Charge and magnetic ordering transitions in stoichiometric 6H-hexagonal perovskite BaFeO3

The newly synthesized oxygen-deficiency-free 6H-type BaFeO3 with unusually high valence Fe4+ is found to show successive first-order structural transitions at approximately 230 and 160 K, in sharp contrast to the absence of structural changes in previously reported oxygen-deficient BaFeO3-δ. The lowering of crystal symmetry at these structural phase transitions suggests sequential charge-ordering transitions. Magnetic order is also observed at 160 K. To reveal the nature of the phase transitions and the accompanying charge and magnetic ordered structures, we propose temperature-dependent neutron diffraction experiments at the WISH diffractometer. The high resolution of WISH is needed to precisely determine localised Fe3/4/5+ states in a large supercell from Fe-O distances, and high flux is needed for the small (300 mg) sample. The expected results will provide new insights into the correlation among charge, spin, and lattice degrees of freedom of the unusually high valence state in novel hexagonal-type perovskites.