Mixed Metallic Ba(Co,Fe)X0.2O3-δ (X = F, Cl) Hexagonal Perovskites: Drastic Effect of Fe-Incorporation on Structural and Electronic Features

Starting from the parent 10H-Ba5Co5X1-xO13-delta (trimeric strings of face-sharing CoO6 octahedra with terminal CoO4 tetrahedra, stacking sequence (chhch')(2)) and 6H-Ba6Co6X1-xO16-delta (similar with tetrameric strings, stacking sequence chhhch') hexagonal perovskites forms (X = F, Cl; c, h = [BaO3] layers; h' = [BaOX1-y] layers), we show here that the Fe incorporation leads to large domains of solid solutions for both X = F and Cl but exclusively stabilizes the 10H-form independently of the synthesis method. In this form, the lowest concentration of h-layers is stabilized by a sensitive metal reduction with increasing the Fe ratio. In a more general context of competition between several hexagonal perovskite polymorphs available for most of the transition metals, this redox change is most probably the key factor driving 1D (face-sharing chains) to 3D (corner-sharing) connectivities. Strikingly, ND data evidence the location of oxygen deficiencies in the tetrahedral (Co/Fe) coordination. This effect is exaggerated at high temperature, while (Co/Fe)O4-delta coordinations are completed by the displacement of X- anions toward the (Co/Fe) sphere of coordination following a push-and-pull mechanism within h'-[BaOX1-y] layers. The Fe-incorporation is also accompanied by increasing conduction gaps with predominant 1D variable range hopping. The full series show antiferromagnetic behavior with increasing T-N as [Fe] increases. For Fe-rich compounds T-N is estimated about 600 K, as rarely observed for hexagonal perovskite compounds. Finally, magnetic structures of all iron-doped compounds show a site-to-site AFM ordering, different of the magnetic structure of Co-only parent compounds. Here, DFT calculations predict low-spin octahedral Co configurations, but high-spin Fe species in the same sites.