First-principles study of the biomineral hydroxyapatite

Abstract

The biomineral hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 is the main mineral constituent of mammal bone. Hydroxyapatite crystallizes in the hexagonal and monoclinic phases, the main difference between them being the orientation of the hydroxyl groups. Using density functional theory we study the energetics of the hexagonal and monoclinic phases along with the several hypothetical crystal structures of hydroxyapatite. The monoclinic phase has the lowest energy, with the hexagonal phase being only 22meV/cell higher in energy. We identify a structural transition path from the hexagonal to monoclinic phase with the activation energy of 0.66 eV per hexagonal cell. At room temperature the transition occurs on a millisecond time scale. The electronic structures of the monoclinic and hexagonal phases are compared. For the hexagonal phase we calculate the phonon frequencies at the Γ-point and elastic constants. Both are in good agreement with available experiment.

Компьютерные Исследования Наноструктур Гидроксиапатита, Их Особенности И Свойства

“…These results are comparable with the data obtained by other authors [10,21] and are very close to the data from [22]. Comparison of the calculated value of the forbidden energy band Eg ~ 4.6 eV [32,33,35], with other data computed show that all Eg data are very close to the data obtained in [35]: Eg ~ 5.4 eV from Calderin and Scott [9], Eg ~ 5.23 eV from Slepko and Demkov [22], Eg ~ 4.51 eV from Rulis et al [26]. Furthermore, the computed values of the forbidden energy gap Eg, are in good agreement with [9, 10, 20-22, 25, 27].…”

“…-the electron energy band structures of initial HAP clusters computed by PM3 methods correspond well to the data reported (both experimental and simulated) [10,21,22,26,41,42,47] (see more details in [30] and in Table 4) -the main values obtained for the energy gap are Eg ∼ 3.54 eV for fixed PC (C3 symmetry), Eg ∼ 4.56 eV for relaxed PC (C1 symmetry) as compared with known data Eg ∼ 4.43 eV for the last structure in [41,42], and Eg ∼ 5.41 eV for the smallest HAP (C1 symmetry) as compared with Eg ∼ 5.38 eV for fixed HAP bulk [42]; -the computed data show some variations of the energies of the electronic band structures (E HOMO and E LUMO) with the addition of hydrogen atoms and corresponding variations of Eg (4.02-5.56 eV, see Table 4), and have a tendency for a decrease of Eg at the finally filled hydrogen bonds -all these data are in good agreement with the experimentally measured Eg ∼ 4.0 eV for a surface charged HAP samples in [47]; -the change of the total dipole moment and polarization are varied with increase of added hydrogen, but average values slightly decrease as compared with the initial state (see Table 3, Table 4). These results are very urgent for practical applications, because recent studies of the influence of electrical charge on HAP specimens surface on the osteoblast behavior and bone tissue growth clearly show that the deposition on the HAP surface of a negative charge enhances attachment and proliferation of the osteoblasts and fabrication of the bone tissue [36][37][38][39][40].…”

“…It is also useful for creating of new HAP nanostructures having wide biomedical applications [13][14][15][16][17][18][19]. The results of these studies [7,9,11,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] allow us to clarify possible mechanisms of structural changes and electrical properties of HAP in the bulk crystal and on the crystal surface (electrical potential, charges and polarization). And this, in turn, will allow one to improve the efficiency of HAP interactions with the living bone cells (osteoblasts, osteoclasts, etc.)…”

“…Beyond HAP crystal bulk model, a special HAP model of the layered slab supercells units, which includes vacuum spaces between the layered slabs forming HAP surface, are considered and analysed too [5]. More computational details are described in [22,24,26,27,[30][31][32][33][34][35]67] and in sections below.…”

“…No 1. doi:10.17537/2017.12.14 these different phase structures of HAP can co-exist at room temperature, and HAP can easily transfer from one form to another, depending on the environmental conditions (and changes of the stoichiometry ratio). Possible mechanism for the phase transition between the hexagonal and monoclinic phases could be thermo-activated switching of OH-dipoles in HAP OH-channels and changes of the proton positions around [13,14,22]. This leads to the possibility for local fluctuations to arise between the phases in space leading to variations of the HAP surface profile due to occasional changes of the neighboring phase.…”