Charge ordering and spin- Peierls transition in alpha'-NaV2O5

Recent X-ray analysis and NMR- experiments have shown that alpha'- NaV2O5 is not a conventional spin-Peierls (SP) compound because above the transition at 33K it is in a homogeneous mixed valent (MV) state and atomic spin chains do not exist. Furthermore, thermal expansion experiments have identified the existence of a double phase transition. We present a theoretical model which qualitatively explains these observations. We derive an effective Hamiltonian for the insulating state which describes the transitions as primary charge order leading to linear spin chains in b- direction and a secondary SP- transition. We show that this scenario explains in a natural way the anomalous BCS-ratio observed in alpha'- NaV2O5.