Visible-Near-Infrared Diffuse Reflectance and Optical Energy Gap of Sodium Niobate Modified by Mn Ions: Na(Nb1-zMnz)O3 and (Na1-xBix)(Nb1-yMny)O3 Compounds Study

The optical and electrical properties of sodium niobate crystals and ceramics doped with Mn atoms are studied. Na(Nb1-zMnz)O-3 crystals exhibit actual z = 0-1.0 wt% Mn content. The Na(Nb1-zMnz)O-3 ceramics contains Mn from the 0-5 wt% range. The (Na1-xBix)(Nb1-yMny)O-3 ceramics is codoped with x = 0.015-0.50 and y = 0.01-0.33. Mn addition decreases the reflectance magnitude. Combined nonstoichiometry and local disorder in the crystal lattice, resulting from Mn doping and oxygen vacancies, induce contributions to the electronic structure. The visible and near-infrared (Vis-NIR) diffuse reflectance, R(lambda), spectra include a linear combination of contributions. The modified Kubelka-Munk function representation allows estimation of the optical energy gap, E-gap. This estimation is qualitative because of the complex structure of the samples. Two optical gaps, showing dependence on Mn content, are discerned when Mn content E-gap,E-high changes toward the Shockley-Queisser limit, from 1.85 to 1.4 eV, when Mn content is low (z < 1 wt%). Simultaneous occurrence of E-gap,E-low of 1.0-1.3 eV suggests tuning possibility. E-gap of approximate to 1 eV dominates optical properties when Mn content is >1 wt%. The optical features are consistent with activation energy attributed to thermally generated electrical conductivity.