10.1021/cm503873x.s001 Junjie Zhang Junjie Zhang Hong Zheng Hong Zheng Christos D. Malliakas Christos D. Malliakas Jared M. Allred Jared M. Allred Yang Ren Yang Ren Qing’an Li Qing’an Li Tian-Heng Han Tian-Heng Han J.F. Mitchell J.F. Mitchell Brownmillerite Ca<sub>2</sub>Co<sub>2</sub>O<sub>5</sub>: Synthesis, Stability, and Re-entrant Single Crystal to Single Crystal Structural Transitions American Chemical Society 2014 2Co Synthesis specimen exhibits neutron powder diffraction 240 K unit cell parameters brownmillerite form brownmillerite structures brownmillerite variant Å. Magnetic susceptibility data Single Crystal Structural TransitionsCa 2Co phase transitions b axis space group Pcmb space groups 140 K zone furnace 2Co crystallizes history dependence room temperature 2014-12-23 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Brownmillerite_Ca_sub_2_sub_Co_sub_2_sub_O_sub_5_sub_Synthesis_Stability_and_Re_entrant_Single_Crystal_to_Single_Crystal_Structural_Transitions/2222791 Ca<sub>2</sub>Co<sub>2</sub>O<sub>5</sub> 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 mm<sup>3</sup> were obtained. At room temperature, Ca<sub>2</sub>Co<sub>2</sub>O<sub>5</sub> crystallizes as a fully ordered brownmillerite variant in the orthorhombic space group <i>Pcmb</i> (No. 57) with unit cell parameters <i>a</i> = 5.28960(10) Å, <i>b</i> = 14.9240(2) Å, and <i>c</i> = 10.9547(2) Å. With decreasing temperature, it undergoes a re-entrant sequence of first-order structural phase transitions (<i>Pcmb</i> → <i>P</i>2/<i>c</i>11 → <i>P</i>12<sub>1</sub>/<i>m</i>1 → <i>Pcmb</i>) 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 Ca<sub>2</sub>Co<sub>2</sub>O<sub>5</sub> occurs near 240 K, corroborated by neutron powder diffraction. Below 140 K, the specimen exhibits a weak ferromagnetic component directed primarily along the <i>b</i> axis that shows a pronounced thermal and magnetic history dependence.