Showing papers by "Colin Greaves published in 1999"
TL;DR: In this paper, the magnetic and electrical properties of sintered manganites of the form of manganite with chromium were investigated and it was shown that the peak in the conductivity moves rapidly to lower temperatures as the chromium content is increased and is completely absent for x > 0.
Abstract: We report measurements of the magnetic and electrical properties of sintered manganites of the form We find that the peak in the conductivity moves rapidly to lower temperatures as the chromium content is increased and is completely absent for x > 02 The Curie temperature, as determined by magnetization measurements, is much less strongly suppressed, disappearing at We interpret this behaviour in terms of the very different effects of the chromium in lifting spin and orbital degeneracy
55 citations
TL;DR: Rietveld structure refinement of the X-ray powder diffraction data recorded from tin-doped Fe============ 3======O============ 4============ prepared by heating metallic iron, α-iron(III) oxide and tin(IV) oxide in sealed evacuated quartz ampoules shows the tin to substitute on the octahedral sites of the inverse spinel-related structure as mentioned in this paper.
Abstract: Rietveld structure refinement of the X-ray powder diffraction data recorded from tin-doped Fe
3
O
4
prepared by heating metallic iron, α-iron(III) oxide and tin(IV) oxide in sealed evacuated quartz ampoules shows the tin to substitute on the octahedral sites of the inverse spinel-related structure. Tin-doped γ-Fe
2
O
3
, prepared by boiling a precipitate derived from iron(III), iron(II) and tin(II) under reflux, also adopts an inverse spinel-related structure containing tin on the octahedral sites and, compared to pure γ-Fe
2
O
3
, is stabilised with respect to conversion to α-Fe
2
O
3
. 57
Fe Mossbauer spectroscopy shows the Neel temperature for γ-Fe
1.9
Sn
0.1
O
3
to be between 750 and 820 K. Tin-doped α-Fe
2
O
3
prepared by hydrothermal methods is shown by Rietveld structure refinement of the X-ray powder diffraction data to contain tin in both the substitutional and interstitial octahedral sites of the corundum-related structure.
31 citations
TL;DR: In this article, the authors describe the synthesis of three new oxide fluorides via processes which involve anion insertion, anion substitution, and cation extraction, and the use of a related strategy to incorporate more complex anions into oxides.
Abstract: The low temperature fluorination of low dimensional mixed metal oxides can be achieved in a variety of ways to provide new structures and modified electronic characteristics The potential of these methods for the synthesis of new materials is considered, with particular reference to the fields of copper oxide superconductors and layered manganese oxide colossal magnetoresistance (CMR) phases; in both areas, careful structural and electronic control is necessary for optimised properties The fluorination mechanisms are quite complex, and this paper describes the synthesis of three new oxide fluorides: La
19
Sr
11
Cu
2
O
5
F
2
, PbSr
2
YCu
3
O
6
F
4
and La
12
Sr
18
Mn
2
O
7
F
2
, via processes which involve anion insertion, anion substitution and cation extraction The use of a related strategy to incorporate more complex anions into oxides is also discussed, and the assimilation of SO
4
2–
ions into Bi
2
O
3
to give phases of composition Bi
2
O
3–x
(SO
4
)
x
is described
26 citations
01 Jan 1999
TL;DR: Tin-doped a-Fe2O3 prepared by hydrothermal methods contains tin in both interstitial and substitutional octahedral sites in the corundum-related matrix.
Abstract: Tin-doped Fe3O4 prepared by solid state reactions is shown by X-ray powder diffraction and extended X-ray absorption fine structure to contain tin in the octahedral sites of the inverse spinel-related structure. In situ 57Fe Mossbauer spectroscopy performed at elevated temperatures in vacuo shows the Curie temperature to decrease with increasing concentrations of tin and, after prolonged heating at elevated temperatures, the partial segregation of tin to form tin dioxide at the magnetite surface. Thermal treatment under oxidising conditions causes the oxidation of tin-doped Fe3O4 to tin-doped γ-Fe2O3 and tin-doped α-Fe2O3, Tin-doped γ-Fe2O3 prepared by precipitation techniques also contains tin in the octahedral sites of the inverse spinel-related structure. 57Fe Mossbauer spectroscopy at elevated temperatures has been used to derive a Neel temperature for tin-doped γ-Fe203 and to examine the stabilising influence of tin on the conversion of γ-Fe2O3 to a-Fe2O3. Tin-doped a-Fe2O3 prepared by hydrothermal methods contains tin in both interstitial-and substitutional-octahedral sites in the corundum-related matrix. The defect structure is unusual and is rationalised by crystallographic-and chemical-arguments.