Superconducting samples of LazCu04+& are shown by neutron powder diffraction to consist of two nearly identical orthorhombic phases. The primary phase has a stoichiometry near La2Cu04. The second phase is an oxygen-rich phase that is superconducting.The abundance of the second phase increases with the oxygen pressure at which the samples are annealed. Neutron-diffraction measurements as a function of temperature show that the phase separation occurs reversibly near 320 K.

Recent advances in the design and synthesis of organic synthetic metals have yielded materials that have the highest superconducting transition temperatures (T(c) approximately 13 kelvin) reported for these systems. These materials have crystal structures consisting of alternating layers of organic donor molecules and inorganic anions. Organic superconductors have various electronic and magnetic properties and crystal structures that are similar to those of the inorganic copper oxide superconductors (which have high T(c) values); these similarities include highly anisotropic conductivities, critical fields, and short coherence lengths. The largest number of organic superconductors, including those with the highest T(c) values, are charge-transfer salts derived from the electron donor molecule BEDT-TTF or ET [bis(ethylenedithio)-tetrathiafulvalene]. The synthesis and crystal structures of these salts are discussed; their electrical, magnetic, and band electronic structure properties and their many similarities to the copper oxide superconductors are treated as well.

Molecular species were found to diffuse readily into the octahedral interstitial sites of the fee lattice of C 60 . The 13 C NMR spectrum of C 60 under magic angle spinning (MAS) conditions consisted of a primary resonance at 143.7 ppm and a minor peak shifted 0.7 ppm downfield. The downfield shift obeys Curie's law and is attributed to the Fermi-contact interaction between paramagnetic oxygen molecules and all 60 carbon atoms of rapidly rotating adjacent C 60 molecules. Exposure of C 6 o to 1 kbar oxygen for 1.75 h at room temperature resulted in a spectrum of seven evenly spaced resonances corresponding to the filling of 0 to 6 of the adjacent octahedral interstitial sites with oxygen molecules. The distribution of site occupancies about a C 60 molecule provided evidence that the intercalation process is controlled by diffusion kinetics. Exposure to 0.14 kbar hydrogen gas at room temperature for 16 h filled a substantial fraction of the interstitial sites of C 60 and C 70 with hydrogen molecules.

The structure of superconducting La,CuO,,o,, has been determined by singlecrystal neutron diffraction data. The excess oxygen, 0 4 , is located between two adjacent La01 layers. The presence of 04 distorts the apical-oxygen 01 sublattice in such that each 0 4 forms a short bond (1.64A) with one 01, which indicates the formation of a strong covalent bond between these two atoms. The Cu and 0 2 sublattices as well as that of La are not affected by 0 4 , however, 1.6% of the La cations increase their coordination from 9 to 10.

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