Color tuning by Si/Ge substitution in Cr4+ doped melilite inorganic pigments

Rockery constitutes one of the significant elements of Chinese royal garden design, and it becomes an emerging interdisciplinary topic covering both science and art. In this work, we report an attempt to restore the color of the ancient rockeries by finding the appropriate inorganic pigments. Chromium is doped as activators into strontium melilites with varying Si/Ge ratio, and the synthesized powders give a series of color changing from ice blue to dark-sea green. With the structure and spectroscopic characterizations, we discuss the substitutions of Cr in Mg/Si/Ge sites of the melilite host. The Tanabe-Sugano diagram of 3d 2 configuration is employed to explain the change of diffuse reflectance spectra caused by Si/Ge substitution. With the work, we hope to inspire the interdisciplinary research and promote the rockery heritage restoration and protection.