First-principles investigations on the visible light photocatalytic activity of NaNbO3 by N and F dopingYong-Qiang Xu, Shao-Yi Wu, Li-Na Wu, Li-Juan Zhang and Si-Ying ZhongEur. Phys. J. B, 92 4 (2019) 68DOI: https://doi.org/10.1140/epjb/e2019-90467-5