen Defects and Novel Transport Mechanisms in Apatite Ionic Conductors: Combined 17O NMR and Modeling Studies

Germanium-based apatite compounds are fast oxide-ion conductors for potential use in fuel cells. A combination of solid-state 17O NMR spectroscopy, atomistic modeling, and DFT techniques help to elucidate oxygen defect sites and novel cooperative mechanisms of ion conduction. The picture shows oxygen diffusion in the studied apatite compound from molecular dynamics simulations.

Synthesis and structure of Ba4CaCu2.24O6.96(CO3)0.5, a perovskite containing carbonate anions, and related phases

Calcination of appropriate mixtures of BaCO3, CaCO3 and CuO has been shown to result in a tetragonal [P4/mmm; a= 5.799(2)A, c= 7.992(3)A] perovskite-related phase containing CO32– anions. The unit-cell composition is Ba4CaCu2.24O6.96(CO3)0.5, and powder neutron diffraction has demonstrated that the octahedral sites are occupied by Ca, Cu and C in an ordered fashion. The structure is compared with the cubic phase Ba4CaCu3O8.61. Related phases in which the Ca ions are replaced by other cations are also reported.

BO3-3 substitutions in phases related to YBa2Cu3O7. superconducting properties of (Y, Ca) (Ba, Sr)2Cu3−z(BO3)zOy

Abstract The effects of partially replacing Cu by borate anions in the system YSr 2− x Ba x Cu 3− z (BO 3 ) z O y (YSr 2− x Ba x Cu 3− z B z O 7−δ ) have been examined. The optimum substitution level to give single-phase samples was found to be z≈0.5. Borate-substituted samples of this composition were found to be non-superconducting as prepared, but superconductivity could be induced by the partial replacement of Y by Ca (for samples with 0.5≤×≤1.5), or by high oxygen pressure treatment (for 1.0≤×≤1.5). The highest T c onset recorded for a single-phase sample was 55 K for Y 0.85 Ca 0.15 Ba 1.5 Cu 2.5 B 0.5 O 7 annealed in high pressure oxygen. The unit cell symmetry changes from orthorhombic for the Ba-free phase ( x =0)to tetragonal at higher Ba contents, which appears to be associated with a randomisation of the orientations of the borate groups.