The crystal structure of superconducting Na(2)CsC(60) was studied by high-resolution powder neutron diffraction between 1.6 and 425 K. Contrary to the literature, the structure at low temperatures is primitive cubic [See equation in the PDF file], isostructural with pristine C(60). Anticlockwise rotation of the C(60) units by 98 degrees about [111] allows simultaneous optimization of C(60)-C(60) and alkali-fulleride interactions. Optimal Na(+)-C(60)(3-) coordination is achieved with each sodium ion located above one hexagon face and three hexagon-hexagon fusions of neighboring fulleride ions (coordination number 12). Reduction of the C(60) molecule lengthens the hexagon-hexagon fusions and shortens the pentagon-hexagon fusions (to approximately 1.43 angstroms). On heating, Na(2)CsC(60) undergoes a phase transition to a face-centered-cubic [See equation in the PDF file] phase, best modeled as containing quasi-spherical C(60)(3-) ions. The modified structure and intermolecular potential provide an additional dimension to the behavior of superconducting fullerides and should sensitively affect their electronic and conducting properties.

The pressure dependence of the structure of solid C70 at ambient temperature has been measured to 25 GPa by energy-dispersive X-ray diffraction. The face-centred cubic structure transforms irreversibly to a rhombohedral phase at 0.35 GPa. Crystalline C70 is stable to 18 GPa when a phase change to amorphous carbon occurs. C70 has a slightly smaller compressibility than C60. The temperature dependence of the structure at ambient pressure has also been investigated by neutron diffraction. A transition to a rhombohedral phase occurs near 280 K with the f.c.c. structure coexisting at least to 200 K. The crystallographic symmetry of the lowest-temperature phase appears to be lower than rhombohedral. On warming, the rhombohedral phase is stable to 350 K. Strong static and dynamic disorder dominates the crystal chemistry of solid C70.

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The crystal and magnetic structures of the perovskite YBaCuFeO5+δ have been studied in the temperature range T = 4.2-300 K by powder neutron diffraction. In addition to the antiferromagnetic ordering transition at TN  = 442 K, a commensurate–incommensurate magnetic transition is detected at TN ′ = 190 K. Below this temperature, two sets of satellite peaks surround the (1/2,1/2,1/2) magnetic peak at d ∼ Å, collapsing into a single set of satellites below 155 K. Polarization analysis has been performed to confirm the magnetic nature of the (1/2,1/2,1/2)± satellites.