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

The layered perovslcite compounds NaLnTiO(4) (Ln = Y, La Tb) have Ln(2)O(2) rock-salt layers alternating with Na2O2 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(x)LnTiO(4) and Li1+xInTiO4 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) Eu3+/Eu2+ 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 Li4Ti5O12 could be attributed to a capacitance energy associated with the formal +2e and -2e 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 spine! titanates.