Illuminant-dependent color change in neodymium tungstate based pigments: role of crystal structure and Judd-Ofelt analysis

Neodymium tungstate materials (i.e., Nd 2 WO 6 , Nd 2 W 2 O 9 , and Nd 2 (WO 4 ) 3 ) as novel allochroic inorganic pigments were synthesized by a solid-state reaction method. The pigments were characterized by X-ray diffraction, UV–visible–NIR spectrophotometry and colorimetry. The results show that the pigment with a monoclinic phase of Nd 2 (WO 4 ) 3 displays noticeable color changes from purple to blue, and then to gray under varying illuminants with a maximum color difference (Δ E∗ ) of 13.58. A relationship between crystal structure characteristics and allochroic function was investigated, and the corresponding mechanism was analyzed based on the Judd-Ofelt model and the crystal structure of the pigments. It is revealed that a lower symmetry in the Nd 3+ crystal environment and a higher covalency increase the probability of 4 f electronic transitions and illuminant sensitivity, thereby enhancing the allochroic effect.