Institute of Materials Science, University of Tsukuba^* ISIS Facility, CLRC Rutherford Appleton Laboratory, U.K.^**
○Miwako Takahashi^* Matthias J Gutmann^**

It is well known that Sn undergoes a phase transition from its usual metallic phase (beta phase) to lower temperature phase (alpha phase). Alpha Sn has the diamond structure and is a semiconductor. Beta Sn has a body-centered tetragonal structure and is metallic. The thermodynamic temperature of the transition is 13C at atmospheric pressure, however, beta Sn dose not transform readily into alpha Sn below 13C. The rate of transition only becomes significant after considerable undercooling, and long periods are necessary to allow alpha Sn to nucleate. Up to now, a detailed microscopic description of this phase transition has not been available. The aim of our study is to understand the mechanism and kinetics of the transition. Neutron diffraction experiments were performed with the beta Sn single crystals to observe a change in diffraction pattern for the beta to alpha phase transition. In the measurement, characteristic rod-like diffuse scattering has been observed in the beta phase. The detail structure and temperature dependence were investigated by using four-circle single crystal diffractometer FONDER at JRR-3M. It was found that the diffuse intensities strongly depend on temperature and distributed around the z.b., thus the origin is considered as temperature diffuse scattering (TDS) with two-phonons process. The intensity distribution was calculated for the 2nd TDS and was found to reproduce the observed diffuse scattering well. The time dependence of the structure in the process of transition was also investigated by using pulsed single crystal diffractometer SXD at ISIS. The results of the structural analysis will also be presented.