Paracrystalline Disorder from Phosphate Ion Orientation and Substitution in Synthetic Bone Mineral

Hydroxyapatite is an inorganic mineral
closely resembling the mineral phase in bone. However, as a biological
mineral, it is highly disordered, and its composition and atomistic
structure remain poorly understood. Here, synchrotron X-ray total
scattering and pair distribution function analysis methods provide
insight into the nature of atomistic disorder in a synthetic bone
mineral analogue, chemically substituted hydroxyapatite. By varying
the effective hydrolysis rate and/or carbonate concentration during
growth of the mineral, compounds with varied degrees of paracrystallinity
are prepared. From advanced simulations constrained by the experimental
pair distribution function and density functional theory, the paracrystalline
disorder prevalent in these materials appears to result from accommodation
of carbonate in the lattice through random displacement of the phosphate
groups. Though many substitution modalities are likely to occur in
concert, the most predominant substitution places carbonate into the
mirror plane of an ideal phosphate site. Understanding the mineralogical
imperfections of a biologically analogous hydroxyapatite is important
not only to potential bone grafting applications but also to biological
mineralization processes themselves.