Common Building Motifs in Ba₂Fe₃(PO₄)₄·2H₂O, BaFe₃(PO₄)_3, and Na₃Fe₃(PO₄)₄: Labile Fe²⁺/Fe³⁺ Ordering and Charge-Dependent\nMagnetism

Abstract

Two new mixed-valence\nFe^2/3+ barium phosphates have been synthesized in hydrothermal\nconditions and characterized: Ba₂Fe^2.66+₃(PO₄)₄·2H₂O (compound <b>1</b>, ratio Fe³⁺/Fe²⁺ = 2:1, orthorhombic\nspace group <i>Pbca</i>, <i>a</i> = 6.71240(10)\nÅ, <i>b</i> = 10.6077(2) Å, <i>c</i> = 20.9975(5) Å, R1 = 3.39%) and BaFe^2.33+₃(PO₄)₃ (compound <b>2</b>, ratio Fe³⁺/Fe²⁺ = 1:2, orthorhombic, space group <i>Imma</i> with <i>a</i> = 10.5236(3) Å, <i>b</i> = 13.4454(4) Å, <i>c</i> = 6.6411(2) Å,\nR1 = 1.63%). <b>1</b> has a two-dimensional crystal structure\nbuilt of [Fe^2.5+₂Fe³⁺₁(PO₄)₄]^4– layers with charge\nsegregation on two individual Fe crystal sites, in contrast to the\nsingle valence on these two sites found in similar layers of Na₃Fe³⁺₃(PO₄)₄. The\ncrystal structure of <b>2</b> is formed of the same layers but\ncondensed into a 3D [Fe²⁺₂Fe³⁺₁(PO₄)₃]^2– framework.\nThe complete Fe²⁺ vs Fe³⁺ charge ordering on\nthe two available sites differs from what was found in the two previous\ncases and denotes a remarkable charge adaptability of the common elementary\nunits. Compared to the antiferromagnetic Na₃Fe³⁺₃(PO₄)₄ the partial iron reduction\ninto Fe²⁺ is responsible for strong ferromagnetic components\nalong the <i>c</i>-easy axis for both <b>1</b> and <b>2</b>. Additionally <b>1</b> shows multiple magnetization\nsteps in the perpendicular direction, giving raise to atypical anisotropic\nmagnetism into a complex magnetic phase diagram.