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Acta Cryst. (2014). A70, C69
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In recent years, a number of novel ceramic oxide materials have emerged that are capable of absorbing CO2 at high temperatures (>500C) while remaining stable over a large number of cycles and a wide range of temperatures [1]. The most promising are been considered for carbon capture applications - specifically, for use in combustion chambers and the smoke stacks of power plants where combustion gases which contain primarily a mixture of CO2 and N2 at high temperature. Compared to other CO2 sequestration technologies, these ceramics have some advantages (eg. chemisorption at high temperatures) and disadvantages (eg. limited kinetics over time) [3]. Examples of oxides already known to show significant CO2 absorption include Li5AlO4, Li6Zr2O7, Na2ZrO3 and Ba4Sb2O9. The phase formations and structural evolution of these metal oxides have been studied under environmental conditions mimicing those found in combustion chambers and power plants, over the temperature range 873-1173 K. CO2 absorption by these materials is believed to proceed through a layering effect of the sorbent material, explained through a core-shell model (see figure). Each phase is represented as a layer covering a particle, with the outermost layer exposed and allowed to react with the environment. Detailed studies into the mechanism of CO2 absorption and the material layers will shed more information that can be used to fine tune the materials to increase their CO2 absorption capacity. Previous work has focused on the identification of phases ex situ and studies of their practical absorption capacity and kinetics. The new work we will present here uses a combination of a x-ray spectroscopy, x-ray and neutron diffraction, to understand both how the sorption process works and how the structural evolution of the phases affects the CO2 sorption of the materials over time in-situ.

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Acta Cryst. (2014). A70, C1363
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This study introduces examples of structure property relationships within the multi-layered Sillen-Aurivillius family (shown in Figure) and aims to investigate the effect of chemical doping and lattice matching effects. The first example involves doping 1/3 of the n = 3 ferroelectric perovskite layers with magnetic transition metal cations in Bi5PbTi3O14Cl [1] with charge balancing by removing Pb2+ for Bi3+. A statistical 1:2 distribution of M3+ and Ti4+ across all three perovskite layers was found in Bi6Ti2MO14Cl, M = Cr3+, Mn3+, Fe3+, resulting in highly strained structures (enhancing the ferroelectricity compared to Bi5PbTi3O14Cl) and pronounced spin-glass behavior below Tirr(0) = 4.46 K. Ferroelectric transitions were observed at high temperature for each of the new compounds. Ferroelectric properties were also measured on Bi6Ti2FeO14Cl using piezoresponse force microscopy showing hysteretic phase behavior. A new n = 2 Sillen-Aurivillius compound Bi3Sr2Nb2O11Br, based on Bi3Pb2Nb2O11Cl [2], was synthesized by simultaneously replacing Pb2+ with Sr2+ and Cl- with Br-. Inter-layer mismatch prevented the formation of Bi3Sr2Nb2O11Cl and Bi3Pb2Nb2O11Br. Sr2+ doping reduces the impact of the stereochemically active 6s2 lone pair found on Pb2+ and Bi3+, resulting in a stacking contraction in the lattice parameters by 1.22 % and an expansion of the a-b plane by 0.25 %, improving inter-layer compatibility with Br-. X-ray Absorption Near Edge Structure spectra analysis shows that the ferroelectric distortion of the B-site cation is less apparent in Bi3Sr2Nb2O11Br compared to Bi3Pb2Nb2O11Cl. Variable-temperature neutron diffraction data show no evidence for a ferroelectric distortion.
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