Mixed metallic Ba(Co,Mn)X0.2-xO3-δ (X=F, Cl) hexagonal perovskites

We show here that the incorporation of Mn in Ba-Co-oxohalide, BaCoX0.2-xO3-delta, hexagonal perovskite stabilizes the 6H-form (stacking sequence (chhhch'); c, h=[BaO3] and h'=[BaOX] layers), with tetramers of face-sharing octahedra) rather than the trimeric 10H-form. On the contrary to previous results on the Fe incorporation in similar system leading to more reduced 10H-compounds, the Mn effect is to increase the mean (Co/Mn) valence better suited to the 6H form. We experienced a poor Mn/Co miscibility during our syntheses leading to great difficulties to isolate mixed Co/Mn single phase materials and/or weak reproducibility. Powder neutron diffraction data shows a mixed Mn/Co octahedral occupancy, while the tetrahedra are filled by Co3+ cations. Anionic vacancies were refined in the h'-[BaO1-zX1-x] layer and the next c-[BaO3-z] layers, while the h-[BaO3] layers are not oxygen deficient. Magnetic properties suggest that a part of Mn cations remain paramagnetic until low temperature, while isolated spin clusters (probably driven by AFM Co tetrahedral dimers) behave as low-dimensional AFM systems. Transport measurements reveal a transition from high-temperature metallic to low-temperature semi-conducting states that could occur from defect shallow donor upon the Mn for Co substitution. (C) 2012 Elsevier Inc. All rights reserved.