C. Greaves

Cation distribution and magnetic structure of the ferrimagnetic spinel NiCo2O4

The compound NiCo2O4, with spinel-related structure, has been prepared by thermal decomposition of metal nitrates and its bulk structural properties examined by means of magnetic measurements, neutron diffraction, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The results suggest a delocalised electron distribution on the octahedral sites with average oxidation states of +3.5 and +2.5 for nickel and cobalt, respectively, and lead to a cation distribution for NiCo2O4 of {Ni3+0.1Co2+0.9}tet[Ni3.5+0.9Co2.5+1.1]octO4. This electronic configuration is consistent with magnetisation measurements if applied magnetic fields cause a charge redistribution on the octahedral sites to favour Co3+ and Ni3+. The surface of NiCo2O4 was examined by X-ray photoelectron spectroscopy (XPS) and found to have a different composition containing Co2+, Co3+, Ni2+, Ni3+ and, probably, Ni4+.

A powder neutron diffraction investigation of vacancy ordering and covalence in γ-Fe2O3

Profile refinement of powder neutron diffraction data has been used to examine the structure of γ-Fe 2 O 3 and to evaluate covalence effects for octahedral and tetrahedral Fe 3+ ions within the structure. The results imply an ordered cation vacancy distribution with tetragonal symmetry (space group P4 3 2 1 2; a = 8.3396(4) A, c = 24.966(2) A). The structural model is compared with the results of electrostatic energy calculations, and the influence of covalence on the structure is discussed.

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 ).

The synthesis and structure of the new phase Ca0.85CuO2

Abstract A new phase, Ca 0.85 CuO 2 , has been synthesised and investigated using powder X-ray and neutron diffraction. The structure is closely related to that of NaCuO 2 with chains of edge-shared CuO 4 units linking layers of octahedrally coordinated Ca ions, but partial occupancy of the Ca sites results in an incommensurate superstructure. Structure refinement based on an orthorhombic subcell (Fmmm; a =6.3206, b =2.8067, c =10.5735 A) has revealed a possible mechanism for the occurrence of the superstructure. Regions of monoclinic symmetry and composition CaCuO 2 are separated by boundaries of Ca vacancies with adjacent domains having CuO 4 chains tilted in opposite directions.

Structural relationships between Bi2O2CO3 and β-Bi2O3

Abstract The structure of Bi 2 O 2 CO 3 has been investigated using powder neutron diffraction. Not all observed peaks could be indexed on the previously reported tetragonal unit cell, and a larger orthorhombic cell was adopted (Pna2 1 , a=5.468(1), b=27.32(2), c =5.468(1) A ). The structure refinement was not totally satisfactory due to apparent orientational disorder of the CO 3 2− groups. Bi 2 O 2 CO 3 of small crystallite size decomposes at about 400°C to β-Bi 2 O 3 , which is thermodynamically unstable relative to α-Bi 2 O 3 . Structural relationships between Bi 2 O 2 CO 3 and β-Bi 2 O 3 are suggested as a means of rationalising this phenomenon.

Synthesis, structure and electrical properties of Sr2CuO2(CO3), an oxide carbonate related to perovskite

The phase Sr2CuO2(CO3) occurs as an intermediate in the reaction between SrCO3 and CuO to form Sr2CuO3. The conditions of formation have been examined, and the structure determined from time-of-flight neutron powder diffraction [tetragonal, P4/mmm; a= 3.9033(2)A, c= 7.4925(4)A]. The structure consists of alternating layers of composition [CuO2]2– and [CO3]2– perpendicular to [001], separated by layers of Sr2+ cations. The carbonate groups are all oriented to have one C—O bond along [100] or [010], and two bonds at ca. 30° to [001]. No clear evidence for long-range ordering of the CO3 groups was found.

Magnetization and resistivity in chromium doped manganites

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 > 0.2. 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.

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.

The synthesis and structural characterization of Sr2CuO4−x, x ∼ 0.1

Abstract Sr2CuO3 (orthorhombic, Immm) is converted at about 400°C under oxygen at 160 bar to Sr2CuO3.9 [tetragonal, I4 mmm ; a = 3.7907(7), c = 12.417(2) A], which is structurally related to La2CuO4. Although the formal Cu oxidation state in this material appears to be higher than any previously reported for Cu in an oxide environment (around + 3.8), powder neutron diffraction data provide no evidence for O2−2 or O−2 ions, but are consistent with a highly defective structure containing Sr, Cu, and O vacancies. The cation vacancies are in a 2:1 ratio to maintain the stoichiometry, and the O vacancies are confined to the CuO2 layers. Additional peaks in the neutron diffraction profile are indicative of an ordering process to give an extended, possibly incommensurate, structure. CuK-edge EXAFS suggests that the resulting local Cu coordination contains two very short CuO bonds (1.755 A).