SMALL POLARONS IN UNDOPED ND2CUO4+ -DELTA

The nature and distribution of the charge carriers in undoped Nd2CuO4/-delta have been investigated by means of conductivity measurements ( 10 less than or equal to T less than or equal to 300 K) and X-ray abso rption spectroscopy (XAS). Whenever it was possible, the conductivity data were obtained at constant carrier concentration, in order to sepa rate the mobility term. In particular, for the T, pO(2) range in which charge carriers are negative (''n'' zone), it was shown that charge c arriers have to be identified with small polarons, hopping adiabatical ly. These small polarons are built up by a self-trapped electron on th e copper site, resulting in Cu(I) formation, and the hopping preferent ially occurs inside the CuO2 planes.