Impact of Harsh Grinding on the Structure and Color of LiCoPO and LiNiPO Pigments.

LiCoPO and LiNiPO 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.