X-ray Scattering and Magnetic Susceptibility Study of doped CuGeO₃

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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 Neel phase.