MODULATED CHARGE ORDERING PROCESS IN SM0.5CA0.5MN1-XCRXO3 MANGANITES

The oxides Sm0.5Ca0.5Mn1-xCrxO3, with 0 less than or equal to x less t han or equal to 0.07, have been studied by electron diffraction and el ectron microscopy versus temperature from 90 K to 300 K, in connection with their magnetic and transport properties. An incommensurate charg e ordering appears at T-CO=275 K for the undoped compound, then a prog ressive transition from an incommensurate to a commensurate modulated structure is observed as T decreases from 275 K to 170 K. But the most important point is that chromium doping enhances the incommensurabili ty of the structure: the q vector of the low temperature form decrease s and T-CO decreases abruptly as the chromium content x increases, so that the charge ordering process is weakened and finally disappears fo r x=0.05. The structure of the low temperature form of these manganite s is characterized by a double supercell, 2a(p) root 2 x 2a(p) x a(p) root 2, similar to that observed for other Ln(0.5)Ca(0.5)MnO(3) mangan ites with Ln=La, Pr, Nd, Tb. However, in these series, one observes si milar space groups (P2mm or Pmmm) for Ln=Nd, Sm, different from those observed for Ln=La, Tb.