Correlation between the twinning process and oxygen insertion in CeNbO4+x phases

This reinvestigation study presents the structural link between the twinning process in the CeNbO4 phase and the commensurate and incommensurate modulations in the CeNbO4+x phases, resulting from their oxidation. The anionic insertion occurs with the formation of oxygen clusters propagating through the structure via a small distortion of the cationic sub-structure associated with the rotation of the coordinating oxygen edges of some niobium sites. These cationic distortions and O–O rotations appear to be a common feature of the twinned CeNbO4 and CeNbO4+x phases. The location of the inserted oxygens is related to a local distortion of the cationic sub-structure, similar to that observed in "nano-twinned" areas, which explains why the directions of the incommensurate vectors of CeNbO4.08 are related to those of the twinning walls. The CeNbO4.08 and CeNbO4.25 phases are discrete and the continuity of the oxidation/reduction process is the result of the mixing of these two phases, which have in common their link with the distortions induced by the twinning process that guides the distribution of the distorted areas.