Inorganic Green Pigments Based on LaSr2AlO5

Abstract

La1.03Sr1.97Al0.97M0.03O5 (M = Fe, Co, Ni, and Cu) samples were synthesized using a citrate sol–gel method to develop a novel environmentally friendly inorganic green pigment. Among them, the Co-doped sample exhibited a vivid yellow, but not green. Then, (La0.94Ca0.06)Sr2(Al0.97Mn0.03)O5 was synthesized and characterized with respect to the crystal structure, optical properties, and color. The sample was obtained in a single-phase form and the lattice volume was smaller than that of the (La0.94Ca0.06)Sr2AlO5 sample, indicating that Mn ions in the lattice of the sample were pentavalent. The sample exhibited optical absorption at a wavelength below 400 nm and around 650 nm. These absorptions were attributed to the ligand, the metal charge transfer (LMCT), and d-d transitions of Mn5+. Because the green light corresponding to 500 to 560 nm was reflected strongly, the synthesized sample exhibited a bright green color. (La0.94Ca0.06)Sr2(Al0.97Mn0.03)O5 showed high brightness (L* = 50.1) and greenness (a* = −20.8), and these values were as high as those of the conventional green pigments such as chromium oxide and cobalt green. Therefore, the (La0.94Ca0.06)Sr2(Al0.97Mn0.03)O5 pigment is a potential candidate for a novel environmentally friendly inorganic green pigment.