STRUCTURAL EVIDENCE FOR A SPIN PEIERLS GROUND-STATE IN THE QUASI-ONE-DIMENSIONAL COMPOUND CUGEO3

X-ray and elastic neutron scattering measurements reveal that the quasi-one-dimensional Cu2+(S = 1/2) antiferromagnet CuGeO3 undergoes a second order structural transition towards a dimerized ground state at the same temperature, T(SP) = 14 K, at which the magnetic susceptibility abruptly decreases. X-ray diffuse scattering measurements show that the critical fluctuations are quasi one dimensional on a large temperature range above T(SP). These results show unambiguously that CuGeO3 undergoes a true spin-Peierls transition triggered by the magnetic chains.