Using a microscopic model and Green's function theory, we have calculated the band gap energy and the polarization of LiNbO3, KNbO3, AgNbO3, and NaNbO3. The effects by substitution of different ions at A or/and B sites for doping concentration x = 0-0.1 are studied. The observed different tuning of these properties is discussed for the possibility of photovoltaic and energy storage applications of these compounds. They should have a large polarization and narrow band gap. It is shown that the band gap of all substances decreases or increases with increasing Fe or Zn dopant at the Nb site, respectively. But the substitution, for example, of Ba at the A site, leads to different behaviors of these materials. The polarization increases by Ba doping at the A site and decreases by Fe doping at the Nb site. For example, by Ba/Fe, Ba/Ni co-doping (Ba at the A site and Fe, Ni at the B site) we observe both an enhanced polarization and reduced band gap.
Keywords: XNbO3; band gap energy; ion doping; microscopic model; polarization.