Showing papers by "Colin Greaves published in 2010"
TL;DR: In this article, two bismuth oxide materials containing large lanthanide and rhenium cations, Bi25La3Re2O49 and Bi25Pr3Re 2O49, have been synthesized in the stabilised face-centred cubic structure.
Abstract: Two bismuth oxide materials containing large lanthanide and rhenium cations, Bi25La3Re2O49 and Bi25Pr3Re2O49, have been synthesised in the stabilised face-centred cubic structure. They exhibit very high oxide ion conductivity but prolonged annealing at 600 °C causes a decrease in conductivity, which has been shown to relate to an order–disorder phase transformation. The details of this transition have been determined using neutron powder diffraction on annealed samples. Upon annealing, the samples undergo cation ordering to form a tetragonal phase (I4/m, a ≈ 8.7 A, c ≈ 17.4 A), which is structurally related to the unsubstituted phase, Bi28Re2O49. Neutron powder diffraction refinement reveals that the lanthanide cations enter only one of the three Bi sites in the ordered structure.
19 citations
TL;DR: The brownmillerite phase LaSrCoFeO5 has been prepared by partially reducing the parent perovskite material in 10% H2/N2 as mentioned in this paper, which is consistent with the presence of high spin Co2+ and Fe3+.
Abstract: The brownmillerite phase LaSrCoFeO5 has been prepared by partially reducing the parent perovskite material LaSrCoFeO6 in 10% H2/N2. The material crystallizes in the Icmm space group with the transition metal ions randomly distributed over the octahedral and tetrahedral sites of the structure. The material shows G-type antiferromagnetic order at room temperature which is consistent with the presence of high spin Co2+ and Fe3+. The perovskite phases LaSrCoFeO5F and LaSrCoFeO5.58 are synthesized by fluorination and room temperature air oxidation of LaSrCoFeO5, respectively. LaSrCoFeO5.56 could be prepared by quenching the sample from 1300 °C into liquid nitrogen. Neutron powder diffraction data, Mossbauer spectroscopy and magnetic susceptibility measurements suggest a G-type antiferromagnetism in these materials at room temperature due to the presence of Co3+ in the high spin state, which is not common in this type of materials.
19 citations
TL;DR: In this article, the reduction behavior, the magnetic properties and the electronic properties of K2NiF4 phases have been examined and oxygen hyperstoichiometry has been achieved in the lanthanum-rich phase La1.5Ni0.08 with retention of the tetragonal symmetry.
Abstract: The K2NiF4 phases La1.5+xSr0.5−xCo0.5Ni0.5O4(+δ) (x = 0, 0.2) have been prepared by solid state reactions and structurally characterized by X-ray and neutron powder diffraction. The reduction behaviour, the magnetic properties and the electronic properties of these materials have also been examined. Oxygen hyperstoichiometry has been achieved in the lanthanum-rich phase La1.7Sr0.3Co0.5Ni0.5O4.08 with retention of the tetragonal symmetry. The excess oxygen occupies the ideal interstitial (0, 0.5, 0.25) sites of the tetragonal structure. The materials withstand reducing conditions (10% H2–N2) up to 800 °C via reduction of the B-site oxidation state to the divalent state (Ni2+/Co2+) and formation of oxide-ion vacancies within the equatorial planes of the structure. Formation of Ni1+ in these materials under reducing conditions is suggested to account for the oxygen stoichiometry and the magnetic behaviour of La1.5Sr0.5Co0.5Ni0.5O3.70. The electrical conductivity of the oxygen-rich semiconductor materials reaches a SOFC applicable limit (>100 S cm−1) at elevated temperatures.
14 citations
TL;DR: The replacement of Co by Fe and Ni may enhance the instability in the low-temperature regime and the influence of the substitution is also reflected in the structure and magnetic behaviour of the doped samples.
