Study of manganese substitutions in hydroxyapatite using density functional theory methods: Optical and magnetic properties

Being the mineral component of the bone tissue, hydroxyapatite (HAP) is widely used in medicine for the bone tissue restoration. The crystal structure of HAP is very flexible and easily integrates various ions, which affects the properties of HAP. This paper presents the data for modeling the Mn-HAP lattice with various Mn/Ca substitutions obtained using calculations of the density functional theory. Experimental data on the synthesis of Mn-HAP by the mechanochemical method are also presented. The calculated and experimental data show good agreement: the unit cell parameters and volume decrease with increasing Mn/Ca substitution. These results on the behavior of structural parameters are similar to the results obtained for Mg/Ca substitutions in Mg-HAP. However, there are significant differences in the changes in the electronic and optical properties for these substituents. Additional electronic energy levels Ei were detected inside the band gap Eg of Mn-HAP, while Mg-HAP did not have energy levels inside the band gap, only the width of the Eg changed. Depending on the concentration of Mn, the photoexcitation energy changes, and its effective value Eg* becomes less than the band gap Eg in the unsubstituted HAP. The arisen magnetic properties of Mn-HAP are proportional to the amount of Mn introduced and energy levels Ei, filled by spin-up electrons. The formation energy of Mn/Ca substitution depends on the position and concentration of Mn. It was found that substitution is more preferable for the Ca2 position.