Structural investigation of composite phases Ba1+x[(NaxMn1-x)O-3] with x similar or equal to 2/7, 5/17 and 1/3; exotic Mn4.5+ valence

International audienceStructural models are proposed for three composite compounds of chemical formula Ba1+x[(NaxMn1-x)O-3] (x similar or equal to 2/7, 5/17 and 1/3) by single crystal X-ray diffraction; superspace formalism is used to obtain an unified description of the three phases. The modulation affecting Ba atoms can be easily designed but the competition "occupational/displacive" modulations relating to the Mn/Na metallic columns were particularly difficult to modelize. However, the large amplitude of the displacive modulation affecting the oxygen atoms (similar or equal to +/- 0.7 angstrom) in comparison with that observed for related compounds (similar or equal to +/- 0.3 angstrom) 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. In addition, even if the sequence between face sharing MnO6 octahedra and NaO6 trigonal prisms can be analytically deduced from the x value, it is clear that the Na/Mn contrasts play in favour of its accurate determination through the XRD single crystal refinement