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

An electron diffraction study of an inorganic spin-Peierls system CuGeO3 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 interpreted as characteristic of a superlattice structure of the space group 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 the 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 observed at the same temperature. The streaks are attributed to low-frequency transverse acoustic waves of the oxygen octahedra and can be precursors to another phase transformation below the spin-Peierls transition.