Structural, electronic, dynamical and thermodynamic properties of Ca10(PO4)6(OH)2 and Sr10(PO4)6(OH)2: First-principles study

Density functional theory (DFT) with optPBE-vdW functional is used to simulate the structural, electronic, dynamical and thermodynamic properties of Ca10(PO4)6(OH)2(Ca-HA) and Sr10(PO4)6(OH)2(Sr-HA). The calculated structural properties within optPBE-vdW functional is found to yield better agreement with the experimental results, which indirectly suggests the important role of weak hydrogen bond in this crystal. The calculated electronic properties indicate that Ca-HA and Sr-HA are insulator materials with indirect band gap of 5.52 eV and 5.10 eV, respectively. The detailed dynamical properties of two apatites are obtained by the linear-response approach. With replacement of Ca by Sr, the librational mode of OH group decreases from 612 cm−1 to 569 cm−1, the stretching mode of OH group increases from 3614.5 cm−1-3649.9 cm−1, which is consistent with the experimental results. Finally, some phonon related thermodynamic properties, such as Helmholtz free energy F, internal energy E, entropy S and heat capacity CV of Sr-HA and Ca-HA are studied according to the phonon calculations within the harmonic approximation. The present calculation results of two apatites with optPBE-vdW functional are in good agreement with the existing experimental.