Increasing A-type CO32− substitution decreases the modulus of apatite nanocrystals

Abstract

Biological apatite mineral is highly substituted with carbonate (CO₃²⁻). CO₃²⁻ can exchange for either phosphate, known as B-type, or hydroxyl groups, known as A-type. Although the former has been extensively studied, A-type CO₃²⁻ substituted apatites are poorly understood. Therefore, A-type CO₃²⁻ apatites with biologically relevant levels of CO₃²⁻ (1.7–5.8 wt%) were prepared and characterized. The addition of A-type CO₃²⁻ into the apatite structure caused the predicted expansion of the a-axis and contraction of the c-axis in the unit cell. This was accompanied by a significant modification in the atomic order, especially along the a-axis plane, and crystallite size. A combination of in situ loading with synchrotron X-ray Diffraction and Density Functional Theory showed that increasing A-type CO₃²⁻ substitutions also reduced the bulk and elastic moduli of the crystals. These results show that although A-type CO₃²⁻ may inhibit lattice changes caused by B-type CO₃²⁻, A-type CO₃²⁻ enhances the reduction in crystal order and mineral stiffness. These results help us to identify the possible contributions of A-type CO₃²⁻ substitutions in biological apatites that contain both A- and B-type CO₃²⁻. In addition, this implies that the stiffness of bioapatite may change with increasing A-type CO₃²⁻ substitutions, potentially altering the fracture mechanics of calcified tissues and biomaterials.