Charge ordering-disordering in the Th-doped CaMnO3

The structural transitions that appear in the manganites Ca1-xThxMnO3 versus temperature are studied in connection with their magnetic and transport properties; and compared to those of the Ca(1-x)Ln(x)MnO(3) manganites. An orthorhombic to monoclinic transition is observed for low x values (x approximate to 0.08); this structural distortion, also observed for Ln-doped oxides, is related to the magnetoresistance properties. For higher x values (0.15 less than or equal to x less than or equal to 0.30), modulated commensurate and incommensurate phases are obtained at low temperature, with a = 1/qa(p)root 2, b = 2a(p) and c = a(p)root 2, which are related to Mn3+/Mn4+ charge ordering (CO) phenomena. T-CO values,determined from electron diffraction,, are in agreement with those determined from the M(T) curves. The low temperature electron microscopy shows that the CO in those oxides is more complex than in Ln-doped manganites. In particular, the destabilisation of CO and consequently of the antiferromagnetic interactions is evidenced as the thorium content increases which may explain the appearance of a spin-glass like behavior for higher x values (x approximate to 0.25 - 0.30) not seen for Ca1-xSmxMnO3 phases (x approximate to 0.50 - 0.60).