One-Dimensional Growth of Li2NiPO4F Single Crystals from Intermediate LiNiPO4 Crystal Surface Using KCl-KI Fluxes

The shape of an inorganic material affects its luminescence, catalytic activity, surface energy, solubility, and conductivity, which are important properties of a functional inorganic material. Herein, we demonstrate the one-dimensional growth of Li2NiPO4F single crystals from a KCl-KI binary flux; these crystals are oriented along the b axis through the conversion of intermediate LiNiPO4. The crystal surface was assigned as {h0l} and included {100} and {001} as major faces. The formation mechanism and growth manner are discussed in terms of phase transitions and morphological changes. Since the LiNiPO4 surface was gradually converted into Li2NiPO4F by reacting with LiF, as evidenced from cross-sectional scanning electron microscopy observations, we speculate that the growth of one-dimensional crystals toward the exterior of LiNiPO4 was driven by the relaxation of compressive stresses formed on the surface as well as by the formation of a steep supersaturation gradient near the LiNiPO4 surface.