Is the surface oxygen exchange rate linked to bulk ion diffusivity in mixed conducting Ruddlesden–Popper phases?

Abstract

The possible link between oxygen surface exchange rate and bulk oxygen anion diffusivity in mixed ionic and electronic conducting oxides is a topic of great interest and debate. While a large body of experimental evidence and theoretical analyses support a link, observed differences between bulk and surface composition of these materials are hard to reconcile with this observation. This is further compounded by potential problems with simultaneous measurement of both parameters. Here we utilize separate techniques, in situ neutron diffraction and pulsed isotopic surface exchange, to examine bulk ion mobility and surface oxygen exchange rates of three Ruddlesden–Popper phases, general form A_n−1A^′₂B_nO_3n+1, A_n−1A^′₂B_nX_3n+1; LaSrCo_0.5Fe_0.5O_4−δ (n = 1), La_0.3Sr_2.7CoFeO_7−δ (n = 2) and LaSr₃Co_1.5Fe_1.5O_10−δ (n = 3). These measurements are complemented by surface composition determination via high sensitivity-low energy ion scattering. We observe a correlation between bulk ion mobility and surface exchange rate between materials. The surface exchange rates vary by more than one order of magnitude with high anion mobility in the bulk of an oxygen vacancy-rich n = 2 Ruddlesden–Popper material correlating with rapid oxygen exchange. This is in contrast with the similar surface exchange rates which we may expect due to similar surface compositions across all three samples. We conclude that experimental limitations lead to inherent convolution of surface and bulk rates, and that surface exchange steps are not likely to be rate limiting in oxygen incorporation.