STACKING FAULTS IN HEXAGONAL C₇₀ SINGLE CRYSTALS STUDIED BY DIFFUSE X-RAY SCATTERING. D. Schwarzenbach⁽¹⁾, E. Blanc⁽¹⁾, R. Restori⁽¹⁾, H.-B. Bürgi⁽²⁾ and P. Ochsenbein⁽²⁾, ⁽¹⁾Institute of Crystallography, University of Lausanne, CH-1015 Lausanne, Switzerland; ⁽²⁾Laboratory of Crystallography, University of Bern, Freiestr. 3, CH-3012 Bern, Switzerland

Pure C₇₀ crystallizes in close-packed crystal structures. Both the hexagonal close packing (hcp) and the cubic close packing (ccp) are known to exist. Most crystals at our disposition (Hoechst Chemical Company) were hexagonal prismatic. Their X-ray diffraction photographs show Bragg-like peaks, primarily in places expected for the hcp ordered material, but also in places expected for the ccp ordered material. The peaks are connected by diffuse rods of intensity which are best observed along the 101 l, 202 l and 213 l lines of the reciprocal lattice. Qualitatively, this indicates the presence of domains of both stacking types [1,2]. The intensities along these three diffuse rods were measured on a single-crystal diffractometer of the Swiss-Norwegian Beam Line at ESRF, Grenoble. The diffuse squared structure amplitude is the product of the squared structure amplitude of a single molecule and an interference function typical of the stacking sequence. The former was calculated assuming a librating spinning top [3]. The latter was calculated as a function of two probabilities, [[alpha]] that during crystal growth a layer is deposited in a c-position on a c-stack (e.g. ...ABC continues with A), and [[beta]] that a layer is deposited in a c-position on an h-stack (e.g. ...ABA continues with C). The corresponding Markov chain was solved explicitely. Quantitative agreement with the observations was obtained with [[alpha]] = 0.77(2) and [[beta]] = 0.17(1). We conclude that an h-stack continues to grow preferentially as an hcp domain, while a c-stack prefers to grow as a ccp domain. In addition, ccp domains are on average smaller. This work has been accepted for publication in Europhysics Letters.

[1] van Tendeloo G. et al. (1993). Europhys. Lett., 21, 329 - 334.

[2] Muto S. et al. (1993). Philos. Mag., B67, 443 - 463.

[3] Blanc, E. et al. (1994). Europhys. Lett., 27, 359 - 364.