Soliton Lattice in Pure and Diluted CuGeO₃

We report a synchrotron x-ray scattering study of the magnetic-field-induced incommensurate phase of the spin-Peierls compound CuGeO₃, in magnetic fields up to 13 T. By measuring first and third harmonics of the incommensurate Bragg reflections as a function of field we show that the lattice modulation has the form of a soliton lattice. The soliton half-width, 13.6 lattice spacings, is surprisingly large. Dilution of the CuO₂ spin- 1/2 chains with spin-0 (Zn) and spin-1 (Ni) impurities results in a short-range-ordered incommensurate state with an anisotropic correlation length comparable to the average impurity separation.