The study of the manganites Ln(0.5)Ca(0.5)Mn(1-x)Ni(x)O(3) (Ln = Pr, Nd, Sm) shows that the introduction of nickel on the Mn site provokes the disappearance of charge ordering (GO). But most important, it is demonstrated for the first time that doping with nickel induces a metal-insulator transition for Ln = Pr or Nd, in spite of the small size of the interpolated cations. All the compounds of these series exhibit colossal magnetoresistance (CMR) properties with resistance ratios (RR) ranging from 5.10(3) at 75 K to 10(8) at 60 K under 7 T, whereas the undoped phases either are not magnetoresistant (Ln = Pr, Sm) or exhibit much smaller RR (20 at 100 K for Ln = Nd). The magnetotransport properties of these Ni-doped manganites are compared to those of Cr- and Co-doped phases and their evolution is discussed, taking into consideration two antagonist factors: the average size of the interpolated cations and the size mismatch between Ln(3+) and Ca3+. (C) 1997 Elsevier Science Ltd.
