Orientational disorder as a function of temperature in the clathrate structure of hydroquinone and C60

In the compound [C(6)H(6)O(2)](3)C(60), hydroquinone (C(6)H(6)O(2)) forms a three-dimensional hydrogen-bonded network enclosing roughly spherical cages with point symmetry 3;m and a diameter of 13.2 A at 100 K. Although C(60) fits tightly into these cages, it shows threefold orientational disorder, the molecular site symmetry being 2/m. The disorder has been studied with single-crystal Mo Kalpha X-ray data at four temperatures, 100, 200, 293 and 373 K. In the refinement, C(60) was restrained to the icosahedral molecular symmetry m3;5; and to rigid-body translational and librational displacements including third- and fourth-order cumulants to account for curvilinear atomic movements, R(|F|) = 3.2-4.7%. At 100 K, bond lengths in C(60) refine to the expected values 1.450 (1) and 1.388 (1) A. The ratio of these values increases with increasing temperature, but the radius of the molecule remains constant at 3. 537 (2) A. The r.m.s. libration amplitudes of C(60) are relatively small (5.5 degrees at 373 K) and the probability function of orientations of C(60) inside the cage shows large values only at the refined positions, indicating that the energy barrier of reorientation is large. Refinement of an ordered twinned structure was unsuccessful; the orientations of neighboring C(60) appear to be uncorrelated.

Anomalous scattering study of the bi distribution in the 2212 superconductor: implications for cu valency.

The distribution of the bismuth atoms over the cation sites in the 2212 Bi-Sr-Ca-Cu-O superconductor has been determined by anomalous scattering synchrotron crystallography. The analysis of reflection pairs measured at wavelengths of 0.9243 and 0.9600 angstrom shows a delocalization of the bismuth atoms over the calcium and strontium sites. The "mixed" plane between the CuO(2) layers contains 6.0(1.4) percent bismuth (where the number in brackets represents the statistical standard deviation derived from the least-squares refinement of the data), and a much smaller amount of strontium than often assumed. The strontium deficiency is charge-compensated by the creation of electron holes in the CuO(2) layer. The result supports the view that neither extra oxygen nor overlap of the bismuth 6p and copper 3d bands is needed to account for the holes, which are an essential feature of the superconductivity mechanism.