Si complexes in calcium phosphate biomaterials.

Silicon complexes in silicon doped calcium phosphate bioceramics have been studied using (29)Si magic angle spinning nuclear magnetic resonance spectroscopy with the objective of identifying the charge compensation mechanisms of silicon dopants. Three different materials have been studied: a multiphase material composed pre-dominantly of a silicon stabilized alpha-tricalcium phosphate(alpha-TCP) phase plus a hydroxyapatite (HA) phase, a single phase Si-HA material and a single phase silicon stabilized alpha-TCP material. NMR results showed that in all three materials the silicon dopants formed Q(1) structures in which two silicate tetrahedra share an oxygen, creating an oxygen vacancy which compensated the substitution of two silicon for phosphorus. This finding may explain the phase evolution previously found where silicon stabilized alpha-TCP is found at low temperature after sintering.

Surface and adsorption properties of alpha-tricalcium phosphate.

The interaction between bone replacement bioceramic materials and the body will take place at the surface of the material. In the present ab initio study, we have investigated surfaces of biologically important alpha-tricalcium phosphate (TCP) and adsorption of biological species including a Ca atom, an H2O molecule, and a SiO2 fragment on the surfaces. The results show that all these species adsorb on the surfaces. The Ca atoms are strongly chemisorbed with newly created Ca-O bonds similar to those in alpha-TCP bulk in bond length; a water molecule is adsorbed and in some cases also dissociated on the surfaces with the dissociative adsorption, the predominant process; a SiO2 fragment initially linear, strongly interacted with two oxygens of different PO4 groups and eventually forms a tetrahedral SiO4-like species which might be a precursor of the silicate present in Si-TCP materials.

Electronic structure and excitations in oligoacenes from ab initio calculations.

Oligoacenes C(4n+2)H(2n+4) (n=2,...,6) are studied using a variety of ab initio methods. Density functional theory (DFT) optimized geometries were in good agreement with experiment. Vertical and adiabatic ionization potentials and electron affinities were computed with DFT and it was found that standard exchange-correlation (xc) functionals underestimate ionization potentials in oligoacenes. Possible reasons for this underestimation are discussed. Low lying electronic excitations were computed using time-dependent density functional theory, configuration interaction singles, and configuration interaction singles with approximate treatment of doubles. In agreement with earlier work, time-dependent DFT in conjunction with standard xc-energy functionals substantially underestimates the lowest (p) singlet-singlet electronic transition.

Interplay between the ionic and electronic density profiles in liquid metal surfaces.

First-principles molecular-dynamics simulations have been performed for the liquid-vapor interfaces of liquid Li, Mg, Al, and Si. We analyze the oscillatory ionic and valence electronic density profiles obtained, their wavelengths, and the mechanisms behind their relative phase shift.

Liquid-vapor interface in liquid binary alloys: an ab initio molecular dynamics study.

We report the results of an ab initio molecular dynamics simulation of the liquid-vapor interface of two binary liquid alloys, Na(0.3)K(0.7) and Li(0.4)Na(0.6), whose bulk behavior exhibits rather differing ordering tendencies. The study has been performed using samples of 2000 and 3000 particles, respectively, in a slab geometry with periodic boundary conditions. In both cases, the total ionic density distributions along the normal to the interface display some layering with a virtually pure monolayer of the lower surface tension component located outermost at the interface. However, the two systems behave very differently below the interface which can be accounted for by their different ordering tendencies in the bulk.

Theoretical insights into bone grafting silicon-stabilized alpha-tricalcium phosphate.

Silicon-stabilized tricalcium phosphate (Si-TCP) is an excellent bone graft substitute being totally resorbed by the body and replaced by natural bone. Experimental studies show that coatings and bulk ceramics based on this material have superior bioactivity not existing in traditional hydroxyapatite materials. However, the mechanisms through which Si and other dopants affect the properties are not known. We have performed ab initio density functional calculations to investigate the effect of Si dopants on these materials. The results show that Si2O7 species can be formed with weak binding in bulk alpha-TCP with an oxygen vacancy for charge compensation, and that 2SiO4 substitution for a pair of PO4 groups with an excess Ca2+ for charge compensation also leads to a stable structure. With an increase of Si concentration, the former is less bound and the latter becomes more stable, and is a good candidate for the form of Si in Si-stabilized alpha-TCP. The stability of the Si-substituted TCP seems to be determined by the P-P distance of the pair of PO4 groups to be replaced before substitution. The Si-doping leads to a pronounced change in the Ca-O bond lengths, and has little effect on the P-O bonds.

