abstract
Hexagonal manganates RMnO3 (R=Y, Ho, Dy) have been recently shown to exhibit oxygen-storage capacities promising for three-way catalysts, air-separation, and related technologies. Here, we demonstrate that Ti substitution for Mn can be used to chemically tune the oxygen-breathing properties of these materials towards practical applications. Specifically, Y(Mn1-xTix)O-3 solid solutions exhibit facile oxygen absorption/desorption via reversible Ti3+<-> Ti4+ and Mn3+<-> Mn4+ reactions already in ambient air at approximate to 400 degrees C and approximate to 250 degrees C, respectively. On cooling, the oxidation of both cations is accompanied by oxygen uptake yielding a formula YMn1-x-y3+Mny4+Tix4+O3+delta. The presence of Ti promotes the oxidation of Mn3+ to Mn4+, which is almost negligible for YMnO3 in air, thereby increasing the uptake of oxygen beyond that required for a given Ti4+ concentration. The reversibility of the redox reactions is limited by sluggish kinetics; however, the oxidation process continues, if slowly, even at room temperature. The extra oxygen atoms are accommodated by the large interstices within a triangular lattice formed by the [MnO5] trigonal bipyramids. According to bond distances from Rietveld refinements using the neutron diffraction data, the YMnO3 structure features under-bonded Mn and even more severely under-bonded oxygen atoms that form the trigonal bases of the [MnO5] bipyramids. The tensile bond strain around the 5-fold coordinated Mn site and the strong preference of Ti4+(and Mn4+) for higher coordination numbers likely provide driving forces for the oxidation reaction. Reverse Monte Carlo refinements of the local atomic displacements using neutron total scattering revealed how the excess oxygen atoms are accommodated in the structure by correlated local displacements of the host atoms. Large displacements of the under-bonded host oxygen atoms play a key part in this lattice-relaxation process, facilitating reversible exchange of significant amounts of oxygen with atmosphere.
keywords
CRYSTAL-STRUCTURE; FERROELECTRICITY
subject category
Chemistry
authors
Levin, I; Krayzman, V; Vanderah, TA; Tomczyk, M; Wu, H; Tucker, MG; Playford, HY; Woicik, JC; Dennis, CL; Vilarinho, PM
our authors
Monika Malgorzata Tomczyk
PhD Student
Paula Maria Lousada Silveirinha Vilarinho
Vice Directoracknowledgements
Portions of this research were carried out at the National Synchrotron Light Source (NIST beamline X23A2), Brookhaven National Laboratory, supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract no. DE-AC02-98CH10886. We thank the STFC (UK) for access to the ISIS facility. The neutron total-scattering data for pure YMnO3 were collected as a part of Prof. Martin Dove's beam time (proposal RB1410591) on the Polaris diffractometer at ISIS; a full analysis of these data will be published in a further article.