Effect of an internal electric field on the redox energies of ALn TiO ₄ (A = Na or Li, Ln = y or Rare-Earth)

The layered perovskite compounds NaLnTiO₄ (Ln = Y, La-Tb) have Ln₂O₂ rock-salt layers alternating with Na ₂O₂ rock-salt layers on either side of a TiO₂ sheet. The cation ordering introduces an internal electric field perpendicular to the layers. Ion exchange of Li⁺ for Na⁺ changes the rock-salt Na₂O₂ layers into OLi₂O antifluorite layers but does not change the charge of the layers or, significantly, the internal electric field. The internal fields induce a ferroic displacement of the Ti(IV) ions toward the alkali-ion layers. Reversible Li insertion is observed in both NaLi_xLnTiO₄ and Li _1+xLnTiO₄ with 0 ≤ x ≤ 1; the voltage profiles show Ti(IV)/Ti(III) redox plateaus at 0.5 and 0.1 V vs Li⁺/Li⁰ and a 4f⁶/4f⁷ Eu³⁺/Eu²⁺ plateau at 0.8 V vs Li⁺/Li⁰. The shift of 1 eV in the Ti(IV)/Ti(III) redox energy relative to that found in the spinel Li₄Ti ₅O₁₂ could be attributed to a capacitance energy associated with the formal +2e and -2e charges on the Ln₂O ₂ layers and the alkali-ion layers, respectively. Optical measurements showed that the energy gap between the Ti 3d⁰ and O 2p⁶ band edges is not significantly different in the layered and spinel titanates.