Abstract
Two new mixed-valence Fe2/3+ barium phosphates have been synthesized in hydrothermal conditions and characterized Ba2Fe2.66+3(PO4)4·2H2O (compound 1, ratio Fe3+/Fe2+ = 21, orthorhombic space group Pbca, a = 6.71240(10) Å, b = 10.6077(2) Å, c = 20.9975(5) Å, R1 = 3.39%) and BaFe2.33+3(PO4)3 (compound 2, ratio Fe3+/Fe2+ = 12, orthorhombic, space group Imma with a = 10.5236(3) Å, b = 13.4454(4) Å, c = 6.6411(2) Å, R1 = 1.63%). 1 has a two-dimensional crystal structure built of [Fe2.5+2Fe3+1(PO4)4]4- layers with charge segregation on two individual Fe crystal sites, in contrast to the single valence on these two sites found in similar layers of Na3Fe3+3(PO4)4. The crystal structure of 2 is formed of the same layers but condensed into a 3D [Fe2+2Fe3+1(PO4)3]2- framework. The complete Fe2+ vs Fe3+ charge ordering on the two available sites differs from what was found in the two previous cases and denotes a remarkable charge adaptability of the common elementary units. Compared to the antiferromagnetic Na3Fe3+3(PO4)4 the partial iron reduction into Fe2+ is responsible for strong ferromagnetic components along the c-easy axis for both 1 and 2. Additionally 1 shows multiple magnetization steps in the perpendicular direction, giving raise to atypical anisotropic magnetism into a complex magnetic phase diagram. © 2016 American Chemical Society.