“…In the case of Ni 2+ , previous research has effectively produced various blue hues by Ni 2+ doping in regular tetrahedral coordination in structures such as CaAl 12 O 19 , BaAl 12 O 19 , MgAl 2 O 4 , and so on. When Ni 2+ resides in distorted tetrahedral positions, it exhibits different colors such as magenta and lilac, , as shown in Figure . However, these pigments have the problems of complicated preparation, expensive price, or poor color performance, and an intense blue hue has not yet been achieved by Ni 2+ into tetrahedral coordination.…”