| Sumario: | Oxygen engineering techniques performed under adequate controlled atmosphere show that the CaMnO[sub3]-CaMnO[sub2] topotactic reduction-oxidation process proceeds via oxygen diffusion while the cationic sublattice remains almost unaltered. Extra superlattice reflections in selected area electron diffraction patterns indicate doubling of the CaMnO[sub2] rock-salt cell along the cubic directions of a distorted rhombohedral cell originated by ordering of Ca[sup2+] and Mn[sup2+] ions distributed in nanoclusters into a NaCI-type matrix, as evidenced by dark field electron microscope images. The local nature of the information provided by the transmission electron microscopy techniques used to characterize the rock-salt type Ca[sub1-x]Mn[subx]O[sub2] solid solution clearly hints at the existence of subtle extra ordering in other upper oxides of the Ca-Mn-O system. The combination of local characterization techniques like electron microscopy with more average ones like powder X-ray and neutron diffraction allows a very complete characterization of the system.
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