Formation and structure of zinc-substituted calcium hydroxyapatite

Zn-substituted Ca hydroxyapatites were synthesized by precipitation method under the specific conditions (pH 8, 90 °C) and their structural properties were investigated. The substituting limit of Zn was estimated at about 15 mol%. The lattice parameter a decreased up to 5 mol% Zn, and started to increase over 5 mol% Zn. The lattice parameter c monotonously decreased with increase in Zn fraction. The increase in lattice parameter a for higher Zn fraction was ascribed to increasing amount of lattice H2O which substituted for OH sites in the apatite structure. The lattice H2O in the Zn-substituted apatites was lost by the heat treatment at 400 °C. As a result, both the lattice parameters a and c of the heat-treated apatites decreased with increasing Zn fraction. Only the difference in ionic radius between Zn2+ (0.074 nm) and Ca2+ (0.099 nm) was decisive to the change in both lattice parameters after the heat treatment at 400 °C.