Identification of a growth defect in solid C 60 by electron diffraction

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X.Q. Wang, D.A. Ricketts-Foot, A.R. McGhie, and W.J. Romanow Laboratory for Research on the Structure of Matter, The University of Pennsylvania, Philadelphia, Pennsylvania 19104-6272 (Received 29 May 1991; accepted 18 September 1991)

The origin of an anomalous sawtooth-shaped feature in x-ray powder diffraction of solid Cgo is explained via electron diffraction analysis. Films sublimed on holey carbon crystallize with close-packed (111) planes parallel to the surface. Rods of diffuse scattering are found along the (111) axis normal to the surface but not along other (111) axes. Powder averaging of these rods, coupled with the x-ray form factor of spherical shells with 3.5 A radius, accounts for the sawtooth feature. We attribute this phenomenon to planar defects parallel to close-packed layers, which form during the growth of solid C60 by sublimation. A possibly related consequence of the growth mode is the observation of strong macroscopic (111) preferred orientation in films sublimed on a variety of substrates.

I. INTRODUCTION Since the publication of a simple method to produce C6o in sufficient quantities for a wide spectrum of experiments, 1>2 significant scientific interest has been generated over this novel form of carbon. The first diffraction study of solid C6o indicated a faulted hexagonal-closepacked (hep) structure.1 It is now widely accepted that the correct structure is face-centered cubic (fee).3'4 Anomalous absences of some normally intense fee reflections have been correlated with the unusual form factor of a spherically averaged shell of charge, namely the zero-order spherical Bessel function jo(QR). This function has coincidental zeros at or near the spatial frequencies of the 200, 400, etc. reflections when R = 3.53 A and a = 14.12 A.4'5 This form factor provides an efficient way to model an fee lattice of randomly oriented C60 molecules.6'7 The structure transforms to simple cubic below 249 K because the molecules are no longer randomly oriented and the 4 molecules per 14.12 A cube are no longer equivalent.5 In this paper we address the identity of a diffraction feature that is not explained by the model described above. This anomalous feature is consistently observed in powder profiles from all solution-grown and sublimed samples, independent of temperature and hydrostatic pressure,4'5 and apparently prompted the hep indexing reported in Ref. 1. By varying the tilt angle in electron diffraction, we observe a rod of diffuse scattering whose geometry in reciprocal space is entirely consistent with the anomalous powder x-ray feature. Detailed examination of the diffuse rod also gives some clues about the growth mode of solid C60 by vacuum sublimation. J. Mater. Res., Vol. 7, No. 2, Feb 1992 http://journals.cambridge.org

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Figure 1 shows part of a high-resolution profile from solution-grown pure C60 powder.5 The sharp peak at Q = 2ir/d = 0.771 A"1 is the fee 111 Bragg reflection. The 5x blowup clearly shows that this "normal" reflection is superposed on a diffuse feature w