Nuclear magnetic resonance study of fluorine–graphite intercalation compounds and graphite fluorides

Influence of fluorination on the properties of carbon fibres

A preliminary phase diagram is given for the system La 2 CuO 4+δ , o ≤ δ ≤ 0.09, prepared electrochemically at room temperature. Room-temperature X-ray data distinguish two orthorhombic phase fields, O I for δ M for δ > 0.055. The orthorhombicity of O I decreases with increasing δ, that of O M increases with δ. A spinodal phase segregation into an oxygen-rich superconductive phase and an oxygen-poor antiferromagnetic phase occurs in the temperature interval 220K T ≤ T sp ; below 220 K, the mobility of the interstitial oxygen becomes too low for a further static phase segregation. The phase segregation causes an increase in resistivity and a decrease in the Seebeck coefficient with decreasing temperature in the interval 220 T T sp . A similar transport behavior occurs in the range T c T T ϱ ≤ 120 K in samples 0.02 ≤ δ ≤ 0.04; from comparisons with the La 2− x Sr x CuO 4 system, we suggest that at lower temperatures this behavior is the signature for a further dynamic hole segregation, induced by atomic displacements, into domains more rich in mobile holes within the superconductive phase. The diamagnetic AC susceptibility associated with this superconductivity is independent of the magnetic field below 0.01 mT. In the compositional range 0.06 ≤ δ ≤ 0.09, the Seebeck data indicate a nearly constant concentration of itinerant holes in the superconductive CuO 2 planes. We conclude that in the O M phase oxygen-oxygen interactions within the superconductive layers trap out at oxygen-atom clusters the excess mobile holes Δ P = 2 δ −0.12 from the CuO 2 planes. An H c1 = 64 mT was obtained for δ = 0.09. A step in the resistivity drop at T c signals a temperature interval T c T T on within which the superconductive pairs are confined to small domains. Whether these domains represent a second superconductive phase correlated to additional oxygen ordering that fails to develop with increasing δ or should be considered a pair-fluctuation phenomenon is not resolved.

Transport and magnetic properties of the superconducting La2CuO4+δ phases (0 < δ < 0.09) prepared by electrochemical oxidation

Graphite fluorides and carbon-fluorine compounds

In the past decade intense research activity in the area of the so-called Ruddlesden-Poper phases, especially the nickelates, has been motivated by the similarity of their structure and many of their electronic properties to the high temperature cuprate superconducting compounds. In the past few years these activities have been further intensified by the discovery of colossal magnetoresistance in the Ruddlesden-Popper manganates.

Ruddlesden-Popper Lnn+1NinO3n+1 nickelates: structure and properties

Transport and Magnetic Properties of La2NiO4+δ(0 ≤ δ ≤ 0.25)

Conductivity measurements, using Ohm's law, are obtained by a current injection and the evaluation of the associated voltage. A computer controlled d.c. resistivity equipment is described in which the injected current is optimized taking into account the non-linearity of the I versus V plot and also the parasitic contact potentials. The application of this automatic system and validity of the algorithm have been tested from 4.2 to 1100 K for various samples with resistivity extending from 10 −4 to 10 5 Ω•m Une methode generale de mesures automatisee de la resistivite est proposee, dans laquelle le courant injecte dans l'echantillon est optimise en tenant compte de la non-linearite eventuelle de la caracteristique I(V) du materiau teste et de l'existence de potentiels de contact

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E. Marquestaut

Papers

Transport and Magnetic Properties of La2NiO4+δ(0 ≤ δ ≤ 0.25)

Dispositif automatique de mesures de résistivité entre 4 et 1 100 K

Fluorine-intercalated carbon fibers-I. structural and transport properties

Electrical properties of room temperature fluorine-intercalated graphite fibers

Superconductivity in the La2CuO4-Nd2CuO4 System after Treatment under Oxidizing Conditions