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The reciprocal lattices of a range of rare earth titanate and zirconate pyrochlores have recently been shown to exhibit an extremely characteristic diffuse intensity distribution identical in form to that characteristic of b-cristobalite implying the existence of b-cristobalite-like orientational disorder of essentially rigid, (O2)(R 3þ ) 4 tetrahedra within the pyrochlore structure type. A single crystal X-ray diffraction study of La 2 Zr 2 O 7 has been carried out utilizing synchrotron radiation in order to investigate this proposed orientational disorder and to determine an unknown oxygen atom fractional co-ordinate. An annular distribution of residual electron density is found around the average La ion positions in f111g planes perpendicular to the local O2--La--O2 directions and interpreted as implying that the La ions never in fact occupy the average 16d positions but rather are dynamically disordered around an annulus perpendicular to that direction in an exactly analogous manner to that which occurs in b-cristobalite.

The radial distribution function of a new form of amorphous diamond, synthesized from C60 fullerene by shock compression and rapid quenching, was obtained from an electron difFraction pattern using an imaging plate. The present amorphous diamond was essentially amorphous material based on difFractometry. Analysis of the local structure around an atom revealed that the amorphous material was distinguishable not only from sp -based amorphous material but also from some tetrahedrally coordinated amorphous carbon and other amorphous semiconductors previously reported. The carbon atoms in the present material were tetrahedrally coordinated, forming regular tetrahedra that additionally were arranged in the same manner as those of crystalline diamond within a unit-cell size region. The implications of producing the new form of amorphous diamond from C60 fullerene also are discussed in the present paper.