Synthesis and color properties of neodymium–doped holmium molybdate pigments with allochroic effect

Abstract

A series of neodymium–doped holmium molybdate (Ho1-xNdx)2MoO6 (x = 0-0.8) color pigments with ‘allochroic effect’ were synthesized by a solid-phase reaction method under different conditions (i.e., sintering temperature, sintering time, and neodymium ions (Nd3+) doping amount). The as-synthesized pigments were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry and differential scanning calorimetry (TG-DSC), ultraviolet–visible–near infrared (UV-VIS-NIR) spectrophotometry and CIE L*a*b* color scale. The results show that the adulteration of Nd3+ ions does not alter the crystal structure of Ho2MoO6 basically, but causes the lattice expansion for the pigments sintered at <1200 °C for 3 h. The pigments as powders or coatings on glass or ceramic tile have an allochroic effect under different illuminant sources (i.e., solar-light D65, incandescent lamp A, cool white fluorescent lamp F2 and three-band fluorescent lamp F11). For the pigment with Nd3+ ions doping amount of 0.2, the color difference (ΔE*) between standard illuminants D65 and F11 is 14.58, and the ΔE* value between any two standard illuminants is greater than 5.6, showing an intense allochroic effect. Based on the analysis of the abundant energy level structure of holmium ions (Ho3+) and Nd3+ ions and the spectral power difference of illuminant sources, the pigments have color variation under different illuminant sources. The pigments with a good thermal stability at high temperatures (i.e., 700–1200oC) can be used as promising functional pigments with ‘allochroic effect’ for glass or ceramic decoration.