We investigated the formation of extended defects in CaFeO2.5, predominantly appearing as antiphase boundaries (APBs), as a function of the synthesis method and temperature. While CaFeO2.5 is known to adopt an ordered oxygen defect structure showing long range order of the (FeO 4)∞ chains in its bulk form, interestingly, we demonstrated that the length of these (FeO4)∞ chains can be considerably scaled down to few nanometers by adopting a modified sol-gel method (low temperature synthesis) while the grain size of the resulting nano-phase CaFeO2.5 is around 50 nm. We discuss the synthesis dependent modulation of the length of APBs, characterized by X-ray diffraction and high resolution TEM, to be at the origin of an amplified switching dynamics of the (FeO4)∞ chains. This can accordingly explain the reduction of the onset temperature for oxygen diffusion to set in from 450 °C for bulk-CaFeO2.5 to 320 °C for nano-CaFeO2.5, as determined by 18O/16O oxygen isotope exchange reactions. © 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.

Mamidanna Sri Ramachandra Rao

Department Of Physics

Antiphase boundaries

BROWNMILLERITES

EXTENDED DEFECT

HIGH-RESOLUTION TEM

Low temperature synthesis

MODIFIED SOL-GEL METHOD

OXYGEN DIFFUSION

OXYGEN ISOTOPE EXCHANGE

Chains

Diffusion in gases

Iron oxides

Isotopes

Oxygen

Perovskite

Sol-gel process

Transmission electron microscopy

X ray diffraction

Defects

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Physica Status Solidi (A) Applications and Materials Science

In this report, we have evolved a new method to synthesize nanocrystalline calcium sulfate dihydrate (gypsum) via electrochemical oxidation. This method provides a facile, direct way of synthesizing nanocrystalline gypsum at room temperature. Field-emission scanning electron microscopy micrographs present an interesting phenomenon of formation of microcracks in the nano-CaFeO2.5 pellet during electrochemical oxidation leading to nucleation of gypsum within the cracks. A known theoretical model has been invoked to explain the gypsum growth within the cracks. Transmission electron microscopy studies indicate electron beam-induced phase transformation of single-crystalline calcium sulfate dihydrate to polycrystalline CaO nanoparticles. The hardness was found to be improved (63% higher) by the addition of a mere 5 wt % of nano-gypsum to commercial gypsum, which is almost 200% higher than nano-gypsum synthesized earlier via an indirect route of flame synthesis and hydration. Moreover, the compressive strength of the gypsum mixture (with 5 wt % of nano-gypsum) was found to be improved by a factor of 1.7 (92.05 MPa), in comparison to commercial gypsum (54.23 MPa). The improved hardness and compressive strength by reduction in particle size of gypsum are noteworthy. © 2016 American Chemical Society.

Crystal Growth and Design

Direct and Facile Room-Temperature Synthesis of Nanocrystalline Calcium Sulfate Dihydrate (Gypsum)

We demonstrate the hitherto unreported field induced magnetoresistance in wet chemical synthesized bulk polycrystalline brownmillerite Ca2Fe2O5. A magnetoresistance of 12% is observed at room temperature, well below its antiferromagnetic transition temperature of 775 K. The variation in resistivity with increasing and decreasing magnetic field shows a significant memory effect. When synthesized in a nanostructured form it exhibits weak ferromagnetism due to the appearance of an Fe oxide phase, besides exhibiting a shift to a higher antiferromagnetic Néel temperature of 820 K. Spectroscopic investigations (X-ray photoelectron and Raman spectroscopy) as well as magnetic measurements have been employed to investigate the above behavior of brownmillerite Ca2Fe2O5. © 2015 The Royal Society of Chemistry.

RSC Advances

Anomalous room temperature magnetoresistance in brownmillerite Ca2Fe2O5

We observed rapid CO2 capture in nanostructured Brownmillerite CaFeO2.5 with no decay in capture capacity even after multiple cycles of carbonation and calcination. Industrial exhaust CO2 gas causes global warming as a greenhouse gas and there is an ever increasing need to develop materials for direct capture of CO2. In this work, we demonstrated CO2 capture effect in an oxygen-deficient perovskite CaFeO2.5 with Brownmillerite structure by first, exposing the compounds (bulk- and nano-CaFeO2.5) to ambient air for different durations and later, by studying fast carbonation-calcination cycles using thermogravimetry. We found that, while the physical properties of bulk-CaFeO2.5 remain unaltered; nano-CaFeO2.5 shows spontaneous capture of CO2 upon exposure to ambient air. Moreover, nano-CaFeO2.5 can be recovered completely by heating in air, showing that the capture of CO2 is reversible. Investigation of bulk and nano-CaFeO2.5 in a 2-min cycle of carbonation and calcination (regeneration) at 500°C revealed that the nano-CaFeO2.5 exhibited rapid CO2 capture ability within few seconds. In addition, almost no decay in CO2 capture capacity was observed even after 30 cycles of carbonation-calcination of nano-CaFeO2.5, which is quite remarkable. Such nanostructured systems can be potential candidates for practical applications of capturing CO2 from hot exhaust gases. © 2014 Elsevier Ltd.

Nano Energy

Fast, reversible CO2 capture in nanostructured Brownmillerite CaFeO2.5

CrystEngComm

Growth and characterization of large high quality brownmillerite CaFeO2.5 single crystals

Journal of the American Chemical Society

CaFeO2: A New Type of Layered Structure with Iron in a Distorted Square Planar Coordination

Journal	Physica Status Solidi (A) Applications and Materials Science
ISSN	18626300
Open Access	No