Effect of an Internal Electric Field on the Redox Energies of ALnTiO4 (A = Na or Li, Ln = Y or Rare-Earth)

The layered perovskite compounds Na Ln TiO 4 ( Ln = Y, La–Tb) have Ln 2 O 2 rock-salt layers alternating with Na 2 O 2 rock-salt layers on either side of a TiO 2 sheet. The cation ordering introduces an internal electric field perpendicular to the layers. Ion exchange of Li + for Na + changes the rock-salt Na 2 O 2 layers into OLi 2 O 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 x Ln TiO 4 and Li 1+ x Ln TiO 4 with 0 ≤ x ≤ 1; the voltage profiles show Ti(IV)/Ti(III) redox plateaus at 0.5 and 0.1 V vs Li + /Li 0 and a 4f 6 /4f 7 Eu 3+ /Eu 2+ plateau at 0.8 V vs Li + /Li 0 . The shift of 1 eV in the Ti(IV)/Ti(III) redox energy relative to that found in the spinel Li 4 Ti 5 O 12 could be attributed to a capacitance energy associated with the formal +2 e and −2 e charges on the Ln 2 O 2 layers and the alkali-ion layers, respectively. Optical measurements showed that the energy gap between the Ti 3d 0 and O 2p 6 band edges is not significantly different in the layered and spinel titanates.