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Structural models are proposed for three composite compounds of chemical formula Ba 1 +  x   [(Na  x  Mn 1 –  x  )O 3 ] ( x  approx. 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 (approx. ±0.7 Å) in comparison with that observed for related compounds (approx. ±0.3 Å) 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 MnO 6  octahedra and NaO 6  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.

Structural investigation of composite phases Ba 1 + x [(Na x Mn 1 – x )O 3 ] with x approx. 2/7, 5/17 and 1/3; exotic Mn 4.5+ valence