Abstract
Reaction of the anion-deficient, cation-ordered perovskite phase Ba2YFeO5 with 80 atm of oxygen pressure at 410 °C results in the formation of the Fe4+ phase Ba2YFeO5.5. The topochemical insertion of oxide ions lifts the inversion symmetry of the centrosymmetric host phase, Ba2YFeO5 (space group P21/n), to yield a noncentrosymmetric (NCS) phase Ba2YFeO5.5 (space group Pb21m (No. 26), a = 12.1320(2) A, b = 6.0606(1) A, c = 8.0956(1) A, V = 595.257(2) A3) confirmed by the observation of second-harmonic generation. Dielectric and PUND ferroelectric measurements, however, show no evidence for a switchable ferroelectric polarization, limiting the material to pyroelectric behavior. Magnetization and low-temperature neutron diffraction data indicate that Ba2YFeO5.5 undergoes a magnetic transition at 20 K to adopt a state which exhibits a combination of ferromagnetic and antiferromagnetic order. The symmetry breaking from centrosymmetric to polar noncentrosymmetric, which occurs during the topochemical oxidation ...
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