Modulated charge ordering process in Sm_0.5Ca_0.5Mn_1–xCr_xO₃ manganites

Abstract

The oxides Sm_0.5Ca_0.5Mn_1–xCr_xO₃ , with 0≤x≤0.07, have been studied by electron diffraction and electron microscopy versustemperature from 90 K to 300 K, in connection with their magnetic and transport properties. An incommensurate charge ordering appears atT_CO=275 K for the undoped compound, then a progressive transition from an incommensurate to a commensurate modulated structure is observed as Tdecreases from 275 K to 170 K. But the most important point is that chromium doping enhances the incommensurability of the structure: theq vector of the low temperature form decreases and T_CO decreases abruptly as the chromium content x increases, so that the charge ordering process is weakened and finally disappears for x=0.05. The structure of the low temperature form of these manganites is characterized by a double supercell, 2a_p √2×2a_p ×a_p √2, similar to that observed for other Ln_0.5Ca_0.5MnO₃ manganites with Ln=La, Pr, Nd, Tb. However, in these series, one observes similar space groups (P2mmorPmmm) for Ln=Nd, Sm, different from those observed for Ln=La, Tb.

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