Figure: Two spin-Peierls domains with different phases of dimerization and domain boundary (red).
Spin-Peierls transition is a transformation of a crystal containing spin S=1/2 chains with a doubling of a crystal lattice period along the magnetic chains. Below the transition temperature the magnetic susceptibility of the crystal drops due to the opening of the spin gap caused by the dimerization of the spin chains. The instability is a collective phenomenon in the interacting spin and elastic subsystems.
Two spin-Peierls states with the different order of the spin chains dimerization are degenerated in the energy of the ground state. Thus the domains of the spin-Peierls phase are possible (see Figure). This domains are defects of the singlet spin-Peierls state and should give the contribution to the residual magnetic susceptibility and ESR signals. We ascribe the regular ESR signals of the pure spin-Peierls crystals CuGeO3 to the domain walls between the spin Peierls domains.