Synthesis, exfoliation, and electronic/protonic conductivity of the dion-jacobson phase layer perovskite HLa₂TiTa₂O ₁₀

Electrochemical impedance spectroscopy was used to study the transport properties of the three-layer Dion-Jacobson phase HLa₂Ti ₂TaO₁₀ in the temperature range of interest (250-475 C) for intermediate temperature fuel cells. The compound was prepared by proton exchange of RbLa₂Ti₂TaO₁₀, which in turn was made by direct solid state synthesis or by an organic precursor-based method. When prepared by the precursor method, HLa₂Ti₂TaO ₁₀·nH₂O (n = 1-2) could be exfoliated by tetrabutylammonium hydroxide to produce rectangular sheets with ∼30 nm lateral dimensions. HLa₂Ti₂TaO₁₀·nH ₂O lost intercalated water at temperatures between 100 and 200 C, but X-ray diffraction patterns up to 500 C did not show evidence of collapse of the interlayer galleries that has been observed with the structurally similar compound HCa₂Nb₃O₁₀. Under humid hydrogen atmosphere, the conductivity of HLa₂Ti₂TaO₁₀ followed Arrhenius behavior with an activation energy of 0.9 eV; the conductivity was in the range of 10^-9 to 10^-5 S cm ^-1 depending on the preparation conditions and temperature. Modification of the stoichiometry to produce A-site or B-site (vacancy or substitution) defects decreased the conductivity slightly. The conductivity was approximately 1 order of magnitude higher in humid hydrogen than in humid air atmospheres, suggesting that the dominant mechanism in the intermediate temperature range is electronic. A-site substitution (Sr²⁺ for La³⁺) beyond the Ruddlesden-Popper phase limit converted the layered pervoskite to a cubic perovskite Sr_2.5□_0.5Ti ₂TaO₉ with 2 orders of magnitude higher conductivity than HLa₂Ti₂TaO₁₀ at 475 C.