Impact of Harsh Grinding on the Structure and Color of LiCoPO₄ and LiNiPO₄ Pigments

LiCoPO₄ and LiNiPO₄ phosphate pigments have colorations very close to the primary colors of the subtractive system: magenta and yellow, respectively. These two pigments are therefore of great interest in a variety of applications, including e-reader devices. However, the need to reduce their crystallite size in order to formulate stable electrophoretic inks has revealed that aggressive milling results in significant color changes, particularly for cobalt-based pigments. Here, we elucidate the origin of this phenomenon by carefully examining the structural changes induced by grinding, utilizing a combination of X-ray diffraction pattern refinement, molecular dynamics simulations, and UV-visible near-infrared, EPR, and NMR spectroscopies. Structural distortion near the crystallite surfaces was shown to be responsible for the change in coloration. As the surface-to-volume ratio increases with a decreasing crystallite size, these atomic reorganizations in the first few atomic layers close to the surface become increasingly significant and even critical for sample coloration.