Origin of the Yellow Color of Complex Nickel Oxides

Single-crystal optical absorption spectra of NiO, NiTiO_3, NiWO_4, NiV_2O_6, NiNb_2O_6, Ni_2SiO_4, Ni_3V_2O_8, LiNiPO_4, Li_2Ni_2MO_3O_(12), SrNiTeO_6, LiScSiO_4:Ni, MgSiO_3:Ni, and (Mg,Ni)_2SiO_4 are presented for the purpose of comparing the spectra of yellow and green Ni^(2+) compounds. Powder spectra of NiTiO_3, NiWO_4, NiV_2O_6, NiNb_2O_6, and Ni_3V_2O_8 in the ultraviolet region help elucidate the more intense charge transfer bands. Bright yellow color results when Ni^(2+) is in a six-coordinated site significantly distorted from octahedral symmetry. Increased absorption intensity occurs when the metal ion d-d bands are in proximity to an ultraviolet charge transfer band