HIGH-PRESSURE synthesis has proved a useful technique for obtaining new, metastable copper oxide superconductors; for example, oxygen insertion into Sr2CuO3 at 6 GPa (ref. 1) yields superconducting Sr2CuO3.i, with transition temperature Tc = 70 K, in which the superconducting CuO2 layers are generated by pressure-induced oxygen migration from apical to equatorial sites. Although the simple structure and high transition temperatures make this family (general formula Srn+1CunO2n + 1+ δ) of interest, the stringent synthesis conditions limit its value for applications. Here we report that fluorine insertion into Sr2CuO3 at ambient pressure causes related structural rearrangements to give superconducting Sr2CuO2F2 + δ with a maximum Tc of 46 K. In this synthesis, the structural changes previously initiated by the thermodynamic effects of high pressure are induced chemically under ambient conditions. The result is a superconducting oxy-fluoride in which fluorine plays a dominant structural role, rather than merely being an electronic dopant as in La2CuO4Fx (ref. 2) and Nd2CuO4−xFy (ref. 3).

Synthesis and superconducting properties of the strontium copper oxy-fluoride Sr2CuO2F2+δ

The structural relations between the α, β, γ and δ-phase of Bi2O3 have been investigated. The analogies between the structures of the various phases and defect fluorite structure are discussed. Conductivity and electrostatic data indicate that in the disordered fluorite related structure of the solid electrolyte δ-Bi2O3 ordering of the vacant sites in the anion sublattice occurs. The α and β-phase can be described with a disported defect fluorite structure with the vacant anion sites separated by 1/2 〈111〉f and 1/2 〈110〉f and by 1/2 〈100〉f, respectively. A model is given for the motion of the atoms in the Bismuthsublattice at the α δ transition.



Phasenbeziehungen im Wismut(III)-oxid



Die strukturellen Zusammenhange zwischen den α, β, γ und δ-Phasen von Bi2O3 sind untersucht worden. Die Ubereinstimmung zwischen diesen verschiedenen Phasen und einer defekten Fluoritstruktur wird besprochen. Leitfahigkeit und elektrostatische Daten weisen darauf hin, das in der mit ungeordnetem Fluorit verwandten Struktur des festem Elektrolyts δ-Bi2O3 eine Ordnung der Gitterleerstellen im anionischen Teilgitter stattfindet.



Die α- und β-Phasen konnen als eine gestorte defekte Fluoritstruktur aufgefast werden, in der die leeren Anionengitterstellen auf 1/2 〈111〉f und 1/2 〈110〉f beziehungsweise 1/2 〈100〉f voneinander entfernt liegen. Es wird ein Modell fur den Bewegungsvorgang der Atome im Wismutteilgitter beim Ubergang von der x-in die δ-Phase gegeben.

Phase Relations in Bismuthsesquioxide

Depending upon the temperature at which spinel oxides are deformed, different dissociation configurations of dislocations have been observed. At elevated temperatures, edge dislocations are dissociated over several hundreds angstroms by a process involving climb of partials. At low temperatures, the long screw dipoles resulting from indentation tests suggest a classical dissociation of dislocations out of the preferred glide plane. These effects are compared to calculations of stacking-fault energy in the spinel structure. A good qualitative agreement is found whatever the dissociation mechanism. Quantitative agreement is satisfactorily obtained only in so far as glide dissociation is concerned. Discrepancies between calculated and observed climb dissociation suggest a strong interaction effect with point defects. The possible effects that such a dissociation dependence upon temperature has on deformation mechanisms in spinels are discussed.

Influence of temperature on dissociation of dislocations and plastic deformation in spinel oxides

The crystal and magnetic structures of the two related phases, Sr2MnGaO5 and Ca2MnAlO5 are reported. Rietveld analysis of neutron powder diffraction has revealed that both phases adopt the brownmillerite structure. Subtle differences in structure lead to the structure of Sr2MnGaO5 being best described by the space group Icmm
(a
= 5.4888(2)
A, b
= 16.2256(6)
A, c
= 5.35450(2)
A at 2 K) while that of Ca2MnAlO5 is best described by Ibm2 (a
= 5.46258(9)
A, b
= 14.9532(3)
A, c
= 5.23135(8)
A at 2 K). Low temperature neutron powder diffraction data show that both phases have a simple antiferromagnetic structure. However, magnetisation data suggest a more complex picture
of the magnetic order within these phases.

J.W. Weenk

Papers

Calculation of electrostatic fields in ionic crystals based upon the Ewald method