Influence of the synthesis route on the formation of 12R/10H-polytypes and their magnetic properties within the Ba(Ce,Mn)O3 family

Abstract

The influence of the Mn precursor on the formation of polytypes was studied within the Ba–Ce–Mn–O system. Minor changes in the mean oxidation state of Mn cations guide the reaction route through different pathways to obtain the iso-stoichiometric 12R-Ba₄CeMn₄O_12−δ or 10H-Ba₅Ce_1.25Mn_3.75O_15−δ compounds. This behaviour was controlled using MnO₂ or MnCO₃ as starting chemical precursors. The understanding of such tendencies allowed the synthesis of the new 10H-Ba₅Ce_0.83Mn_4.17O_15−δ composition through a two-step reaction route. This compound crystallizes in a hexagonal symmetry with space group P6₃/mmc (No. 194) and cell parameters a = b = 5.77650(6) Å and c = 23.8590(4) Å. Its main characteristic is the presence of pure [Mn₄O₁₅] oligomers, establishing a difference with respect to the already known 10H-Ba₅Ce_1.25Mn_3.75O₁₅ polytype for which the principal feature was a mixed metal occupancy within [Ce_0.25Mn_3.75O₁₅] tetramers. This difference generates strong antiferromagnetic (AFM) exchanges, contrary to the metamagnetic-like transition observed in 10H-Ba₅Ce_1.25Mn_3.75O₁₅. It is concluded that full Mn occupancy in [Mn₃O₁₂] or [Mn₄O₁₅] sub-units induces a robust AFM character.