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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.

The infinite-layer compound CrCl3 is found to undergo a first-order phase transformation near 240°K. The crystal structures of the high- and low-temperature phases have been elucidated by detailed x-ray diffraction and 35Cl nuclear quadrupole resonance studies. The previously reported structure for CrCl3 is shown to be incorrect. The present single-crystal diffraction results at 298° and 225°K give monoclinic (C2/m: a0=5.959 Å, b0=10.321, c0=6.114 Å, β=108.49°) and rhombohedral (R3̄: a0=5.942 Å, c0=17.333 Å) structures, respectively. The 35Cl quadrupole resonance frequencies, measured in polycrystalline samples, have been compared with results of field-gradient calculations based on the ionic point-charge model. The ionic contribution accounts for a large fraction of the total gradient making it impossible to evaluate the covalency parameters of the Cr–Cl bonds from the measured quadrupole coupling constants.

Results from structure refinement using neutron powder diffraction data for boron carbide samples with 10, 13, 16, and 20 at. % carbon are reviewed. Those obtained for ceramic powder samples show an apparent large vacancy concentration (as high as 25%) at the central atom position of the linear three-membered chains. A model deduced from the previous X-ray structure of the boron-rich end member, B 9 C, suggests the shift of sufficient scattering density from the central chain position into adjacent voids to account for the observed vacancies. On the other hand, neutron powder diffraction from ground, Cu-melt-grown single crystals gives a structure which shows no such vacancies. This, together with results from sequential runs with the same ceramic sample, suggests that the vacancies in the ceramic materials are intrinsic to their synthesis and do not result from radiation damage, as has been suggested. Our results support the more recent interpretation of other experimental results concerning the nature of the three-membered C-B-C chains in the boron carbides, i.e., that the bonding to the central atom is very weak. Further, low-temperature structural studies of the ground single crystals suggest that even at the nominal B 4 C composition, there is site disorder at the chainend "C" site. It is most likely that this disorder arises from the presence of boron (or C-B-B) chains.

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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.

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The room-temperature structure of A12TiOs has been re-examined and is compared with the 600°C structure. This material is isomorphous with pseudobrookite, crystallizing in space group Bbmm with cell dimensions a0= 9-429, b0= 9.636, and co = 3"591 A at room temperature and a0 = 9-481, b0 = 9.738, and c0=3"583 A at 600°C. Our results, as well as refinement of previously published data, indicate that there is complete disorder in the metal sites for this compound. Provided that allowances are made concerning electrostatic interactions, the differences in the structure at the two temperatures appear to support the suggestion that the coordination polyhedra about the metal ions tend towards more regular configurations with increasing temperatures. The importance of cleavage planes, resulting from edge-shared octahedra in the hysteresis of the thermal expansion of the ceramic, is pointed out.