Increasing A-type CO₃^2- substitution decreases the modulus of apatite nanocrystals

Biological apatite mineral is highly substituted with carbonate (CO₃^2-). CO₃^2- 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 CO₃^2- substituted apatites are poorly understood. Therefore, A-type CO₃^2- apatites with biologically relevant levels of CO₃^2- (1.7-5.8 wt%) were prepared and characterized. The addition of A-type CO₃^2- 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 CO₃^2- substitutions also reduced the bulk and elastic moduli of the crystals. These results show that although A-type CO₃^2- may inhibit lattice changes caused by B-type CO₃^2-, A-type CO₃^2- enhances the reduction in crystal order and mineral stiffness. These results help us to identify the possible contributions of A-type CO₃^2- substitutions in biological apatites that contain both A- and B-type CO₃^2-. In addition, this implies that the stiffness of bioapatite may change with increasing A-type CO₃^2- substitutions, potentially altering the fracture mechanics of calcified tissues and biomaterials.