Brownmillerite Ca₂Co₂O₅: Synthesis, stability, and re-entrant single crystal to single crystal structural transitions

Ca₂Co₂O₅ in the brownmillerite form was synthesized using a high-pressure optical-image floating zone furnace, and single crystals with dimensions up to 1.4 × 0.8 × 0.5 mm³ were obtained. At room temperature, Ca₂Co₂O₅ crystallizes as a fully ordered brownmillerite variant in the orthorhombic space group Pcmb (No. 57) with unit cell parameters a = 5.28960(10) Å, b = 14.9240(2) Å, and c = 10.9547(2) Å. With decreasing temperature, it undergoes a re-entrant sequence of first-order structural phase transitions (Pcmb → P2/c11 → P12₁/m1 → Pcmb) that is unprecedented among brownmillerites, broadening the family of space groups available to these materials and challenging current approaches for sorting the myriad variants of brownmillerite structures. Magnetic susceptibility data indicate antiferromagnetic ordering in Ca₂Co₂O₅ occurs near 240 K, corroborated by neutron powder diffraction. Below 140 K, the specimen exhibits a weak ferromagnetic component directed primarily along the b axis that shows a pronounced thermal and magnetic history dependence.