Bystrov Vladimir S, Paramonova Ekaterina V, Avakyan Leon A, Eremina Natalya V, Makarova Svetlana V, Bulina Natalia V
Institute of Mathematical Problems of Biology-Branch of Keldysh Institute of Applied Mathematics, Russian Academy of Sciences, 142290 Pushchino, Russia.
Physics Faculty, Southern Federal University, 344090 Rostov-on-Don, Russia.
Materials (Basel). 2023 Aug 30;16(17):5945. doi: 10.3390/ma16175945.
Hydroxyapatite (HAP) is the main mineral component of bones and teeth. It is widely used in medicine as a bone filler and coating for implants to promote new bone growth. Ion substitutions into the HAP structure highly affect its properties. One of the most important substituents is magnesium. This paper presents new results obtained using high-precision hybrid density functional theory calculations for Mg/Ca substitutions in HAP in a wide magnesium concentration range within a 2 × 2 × 2 supercell model. Experimental data on the mechanochemical synthesis of HAP-Mg samples with different Mg concentrations are also presented. A comparison between the experiment and the theory showed good agreement: the HAP-Mg unit cell parameters and volume decreased with increasing degree of Mg/Ca substitution. The changes in the distances between the Ca and O, Ca and H, and Mg and O ions upon Mg/Ca substitution in different calcium positions was analyzed. The resulting asymmetry and distortion of the cell parameters were evaluated. It was shown that bulk modulus, energy levels, and band gap depend on the degree of Mg substitutions in the Ca1 and Ca2 positions. The formation energies of Mg/Ca substitutions showed non-monotonic behavior that was different for Ca1 and Ca2 positions. The Ca2 position had a slightly higher probability (~5 meV/f.u.) of substitution than Ca1 position at a Mg concentration x = 0.5. At x = 1, substitution in both positions can coexist. The simulated IR spectra for different Mg/Ca substitutions showed that Mg in the Ca2 position changes the IR spectrum more significantly than Mg in the Ca1 position. Similar changes were recorded in the IR spectra of the synthesized samples. The electronic structure is shown to be sensitive to the number and position of substitutions, which may be used to tweak the optical properties of the HAP-Mg material.
羟基磷灰石（HAP）是骨骼和牙齿的主要矿物质成分。它作为骨填充剂和植入物涂层在医学中广泛应用，以促进新骨生长。离子取代进入HAP结构会极大地影响其性能。最重要的取代基之一是镁。本文展示了在2×2×2超晶胞模型中，使用高精度混合密度泛函理论计算在宽镁浓度范围内对HAP中Mg/Ca取代所获得的新结果。还展示了不同Mg浓度的HAP-Mg样品机械化学合成的实验数据。实验与理论之间的比较显示出良好的一致性：HAP-Mg晶胞参数和体积随着Mg/Ca取代程度的增加而减小。分析了在不同钙位置进行Mg/Ca取代时Ca与O、Ca与H以及Mg与O离子之间距离的变化。评估了由此产生的晶胞参数不对称性和畸变。结果表明，体模量、能级和带隙取决于Ca1和Ca2位置的Mg取代程度。Mg/Ca取代的形成能表现出非单调行为，且Ca1和Ca2位置不同。在Mg浓度x = 0.5时，Ca2位置的取代概率比Ca1位置略高（约5 meV/原子单位）。在x = 1时，两个位置的取代可以共存。不同Mg/Ca取代的模拟红外光谱表明，Ca2位置的Mg比Ca1位置的Mg更显著地改变红外光谱。在合成样品的红外光谱中也记录到了类似的变化。电子结构对取代的数量和位置敏感，这可用于调整HAP-Mg材料的光学性质。
