Heretofore unknown cobalt oxide with brownmillerite structure discovered in Argonne National Lab using the ScIDre HKZ floating zone furnace.
Various complex oxides and intermetallic compounds have been successfully grown with the HKZ floating zone furnace, yielding surpassing quality single crystals. However, the discovery of new materials with exceptional properties remains a claim to fame. Recently, a completely new cobalt oxide was synthesized in the Emerging Materials Group at Argonne National Laboratory, Illinois, USA. The scientists were able to grow single crystals of heretofore elusive compound Ca₂Co₂O₅ in the brownmillerite form. Junjie Zhang et al. write in a recent publication (Chem. Mater. 2014, 26, 7172 − 7182, full text at ResearchGate site of Junjie Zhang), that many attempts have been made so far to synthesize the pseudobinary CaO − CoO system with a ratio Ca/Co = 1. The successful synthesis of the brownmillerite form of Ca₂Co₂O₅ would allow direct comparison to the extensive understanding of the chemical, magnetic, and electronic structure of this rich family of compounds. The group of John F. Mitchell could achieve this goal using the ScIDre HKZ floating zone furnace with high-pressure (145 bar) oxygen atmosphere: „We found that pO₂ indeed is a key factor for Ca₂Co₂O₅ formation, with pO₂ ≥ 50 bar required to obtain high-purity Ca₂Co₂O₅ crystals“. With decreasing temperature, Ca₂Co₂O₅ undergoes re-entrant phase transition series, which is unique among brownmillerites. Temperature-dependent magnetic susceptibility data reveal that Ca₂Co₂O₅ is antiferromagnetic below ∼ 240 K. In their conclusion, the authors write „Finally, the present study demonstrates a robust platform for the synthesis of new ordered anion-deficient Co-based perovskites using high-pressure oxygen gas in crystal growth. We also note that these anion-deficient perovskites provide a route to other new materials and properties.“ They close with the sentence: „More generally, the successful synthesis of bulk brownmillerite Ca₂Co₂O₅ provides a first glimpse at the opportunity space for new, metastable perovskites, and other oxides opened by high-pressure optical image growth.“ All ScIdre staff members congratulate the authors on the remarkable publication and thank for the emphasis on our high-pressure furnace which helped to make the discovery.
