Oxygen-storage behavior and local structure in Ti-substituted YMnO3

Abstract

Hexagonal manganates RMnO 3 (R=Y, Ho, Dy) have been recently shown to exhibit oxygenstorage 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 oxygenbreathing properties of these materials towards practical applications. Specifically, Y(Mn 1-x Ti x )O 3 solid solutions exhibit facile oxygen absorption/desorption via reversible Ti 3+ Ti 4+ and Mn 3+ Mn 4+ reactions already in ambient air at ≈400 °C and ≈250 °C, respectively. On cooling, the oxidation of both cations is accompanied by oxygen uptake yielding a formula YMn 3+ 1-xy Mn 4+ y Ti 4+x O 3+ . The presence of Ti promotes the oxidation of Mn 3+ to Mn 4+ , which is almost negligible for YMnO 3 in air, thereby increasing the uptake of oxygen beyond that required for a given Ti 4+ 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 [MnO 5 ] trigonal bipyramids. According to bond distances from Rietveld refinements using the neutron diffraction data, the YMnO 3 structure features under-bonded Mn and even more severely under-bonded oxygen atoms that form the trigonal bases of the [MnO 5 ] bipyramids. The tensile bond strain around the 5-fold coordinated Mn site and the strong preference of Ti 4+ (and Mn 4+ ) 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.

Pure and Zr-doped YMnO3+δas a YSZ-compatible SOFC cathode: a combined computational and experimental approach

“…Beyond the decrease of the c parameter and lattice volume, already observed and discussed for the samples prepared by solid-state synthesis, Zr-doped YMnO3 adopts a P63/mmc space group. Such behaviour has already been observed in a close compound (but doped with Ti on the Mn site, instead of Zr on the Y site in our case): the (102) super-reflection at 2θ≈23°, present for the pattern of the as-prepared sample, disappears with the oxygen uptake at high temperature32 . Thus, it is possible to infer that the structural change in our Y0.9Zr0.1MnO3+δ can be also associated with an oxygen uptake, as evidenced by TGA.…”

“…16 Therefore, considering that some of the original equatorial sites of the MnO5 polyhedra are now partially empty and that new interstitial positions (with possibly higher energy) are, in change, partially occupied too, such situation seems to create perfect conditions for oxygen atoms to move easily within the structure. The mobility of oxygen ions is a crucial point in the search of MIEC compounds 32 . In Figure 1d, the simulated X-Ray diffractograms of the aforementioned structures are given for comparison.…”

Electronic bulk and domain wall properties in B -site doped hexagonal ErMnO3

“…Their bulk properties are well-characterized in experiment [34][35][36] and theory [37,38] and the system naturally provides stable charged head-to-head (→←) and tail-to-tail (←→) DWs in the as-grown state, making it an excellent model template material [11,39]. Furthermore, the system has enough chemical flexibility to allow doping, as reflected by previous investigations on the bulk level -reporting chemical substitution on the A-site [40,41] and B-site [42][43][44], as well as oxygen off-stoichiometry [45][46][47][48]. Recently, the effect of doping has been extended to the micro-and nanoscopic length scales: specifically, aliovalent substitution of A-site cations was scrutinized as a control parameter for adjusting the electronic DW behavior [27,28,39] In this work, electronic DW conductance in Ti-doped erbium manganite, Er(Mn 1−x ,Ti x )O 3 (x = 0.002), is reported.…”

The Influence of the Elemental Composition, Crystal Structure, and Grain Structure of the Ferro-Piezoceramics of Various Degrees of the Ferro-Hardness on the Stability of the Polarized State

“…These vacancies, weakly related to the structure of the material, are the sources of the conductivity and make an additional contribution to the formation of the dielectric properties of the solid solutions. A sharp increase in ε/ε 0 in the paraelectric region (in the vicinity of (670-820)K) may be a consequence of the redox processes associated with variable valences of Nb (5+↔4+) and Ti (4+↔3+) [21][22][23], and as a consequence, the appearance of the oxygen vacancies, which form the anion-deficient nonstoichiometry. These vacancies, weakly related to the structure of the material, are the sources of the conductivity and make an additional contribution to the formation of the dielectric properties of the solid solutions.…”