Magnetic Structures of Mn 11 Ta 4 O 21 and Interpretation as an Hexagonal A-site Manganite

International audienceMn11Ta4O21 is presented as the first Hexagonal A-site Manganite. Based on simple rules, the structure is compatible with a 14Hlayer (cchchch)2 stacking sequence; related to BaVO3 and BaCrO3 high pressure polymorphs. The A-site over-stoichiometry is explained through difference in ionic radii sizes between Ba and Mn. Magnetic properties show two transitions at TN1 = 88 K and TN2 = 56 K. Neutron powder diffraction evidence two magnetic structures with purely antiferromagnetic and ferrimagnetic orders below TN1 and TN2 respectively. A complementary description with 14H-(hhccccc)2 sequence of only Mn octahedra provides a direct comparison with BaMnO3-δ hexagonal perovskites and naturally explains the AFM order. Below TN2 a magneto-elastic coupling along with uniaxial negative thermal expansion are observed