X-ray diffraction evidence for the formation of a discotic phase during graphitization

The low-angle peak with a spacing of 10 Å observed in X-ray diffraction scans of partially pyrolyzed aromatic molecules is attributed to the formation of a liquid crystalline pre-graphitic phase that we identify as discotic. This conclusion is substantiated using data obtained for partially pyrolyzed 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). The crystalline reflections of PTCDA are replaced by amorphous scattering during pyrolysis at 530°C. In addition to the commonly observed amorphous carbon peak providing an interlayer spacing of about 3.5 Å, a low-angle peak corresponding to a 10 Å spacing is observed in partially pyrolyzed PTCDA. Based on simulation of structure and diffraction patterns using molecular dynamic calculations, this peak is attributed to the distance between columns of rotationally disordered, partially pyrolyzed PTCDA molecules in a discotic phase.