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 HLa 2 Ti 2 TaO 10 in the temperature range of interest (250–475 °C) for intermediate temperature fuel cells. The compound was prepared by proton exchange of RbLa 2 Ti 2 TaO 10, which in turn was made by direct solid state synthesis or by an organic precursor-based method. When prepared by the precursor method, HLa 2 Ti 2 TaO 10 · n H 2 O ( n = 1–2) could be exfoliated by tetrabutylammonium hydroxide to produce rectangular sheets with ∼30 nm lateral dimensions. HLa 2 Ti 2 TaO 10 · n H 2 O 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 HCa 2 Nb 3 O 10 . Under humid hydrogen atmosphere, the conductivity of HLa 2 Ti 2 TaO 10 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 2+ for La 3+ ) beyond the Ruddlesden–Popper phase limit converted the layered pervoskite to a cubic perovskite Sr 2.5 □ 0.5 Ti 2 TaO 9 with 2 orders of magnitude higher conductivity than HLa 2 Ti 2 TaO 10 at 475 °C.