cm503873x_si_001.pdf (1.61 MB)
Brownmillerite Ca2Co2O5: Synthesis, Stability, and Re-entrant Single Crystal to Single Crystal Structural Transitions
journal contribution
posted on 2014-12-23, 00:00 authored by Junjie Zhang, Hong Zheng, Christos
D. Malliakas, Jared M. Allred, Yang Ren, Qing’an Li, Tian-Heng Han, J.F. MitchellCa2Co2O5 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.
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2Co Synthesisspecimen exhibitsneutron powder diffraction240 Kunit cell parametersbrownmillerite formbrownmillerite structuresbrownmillerite variantÅ.Magnetic susceptibility dataSingle Crystal Structural TransitionsCa 2Cophase transitionsb axisspace group Pcmbspace groups140 Kzone furnace2Co crystallizeshistory dependenceroom temperature
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