ELECTRON-DIFFRACTION STUDY OF THE INORGANIC SPIN-PEIERLS SYSTEM CUGEO3

An electron diffraction study of an inorganic spin-Peierls system CuGe O3 was performed. Weak superlattice reflections indexed by (h/2)k(l/2) (h, k, l: odd) are observed in electron diffraction patterns taken at about 10 K with the [100] electron incidence. The reflections are inte rpreted as characteristic of a superlattice structure of the space gro up Bbcm or Bbmm with cell dimensions a' congruent-to 2a, b' congruent- to b and c' congruent-to 2c and with dimerization of Cu atoms along th e quasi-one-dimensional [001] or the [010] direction, which is strong evidence of the spin-Peierls transition. Strong diffuse streaks which run along the c-direction through the fundamental reflections are obs erved at the same temperature. The streaks are attributed to low-frequ ency transverse acoustic waves of the oxygen octahedra and can be prec ursors to another phase transformation below the spin-Peierls transiti on.