Electronic properties of Bi2O3 based solid electrolytes

Bi2O3 based solid electrolytes with partial substitution of Bi by Y, Zr, or Pr were prepared and studied by UV and X-ray photoelectron spectroscopies (UPS, XPS) as well as electron energy loss spectroscopy (EELS). Changes of the band gap and of the position of the Fermi level in the band gap with respect to the band edges were determined as a function of composition. An increase of the band gap is desirable for Bi2O3 based solid electrolytes in order to decrease the electronic conductivity and enlarge the stability and electrolytic domain of these materials. The band gap increases with Y concentration from E-gap = 2.8 eV for Bi0.75Y0.25O1.5 to E-gap = 3.2 eV for Bi0.60Y0.4O1.5 Dissolving PrO1.83 in Bi0.75Y0.25O1.5 up to a composition (Bi0.75Y0.25O1.5)(0.98)(PrO1.83)(0.02) yields no change of the band gap, whereas Zr substitution causes an increase by 0.4 eV to a value of E-gap = 3.2 eV for (Bi0.75Y0.25O1.5)(0.9)(ZTO(2))(0.1). The position of the Fermi level was 2.1 eV above the valence band edge for Bi0.75Y0.25O1.5 in equilibrium with Fel FeO reference contacts slightly increasing to 2.3 eV for Bi0.60Y0.4O1.5. The values were temperature independent in the range 650 K to 900 K. Band tails were observed at the band edges extending into the band gap and indicating the high ionic disorder in the lattice. Additional O1s XPS data are given with regard to the surface oxygen species.