High Conductivity and Diffusion Mechanism of Oxide Ions in Triple Fluorite-Like Layers of Oxyhalides

Oxide ion conductors are attractive materials because of their wide range of applications, such as solid oxide fuel cells. Oxide ion conduction in oxyhalides (compounds containing both oxide ions and halide ions) is rare. In the present work, we found that Sillén oxychlorides, Bi_2-TeLuO₄₊Cl (x = 0, 0.1, and 0.2), show high oxide ion conductivity. The bulk conductivity of Bi_1.9Te_0.1LuO_4.05Cl reaches 10^-2 S cm^-1 at 431 °C, which is much lower than 644 °C of yttria-stabilized zirconia (YSZ) and 534 °C of La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 (LSGM). Thanks to the low activation energy, Bi_1.9Te_0.1LuO_4.05Cl exhibits a high bulk conductivity of 1.5 × 10^-3 S cm^-1 even at a low temperature of 310 °C, which is 204 times higher than that of YSZ. The low activation energy is attributed to the interstitialcy oxide ion diffusion in the triple fluorite-like layer, as evidenced by neutron diffraction experiments (Rietveld and neutron scattering length density analyses), bond valence-based energy calculations, static DFT calculations, and ab initio molecular dynamics simulations. The electrical conductivity of Bi_1.9Te_0.1LuO_4.05Cl is almost independent of the oxygen partial pressure from 10^-18 to 10^-4 atm at 431 °C, indicating the electrolyte domain. Bi_1.9Te_0.1LuO_4.05Cl also exhibits high chemical stability under a CO₂ flow and ambient air at 400 °C. The oxide ion conduction due to the two-dimensional interstitialcy diffusion is considered to be common in Sillén oxyhalides with triple fluorite-like layers, such as Bi_1.9Te_0.1RO_4.05Cl (R = La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu) and Bi_6-2Te₂O₈₊Br₂ (x = 0.1, 0.5). The present study opens a new field of materials chemistry: oxide ion-conducting Sillén oxyhalides with triple fluorite-like layers.