Structure and stability of hydroxyapatite: Density functional calculation and Rietveld analysis (original) (raw)
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Nanomaterials
Simulation and computer studies of the structural and physical properties of hydroxyapatite (HAP) with different defects are presented in this review. HAP is a well-known material that is actively used in various fields of medicine, nanotechnology, and photocatalytic processes. However, all HAP samples have various defects and are still insufficiently studied. First of all, oxygen and OH group vacancies are important defects in HAP, which significantly affect its properties. The properties of HAP are also influenced by various substitutions of atoms in the HAP crystal lattice. The results of calculations by modern density functional theory methods of HAP structures with these different defects, primarily with oxygen and hydroxyl vacancies are analyzed in this review. The results obtained show that during the structural optimization of HAP with various defects, both the parameters of the crystallographic cells of the HAP change and the entire band structure of the HAP changes (change...
Rietveld Refinement of the Crystal Structure of Hydroxyapatite Using X-ray Powder Diffraction
The stoichiometric hydroxyapatite, Ca 10 (PO 4) 6 (OH) 2 is prepared by an aqueous precipitation method at room temperature, the main reactants were Ca(OH) 2 and H 3 PO 4 without addition of ammonia solution. The sample was analyzed by Fourier transformed infrared spectroscopy (FTIR) which reveals the presence of a small amount of carbonate due to absorbance of carbon dioxide from the air during synthesis process. The crystal structure was carried out by X-ray powder diffraction data and the Rietveld method using FullPROF software. We confirmed that this material has a hexagonal structure (space group P63/m; Z = 1). Unit-cell parameters with higher precision (a = b = 9.4159(4) Å , c = 6.8819(3) Å, α = β = 90°; and γ = 120°).
Computational study of hydroxyapatite structures, properties and defects
Hydroxyapatite (HAp) was studied from a first principle approach using the local density approximation (LDA) method in AIMPRO code, in combination with various quantum mechanical (QM) and molecular mechanical (MM) methods from HypemChem 7.5/8.0. The data obtained were used for studies of HAp structures, the physical properties of HAp (density of electronic states—DOS, bulk modulus etc) and defects in HAp. Computed data confirmed that HAp can co-exist in different phases—hexagonal and monoclinic. Ordered monoclinic structures, which could reveal piezoelectric properties, are of special interest. The data obtained allow us to characterize the properties of the following defects in HAp: O, H and OH vacancies; H and OH interstitials; substitutions of Ca by Mg, Sr, Mn or Se, and P by Si. These properties reveal the appearance of additional energy levels inside the forbidden zone, shifts of the top of the valence band or the bottom of the conduction band, and subsequent changes in the width of the forbidden zone. The data computed are compared with other known data, both calculated and experimental, such as alteration of the electron work functions under different influences of various defects and treatments, obtained by photoelectron emission. The obtained data are very useful, and there is an urgent need for such analysis of modified HAp interactions with living cells and tissues, improvement of implant techniques and development of new nanomedical applications.
Ceramics International, 2017
We describe the temperature dependence of the vibrational, rotational and translational partition functions of the activated complex of hydroxyapatite (HAp). Computed data show that the vibrational modes have a larger contribution towards the partition function of HAp compared to the rotational and translational contributions. X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy have been applied for HAp in the temperature range from 730 °C to 1030 °C at steps of 100 °C. Temperature dependent density functional theory (DFT), B3LYP, and Ground State Hartree-Fock (HF) with 6-311G basis set calculations were also applied to HAp to calculate FTIR spectra, HOMO, and LUMO energies, and density of states (DOS), and the results have been compared to experimental findings. The present results underline that experimental measurements and theoretical calculations of unit cell parameters, and the intensities of most of the FTIR data for HAp are nearly independent on temperature.
A Computational Study of the Properties and Surface Interactions of Hydroxyapatite
Ferroelectrics, 2013
Hydroxyapatite (HAP, Ca 10 (PO 4 ) 6 (OH) 2 ) was studied from first principles approaches using the local density approximation (LDA) method in combination with various quantum-chemical (QM) and molecular mechanical (MM) methods from HypemChem 7.5/8.0. The data then were used for studies of HAP structures, and the interactions of HAP clusters with ionic species such as citrates. Computed data show that HAP can co-exist in different phases at room temperature, as both hexagonal and monoclinic. Special interest is connected with the ordered monoclinic structure, which could reveal piezoelectric properties. Obtained data on HAP interactions with citrates show the formation of differing HAP nanostructure forms, depending upon the concentration of citrate present.
