Synthesis, Exfoliation, and Electronic/Protonic Conductivity of the Dion−Jacobson Phase Layer Perovskite HLa2TiTa2O10

Electrochemical impedance spectroscopy was used to study the transport properties of the three-layer Dion−Jacobson phase HLa2Ti2TaO10 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, HLa2Ti2TaO10·nH2O (n = 1−2) could be exfoliated by tetrabutylammonium hydroxide to produce rectangular sheets with ∼30 nm lateral dimensions. HLa2Ti2TaO10·nH2O 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 (Sr for La) beyond the Ruddlesden−Popper phase limit converted the layered pervoskite to a cubic perovskite Sr2.5□0.5Ti2TaO9 with 2 orders of magnitude higher conductivity than HLa2Ti2TaO10 at 475 °C.