X-RAY DIFFRACTION STUDY OF PRESSURE- INDUCED POLYMERIZATION IN SIMPLE MOLECULES WITH TRIPLE BONDS. K. Aoki, H. Yamawaki, M. Sakashita, H. Fujihisa, National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan

Molecules with multibonds often undergo polymerization under pressure. Intramolecular covalent bonding, which is stronger by a factor of several tens than intermolecular bonding, is influenced slightly at modest pressures. However, when molecules are forced to approach beyond threshold distances at sufficiently high pressures, new bonding are formed between adjacent molecules with reconstruction of chemical bonding. Acetylene with a C-C triple bond is one example, which shows successive polymerizations with rehybridization from sp to sp2 and finally sp3. The knowledge of the molecular arrangement at polymerization pressure is essential for understanding reaction mechanism, and hence structural investigation of molecular solid by x-ray diffraction is required.

We have started x-ray diffraction study of simple molecules with triple bonds, which are already found to polymerize under pressure. Attention has carefully been paid for optimization of incident beam intensity and preparation of specimen inside a DAC. Radiation of too much strong x- ray immediately induces polymerization and molecules tend to grow large domains. Preliminary powder diffraction measurement on acetylene with a IP detector gave Debye rings but not sufficiently good for intensity analysis. Mixing of amorphous boron with the specimen is being attempted to prevent particle growth. Besides powder diffraction, we have recently started a single-crystal diffraction measurement with a four-circle device. The results will be presented.