A Crystal Chemistry of Hydroxyapatite : A Review
International Journal of Advanced Research, 2021
In chemistry of inorganic crystals, the octacalcium phosphate (OCP) is an apatite based crystals and having a hydrated layers which used in producing of needle or plate-shaped hydroxyapatite (HAP) nanocrystals. Although, the crystals is prepared by a dissolution precipitation reaction. These reaction led to a hexagonal HAP nanocrystals formation under hydrothermal condition from OCP at 180 for 3 hours with pH of solution adjusted to 5.5 and incorporating dicarboxylate e.g. succinate (OOC.(CH2)2.COO)2- ions having Ca/P molar ratio is expected to be 1.56±0.02, where the morphology of OCP are retained. During incorporating of succinate ions in OCP crystals, the hydrogen phosphate (HPO42-) ions in the hydrated layers of OCP are being substituted by succinate ions. Since the crystal system of HAP is hexagonal and its crystalline size in the longitudinal direction of various (a,b,c) axes depending on the thickness of the laminated plate-shaped HAP crystals. Here, their size as perpendicu...
Experimental and theoretical characterization of Dy-doped hydroxyapatites
The effects of adding Dy to the hydroxyapatite (HAp) structure were investigated experimentally and theoretically. The as-obtained experimental results with an increasing amount of Dy are as follows. X-ray diffraction, Raman, and Fourier transform infrared measurements verified the HAp structure for each specimen. The crystallinity, lattice parameters, lattice stress, strain, and anisotropic energy density were affected. Thermal stability and stoichiometry were not affected. It was observed that all the Dy-doped HAps have smaller crystallite size values compared to the un-doped HAp. The cell viability obtained from mouse fibroblast cell (L929) was higher than 82%, indicating all the samples were biocompatible. The theoretical findings, obtained from the density functional theory (DFT) calculations, exhibited a continuous decrease in the bandgap from 4.7109 to 3.7982 eV, an increase in the density from 3,155 to 3,189 kg m −3 , and an increase in the linear absorption coefficient.
Revista Mexicana de …, 2005
The structure of natural hydroxyapatite nHAP (i.e. the hydroxyapatite found in teeth and bones) has not been completely characterized experimentally until now. This involves the study of the structural characteristics of synthetic hydroxyapatite sHAP (i.e. one whose stoichiometric formula is Ca 10 (PO 4) 6 (OH) 2) using many techniques, in particular electron diffraction computer simulation. Thus, any variation presented in its structure will be easily detected. In this work we comment on the crystallographic elements presented in simulated convergent beam electron diffraction (CBED) patterns for sHAP, in its versions of Laue Zones of Zero Order (ZOLZ), First Order (FOLZ), Second Order (SOLZ) and Higher Order (HOLZ), and the deduction of its space group P63/m. These results are compared with those reported experimentally.
Journal of the Australian Ceramic Society, 2020
Five samples of hydroxyapatite (HAp) doped with praseodymium (Pr) at various amounts (2, 4, 6, 8, and 10 at.%) were synthesized by using the wet chemical route. The effects of Pr doping on the structural and thermal properties, as well as on the in vitro performance of HAp, were investigated experimentally. The band structure and density of states (DOS) of HAp were studied theoretically. Incorporation of Pr into the crystal lattice of HAp was observed. A gradual increase in the crystallite size, lattice parameter a, and unit cell volume was found, and a gradual decrease in the crystallinity degree was seen. Pr content from 2 to 10 at.% did not affect the thermal stability of HAp. The theoretical results showed that the bandgap energy of HAp decreased steadily from 3.82 to 1.32 eV with the adding of Pr, and the DOS was also affected by the Pr content. The cell viability tests showed that among all the as-synthesized samples, the best biocompatible properties were found for the sample which was doped with 10 at.% Pr, and the amount of Pr affected significantly the cell viability property of HAp. Except for the sample having 6 at.% Pr, all the remaining samples appeared to be potentially good candidates for biomedical applications.