Structural investigation of composite phases Ba1+x[(NaxMn1−x)O3] with x= 2/7, 5/17 and 1/3 ; exotic Mn4.5+ valence in perovskite-like structure

Abstract

International audienceStructural models are proposed for three composite compounds of chemical formula Ba 1+x[(NaxMn1-x)O3] (x2/7, 5/17 and 1/3) by single crystal X-ray diffraction ; super space formalism is used to obtain an unified description of the three phases. The [(NaxMn1-x)O3] framework is built up from the stacking of face sharing MnO6 octahedra and NaO6 trigonal prisms. The displacive modulations relating to these metallic columns were particularly difficult to modelize. Thus a model, including both discontinuous crenel-like functions for Ba atoms and continuous atomic strings for the Mn/Na sites has finally been retained. The large displacement of the oxygen atoms ( ±0.7 ° A) in comparison with that observed for related compounds ( ±0.3 ° A) makes it a direct consequence of the Na+ alkali insertion inside the trigonal prisms. Owing to this insertion, the Mn atoms exhibit, in the three phases, an ”exotic” oxidation state of about + 4.5