Ca-vacancy effect on the stability of substitutional divalent cations in calcium-deficient hydroxyapatite |
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Authors: | Tatsushi Saito Yuto Ishikawa Yusuke Noda Tatsuya Yokoi Yu Oshima Atsutomo Nakamura Katsuyuki Matsunaga |
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Affiliation: | 1. Department of Materials Physics, Nagoya University, Nagoya, Japan;2. Department of Information and Communication Engineering, Okayama Prefectural University, Soja, Japan;3. Department of Mechanical Science and Bioengineering, Osaka University, Toyonaka, Japan |
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Abstract: | First-principles calculations were performed to reveal an effect of Ca vacancies on the stability of substitutional divalent cations M2+ (M = Mg, Zn, Sr) in Ca-deficient hydroxyapatite (dHAp). M2+ concentrations up to 20 mol% in dHAp were considered, and the most stable substitutional sites and their lowest energy configurations in the dHAp lattice were examined with the aid of a generic algorithm method. It was found that defect formation energies of substitutional M2+ are lower in dHAp than in stoichiometric HAp (sHAp) at all M2+ concentrations. This indicates that these M2+ ions are more favorably involved in dHAp than in sHAp, which is in reasonable agreement with experiment. Detailed analyses on atomic structures in dHAp show that the presence of a Ca vacancy varies its surrounding Ca–O bond lengths over a wide area so that Ca–O polyhedrons with various sizes are produced. As a result, M2+ ions can predominantly occupy Ca sites at which M2+ fits better, depending on the ionic radii of M2+. For Zn2+ substitution in dHAp, its defect formation energy decreases more with the increasing concentrations and has the minimum value at 15 mol%. Such a trend can be understood from changes in effective coordination numbers of Zn in dHAp. |
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Keywords: | atomic structure bioceramics defects density functional theory impurities |
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