Microscopic property of Li impurity in biphasic calcium phosphate

Recently Li doping is employed to control the properties of bioceramic biphasic calcium phosphate (BCP), which consists of \(\beta -\hbox {Ca}_{3}(\hbox {PO}_4)_2\) (\(\beta\)-TCP) and hydroxyapatite(HAp). We investigated the microscopic properties of Li in BCP through first-principles electronic structure calculations. The stable microscopic structure of Li-driven state is identified by comparing the formation enthalpies of Li-driven structures: interstitial (\(\hbox {Li}_{int}\)) and substitutional (\(\hbox {Li}_{Ca}\)). It is found that the \(\hbox {Li}_{int}\) is more stable than the substitutional state \(\hbox {Li}_{Ca}\). The \(\hbox {Li}_{int}\) atom is strongly coupled to three or four O atoms, which stabilizes the interstitial state. The \(\hbox {Li}_{int}\) is calculated to be relatively more stable in \(\beta\)-TCP than in HAp, because the electronic level of \(\hbox {Li}_{int}\) is much lower in \(\beta\)-TCP than in HAp.