Abstract
We report comprehensive synchrotron x-ray scattering and magnetic susceptibility studies of the doped spin–Peierls materials Cu1−xZnxGeO3 and CuGe1−ySiyO3. Temperature versus dopant concentration phase diagrams are mapped out for both Zn and Si dopants. The phase diagrams of both Cu1−xZnxGeO3 and CuGe1−ySiyO3 closely resemble that of Cu1−xMgxGeO3, including the observation that the spin gap is established at a much higher temperature than the temperature at which the spin–Peierls dimerization attains long-range order. The spin–Peierls transitions in doped samples exhibit unusual phase transition behavior, characterized by highly rounded phase transitions, Lorentzian squared lineshapes, and very long relaxation times. Phenomenological explanations for these observations are given by considering the effects of competing random bond interactions as well as random fields generated by the dopants. We have also confirmed the reentrance of the spin–Peierls phase when the temperature is lowered through the antiferromagnetic ordering transition. The low temperature re-entrance of the spin–Peierls phase has been explained speculatively using a local phase separation scheme between the spin–Peierls phase and the Néel phase.
