Structural Components and Anisotropic Dissolution Behaviors in One Hexagonal Single Crystal of β-Tricalcium Phosphate

Abstract

Large-scale β-tricalcium phosphate (β-TCP) hexagonal single crystals were synthesized at a relatively low temperature (150 °C) by using a solution-phase method. The solvent, ethylene glycol, played an important role during the formation of the homogeneous submicron-sized crystals. Unlike the conventional understanding of a single crystal, the wall of the formed β-TCP hexagonal was well crystallized, showing different physicochemical properties from the bulk part. The dissolution spots were anisotropically distributed throughout the single crystal. The bulk part dissolved readily from the top and bottom planes in the undersaturated solutions, but the thin hexagonal wall could be stable against any dissolution even in pure water. These differences between the wall and the bulk part were attributed to the different crystallinities and defect densities in their structures. It was suggested that the low defect number might stem from the solvent−interface exchange that was allowed the edge surfaces in contact with the solution. And the rapid growth of the particles resulted in the randomly distributed defects in the bulk part, which induced a selective dissolution along the c-axis of β-TCP. Furthermore, the stability of wall could be explained by a size effect during the nanodemineralization. It was interesting that both the wall and the bulk part shared the exact same lattice fringes under the transmission electron microscope. This phenomenon implied that both components were crystallographically identical so that they were constructed into an integral single crystal of β-TCP. The distinct dissolution behaviors of these two parts in one single crystal resulted in the formation of porous, gearlike, and ringlike single crystals at different demineralization stages, which demonstrated an easy control of crystal morphology patterns by using the anisotropic dissolution behavior.