First-principles calculations of Zn-K XANES in Ca-deficient hydroxyapatite

The local environment of substitutional Zn2+ in Ca-deficient hydroxyapatite (HAp) was investigated using experimental and theoretical analyses of the x-ray absorption near edge structure (XANES). For Zn-K XANES calculations, two situations of Zn2+ were considered. One was Zn2+ substituted for Ca sites in perfect HAp, and the other was a Ca-deficient HAp model of substitutional Zn2+ associated with a Ca2+ vacancy charge compensated by two protons. The model of Zn2+ in perfect HAp did not reproduce the experimental Zn-K XANES spectrum. In contrast, the Ca-deficient HAp model agreed well with the experimental spectrum. This indicates that substitutional Zn2+ in Ca-deficient HAp is associated with the Ca2+ vacancy complex in HAp.