Role of the A-site size and oxygen stoichiometry in charge ordering commensurability of Ln0.50Ca0.50MnO3 manganites

The commensurability of the charge ordering process of the ${\mathrm{Ln}}_{\mathrm{0.50}}{\mathrm{Ca}}_{\mathrm{0.50}}{\mathrm{MnO}}_{\mathrm{3}}$ manganites ($\mathrm{Ln=Pr},$ Nd, Sm, Eu, and Gd) has been studied by electron diffraction and lattice imaging versus temperature. For the as-synthesized phases, it is shown that an incommensurate state at low temperature (92 K) is favored as the size of the lanthanide increases. A large transition width ($\mathrm{\Delta T}$ ranging from 40 to 110 K) is observed which is closely related to the size of ${\mathrm{Ln}}^{\mathrm{3+}},$ and also to the size mismatch between ${\mathrm{Ln}}^{\mathrm{3+}}$ and ${\mathrm{Ca}}^{\mathrm{2+}}$ cations. The most important result concerns the effects of oxygen stoichiometry, varied via different annealings carried out at low temperature $\text{(T⩽600\hspace{0.17em}°}\mathrm{C})$ under ${\mathrm{Ar/H}}_{\mathrm{2}}$ flow, air or ${\mathrm{O}}_{\mathrm{2}}$ at $P_{{\mathrm{O}}_{\mathrm{2}}}\mathrm{=100b}.$ It is shown that very tiny deviations in the oxygen stoichiometry, which are not measurable using conventional techniques, affect significantly the transition width and commensurability, keeping the magnetic properties nearly unchanged. It is demonstrated that oxidized samples are systematically incommensurate whereas stoichiometric or the less oxidized ones are commensurate. A structural model is proposed to account for the modulated distribution of the additional $[{\mathrm{Mn}}^{\mathrm{4+}}{]}_{\infty }$ layers, with respect to the 1:1 ordering that should be obtained for the perfectly stoichiometric ${\mathrm{Ln}}_{\mathrm{0.50}}{\mathrm{Ca}}_{\mathrm{0.50}}{\mathrm{MnO}}_{\mathrm{3}}$ phase. The concomitant influence of oxygen stoichiometry, cationic size, and mismatch effect upon the charge ordering phenomena in these oxides is discussed.