Microscopic dynamics in the liquid Li-Na alloy: an ab initio molecular dynamics study.

We present results for several structural and dynamical properties of the liquid Li(1-x)Na(x) alloy. The study has been carried out by means of the orbital-free ab initio molecular dynamics method, combined with local ionic pseudopotentials constructed within the same framework. We obtain good agreement with the available experimental data, reproducing accurately, the strong homocoordinating tendencies exhibited by this alloy. The calculated partial dynamic structure factors exhibit clear side peaks whose frequencies, for q

Surface structure of liquid Li and Na: an ab initio molecular dynamics study.

Molecular dynamics simulations of the liquid-vapor interfaces of liquid metals have been performed using first principles methods. Results are presented for liquid lithium and sodium near their respective triple points, for samples of 2000 particles in a slab geometry. The atomic density profiles show a pronounced stratification extending several atomic diameters into the bulk, which is similar to that already experimentally observed in liquid K, Ga, In, and Hg.

Collective ionic dynamics in the liquid Na-Cs alloy: an ab initio molecular dynamics study.

We present results for several structural and dynamical properties of the liquid Na-Cs alloy. The study has been carried out by means of the orbital-free ab initio molecular dynamics method, combined with local ionic pseudopotentials constructed within the same framework. The results show good agreement with the available experimental data, reproducing the homocoordinating tendency exhibited by this alloy.

Structure and composition of silicon-stabilized tricalcium phosphate.

Silicon stabilized tricalcium phosphate [Si-TCP] is formed within the calcium hydroxyapatite (HA)-tricalcium phosphate (TCP) system when a stoichiometric precipitate of hydroxyapatite is fired at 1,000 degrees in the presence of SiO(2). This paper proposes a composition range and crystallographic structure for Si-TCP. Reitveld XRD powder diffraction, transmission electron microscopy, infrared and proton nuclear magnetic resonance measurements show that crystalline Si-TCP is associated with the displacement of OH from an initial hydroxyapatite structure. The resulting calcium phosphate is modified by the incorporation of silicon into its structure with excess silica contributing to an amorphous component. Si-TCP has a monoclinic structure with a space group P2(1)/a akin to alpha-TCP with estimated lattice constants of a=12.863+/-0.004 A, b=9.119 +/-0.003 A, c=15.232+/-0.004 A, beta=126.3+/-0.1 degrees. It is proposed that Si(4+) substitutes for P(5+)in the TCP lattice with the average chemical composition of Si-TCP set primarily by the mechanisms available for charge compensation. While the formation of OH vacancies in HA initiates the transformation to Si-TCP, two mechanisms of charge compensation in the Si-TCP structure are plausible. If O(2-) vacancies provide charge compensation, the composition of Si-TCP is Ca(3)(P(0.9)Si(0.1)O(3.95))(2) derived for the addition of 0.33 mol SiO(2):mol HA. If excess Ca(2+) compensates, the composition is Ca(3.08)(P(0.92)Si(0.08)O(4))(2) derived for the addition of 0.25 mol SiO(2):mol HA. The reaction occurs most effectively when SiO(2) is added as a colloidal suspension rather than by the in-situ thermal decomposition of a silicon metallorganic compound. The material is a bioceramic of major biological interest because of its osteoconductivity and unique influence on skeletal tissue repair and remodeling.

Density functional study of structural, electronic and vibrational properties of mg- and zn-doped tricalcium phosphate biomaterials.

Zn- and to a lesser extent Mg-releasing tricalcium phosphate (Zn- and Mg-TCP) have excellent bioactivities which do not exist in their parent TCP base. However, the mechanisms through which the dopants affect the properties are not known. In order to gain insight from geometrical and electronic structures and chemical bonding, ab initio density functional calculations have been performed for these materials using cluster models. The results show a distorted structure for Zn-TCP which may be related to its bioactivity, whereas no such distortion was found for TCP and Mg-TCP. The infrared spectra of these materials has been calculated, and the relationship to the structure investigated.