Abstract: This paper discusses the effects of cation substitutions on the structural (and linked electronic) transition which has been observed in Na0.63CoO2. The effects of the following substitutions are reported: Ca on the Na site; Fe and Ni on the Co site. Ca doping suppresses the transition and is suggested to interfere with the Na ordering and hence causes a variation in the electronic structure. Fe and Ni doped samples all show transitions, but the transition temperature decreases with the dopant cation concentration. This implies that the replacement of Co by Fe and Ni may enhance the instability in the low-temperature regime. The influence of the substitution is also reflected in the structure and magnetic behaviour of the doped samples.
9 citations
TL;DR: In this article, the structure and ionic conductivity of fluorite-related Bi 20 Ca 7 NbO 39.5, Bi 10.75 Ca 4.375 GaO 22 and the high temperature form of Bi 9 ReO 17, formed by quenching from 800 −°C, were studied by neutron powder diffraction, X-ray powder diffusion and impedance spectroscopy.
5 citations
01 Mar 2010
TL;DR: In this article, the 57Fe Mossbauer spectrum was recorded from a material of identical composition synthesised by a solid state reaction during the course of this work and the results were consistent with crystal structure determinations and the formulation of apuanite as Fe42+Fe163+Sb16O48S4.
Abstract: Iron-57 Mossbauer spectra have been recorded from three minerals containing both iron and antimony. Schafarzikite of composition FeSb2O4 contains Fe2+. The 121Sb Mossbauer spectrum shows only the presence of Sb3+. The 57Fe Mossbauer spectrum corresponds with that recorded from a material of identical composition synthesised by a solid state reaction during the course of this work. Apuanite of formulation Fe20Sb16O48S4 contains both Fe2+ Fe3+ in the ratio 1:3.35 The result is consistent with crystal structure determinations and the formulation of apuanite as Fe42+Fe163+Sb16O48S4. Versiliaite of composition Fe12Sb12O32S2 contains Fe2+and Fe3+ in the ratio 1:2.12 and, also consistent with structural characterisations, can be formulated Fe42+Fe83+Sb123+O32S2.
5 citations
TL;DR: A new bismuth oxide sulfate of stoichiometry Bi14O20(SO4) was synthesized from thermal treatment of an intimate mixture of α-Bi2O3 and (NH4)2SO4 in the mole ratio 1:0.14 as discussed by the authors.
Abstract: A new bismuth oxide sulfate of stoichiometry Bi14O20(SO4) has been synthesized from thermal treatment of an intimate mixture of α-Bi2O3 and (NH4)2SO4 in the mole ratio 1:0.14, and its structure determined using a combination of electron diffraction, X-ray powder diffraction, and neutron powder diffraction. The body-centered tetragonal unit cell (I4/m; a = 8.664(1) A, c = 17.282(2) A) is a commensurate superstructure of the cubic fluorite subcell of δ-Bi2O3 (a0): a = ( /2)a0, c = 3a0. The structure contains discrete sulfate anions at the unit cell corners and body center, and a Bi−O framework of linked BiO4e (e = lone pair of electrons) square pyramids and trigonal bipyramids. The square pyramids are aggregated into clusters of composition Bi6O8, comprising O2- ions arranged in the form of a cube, the faces of which are capped by Bi3+ ions. In contrast, the trigonal bipyramids are corner linked to form layers perpendicular to [001]. The structure is thought to represent just one of several related phases ...
3 citations
TL;DR: The n = 2 Ruddlesden-Popper phases LaSr2CoMnO7 and La1.2Sr1.8CoMoNO7 were prepared by a sol-gel method from acetic acid solutions of stoichiometric amounts of La2O3, SrCO3, Co(OAc)2, and Mn(Oac)2 (calcination in flowing oxygen at 1350 °C, 18 h).
Abstract: The n = 2 Ruddlesden—Popper phases LaSr2CoMnO7 and La1.2Sr1.8CoMnO7 are prepared by a sol—gel method from acetic acid solutions of stoichiometric amounts of La2O3, SrCO3, Co(OAc)2, and Mn(OAc)2 (calcination in flowing oxygen at 1350 °C, 18 h).