The effect of the B-site cation and oxygen stoichiometry on the local and average crystal and magnetic structures of Sr2Fe1.9M0.1O5+y (M = Mn, Cr, Co; y = 0, 0.5)

Abstract

Six compounds with formula Sr₂Fe_1.9M_0.1O_5+y (M = Mn, Cr, Co; y = 0, 0.5) were synthesized in air and argon, exhibiting surprisingly different properties depending on the B-cation type in spite of the low (5%) doping level. All argon synthesized phases, y ∼ 0, have long range brownmillerite ordering of oxygen vacancies with Icmm symmetry as shown by neutron diffraction (ND). All show long-range G-type antiferromagnetic order with Néel temperatures, T_N, from variable temperature ND of 649(3)K, 636(2)K and 668(5)K for Cr, Mn and Co-compounds, respectively, compared with Sr₂Fe₂O₅, T_N = 693 K. Competing ferromagnetic interactions may be responsible for the anomalously low value in the M = Mn case. The air synthesized phases with y ∼ 0.5 show surprising variation with M as investigated by X-ray, TOF and constant wavelength neutron diffractions. The M = Co compound is isostructural with Sr₄Fe₄O₁₁ (Sr₂Fe₂O_5.5), Cmmm, while the M = Cr phase is cubic, Pm-3m, and that for M = Mn appears to be cubic but the reflections are systematically broadened in a manner which suggests a local Cmmm structure. NPDF studies show that the local structure of the Cr phase is better described in terms of a Cmmm ordering of oxygen vacancies with Fe–O coordination numbers of five and six. The M = Co material shows C-type antiferromagnetic long-range magnetic order at 4 K as found for Sr₄Fe₄O₁₁. T_N ∼ 230 K is inferred from a ZFC-FC magnetic susceptibility divergence compared with T_N = 232 K for un-doped Sr₄Fe₄O₁₁. The M = Cr and Mn compounds show no long-range magnetic ordering down to 4 K, but the divergence of ZFC and FC susceptibility data indicative of spin glass-like transitions occur at ∼60 K and ∼45 K for Cr and Mn, respectively. ND shows both diffuse and sharp Bragg magnetic reflections at positions consistent with a Cmmm cell for the M = Mn phase. For the M = Cr material, a very weak magnetic Bragg peak indexed as (1/2 1/2 1/2), consistent with a G-type AF order, is found at 4 K. These results rule out a spin glass-like ground state for both materials.