Investigation of the effect of doped Zn atom to the hydroxyapatite based on experimental method and first-principles calculations

In this paper, experimental method and first-principles calculations were combined to study the properties of pure hydroxyapatite and Zn-doped hydroxyapatite. Pure and Zn-doped hydroxyapatite (HA) powders were synthesized using the hydrothermal method. The FTIR spectra show that the adding of Zn atoms makes the intensity of PO43− stretching peaks become weaken and obscure. In addition, the O-H bending peaks at 3570 and 632 cm−1 become weaker and broader. The XRD patterns show that the replacement of Zn ion in the HA structure leads to the distortion in HA lattice, which is shown and proved by the TEM analysis. First-principles calculations based on density functional theory (DFT) method were applied to investigate the Zn-doped hydroxyapatite. The elastic properties of HA, HA with Ca2+(II) vacancy, and Zn doped HA systems were calculated. Through the calculation, we found that the doping of Zn atoms made the elastic modulus of HA crystal becoming soft. The elastic properties dropped dramatically when there is a vacancy in the Ca2+(II) position. Compared with the HA-VCa2+ structure, the Zn-doped HA has better elastic properties and stability. The local structure of the substitution position was presented and analyzed. In the Zn-doped HA structure, the OH− group adjacent to Zn2+ is significantly displaced from the c axis toward to the Zn2+, making the lattice distorted. The distortion of the crystal lattice is responsible for the weakness of the P-O stretching peaks and O-H bending peaks in the FTIR spectra of the Zn doped HA crystal. The softness of the elastic modulus is also caused by the distortion of the lattice and the weak bonding force between Zn atom and O atom.