Lanthanum-strontium copper silicates as intense blue inorganic pigments with high near-infrared reflectance

Abstract

Novel non-toxic intense blue near-infrared reflecting inorganic pigments having the general formula Sr1-xLaxCu1-yLiySi4O10 (x = y ranges from 0 to 0.5) were developed as viable alternatives to existing toxic cobalt based blue colorants. The pigment powders were characterized by XRD and UV-vis-NIR diffuse reflectance spectroscopy. The substitution of La3+ for Sr2+ and Li+ for Cu2+ in SrCuSi4O10 gently changes the color of the pigment from sky-blue to intense blue and as a result the band gap of the pigment powders increases from 2.59 to 2.68 eV. The coloring mechanism is based on the crystal field splitting of the Cu2+ d-orbitals in a square planar environment. Finally the ability of the pigments to transfer the color as well as NIR reflectance properties was demonstrated by coating on to concrete cement block and PMMA. Most importantly, the developed pigments exhibit intense blue color with impressive NIR solar reflectance (67%) and thermally stable.