Solar reflective brick brown stable iron doped magnesium aluminate: Combustion synthesis, structure and chromatic properties

Abstract

Spinel oxide host MgAl2O4 was chemically engineered to derive solar reflective colored pigments. The spinel oxides of the formula MgAl2-xFexO4 (x = 0.5–2.0) and Mg1-xMxAl₂O₄ (x = 0.1–0.5, M = Cu, Ni, Mn) were synthesized by combustion route. Cationic disorder in the parent MgAl2O4 was noted to transform into ordered arrangement with Fe3+ and Mn2+ ionic substitution. Fe3+ ion substitution imparted brick brown color of varied hue. Whereas, substitution of Mg2+ ion by Mn2+ and Ni2+ (x = 0.1) ions resulted in coffee brown color of light and dark shades and cyan color respectively. The optical band gap of the host (3.9 eV) decreased to 2.6–2.0 eV with Fe3+ substitution, 2.5–2.0 eV with Mn2+ ion substitution and with Ni2+ ion substitution showed slight decrease to 3.8 eV. The % solar reflectance of the phases MgAl2-xFexO4 was determined to be 73–80% with 80% in the case of MgAl1·0Fe1·0O4.