Brownmillerite CaCoO$_{2.5}$: Synthesis, Re-entrant Structural Transitions and Magnetic properties*

Cobalt oxides attract both fundamental and technological attention due to their physical properties including thermoelectricity, giant magnetoresistance, superconductivity and multiferroicity [1]. Here we report the first synthesis of CaCoO$_{2.5}$ single crystals using a high pressure optical-image floating zone technique. We find that it is an ordered oxygen-deficient perovskite of the brownmillerite type, and it undergoes an unprecedented re-entrant structural phase transitions (Pcmb$\to $ P2/c11$\to $ P12$_{1}$/m1$\to $ Pcmb) with decreasing temperature. We describe its temperature-dependent structural, thermal, and magnetic properties, including AFM ordering near 240 K, with a weakly spin canted ferromagnet ground state below 140 K. The magnetic response of CaCoO$_{2.5}$ depends markedly on the cooling rate and field history. Magnetization data also imply the potential of a distinct, field-induced phase arising uniquely from the P12$_{1}$/m1 structure, revealed as kinetically trapped by a rapid-cooling protocol.\\[4pt] [1] Raveau, B.; Seikh, M. M. Cobalt Oxides: From Crystal Chemistry to Physics; Wiley-VCH: Weinheim, 2012.