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

Junjie Zhang*, Hong Zheng, Christos D Malliakas, Jared M. Allred, Yang Ren, Qing'An Li, Tian Heng Han, J. F. Mitchell

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

21 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)7172-7182
Number of pages11
JournalChemistry of Materials
Issue number24
StatePublished - Dec 23 2014

ASJC Scopus subject areas

  • Chemistry(all)
  • Chemical Engineering(all)
  • Materials Chemistry


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