[Show abstract][Hide abstract] ABSTRACT: Six compounds with formula Sr 2 Fe 1.9 M 0.1 O 5+y (M ¼ Mn, Cr, Co; y ¼ 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 N eel 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 Sr 2 Fe 2 O 5 , T N ¼ 693 K. Competing ferromagnetic interactions may be responsible for the anomalously low value in the M ¼ 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 ¼ Co compound is isostructural with Sr 4 Fe 4 O 11 (Sr 2 Fe 2 O 5.5), Cmmm, while the M ¼ Cr phase is cubic, Pm-3m, and that for M ¼ 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 ¼ Co material shows C-type antiferromagnetic long-range magnetic order at 4 K as found for Sr 4 Fe 4 O 11 . T N $ 230 K is inferred from a ZFC-FC magnetic susceptibility divergence compared with T N ¼ 232 K for un-doped Sr 4 Fe 4 O 11 . The M ¼ 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 ¼ Mn phase. For the M ¼ 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.