M. J. Stott

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.

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.

Biological calcium phosphates and Posner’s cluster

A calcium phosphate amorphous to x-ray diffraction (ACP) exists in bone mineral in addition to the main bone apatite component, such as hydroxyapatite (HA). Experimental studies found that ACP has definite local atomic order and contains microcrystallites about 9.5 A in extent rather than a random network structure. Experimental evidence indicates that Posner’s cluster (PC), Ca 9 ( PO 4 ) 6 , could be the basic component of ACP. In addition, it is present in various simulated body fluids and could be the growth unit of HA. In the transformation from ACP to HA, ACP need only dissociate into the clusters rather than undergo complete ionic solvation. Although PC could bridge the biologically important gap between ACP and HA, the form it is likely to take in body fluids is not known. In this study, we have performed ab initiodensity functional calculations to investigate the structure and stability of PC alone in vacuum and in the presence of H + , OH − , Na + , and Cl − ions mimicing the interaction with water and other constituents of body fluids. We find that the cluster with C 1 symmetry is the most stable isomer in vacuum. The interaction of PC with sodium ions and especially with protons leads to a great increase in its stability and surprisingly, the cluster with six protons and six OH − recovers the C 3 symmetry and similar atomic arrangement it has as a structural unit in HA crystal. This may be a key factor in the transformation from ACP to HA crystal.

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.

Orbital-free molecular dynamics simulations of melting in Na8 and Na20: Melting in steps

The melting-like transitions of Na8 and Na20 are investigated by ab initio constant energy molecular dynamics simulations using a variant of the Car–Parrinello method which employs an explicit electronic kinetic energy functional of the density, thus avoiding the use of one-particle orbitals. Several melting indicators are evaluated in order to determine the nature of the various transitions, and are compared with other simulations. Both Na8 and Na20 melt over a wide temperature range. For Na8, a transition is observed to begin at ∼110 K, between a rigid phase and a phase involving isomerizations among the different permutational isomers of the ground state structure. The “liquid” phase is completely established at ∼220 K. For Na20, two transitions are observed: the first, at ∼110 K, is associated with isomerization transitions among those permutational isomers of the ground state structure which are obtained by interchanging the positions of the surface-like atoms; the second, at ∼160 K, involves a struc...

Dynamical properties of liquid Al near melting: An orbital-free molecular dynamics study

The static and dynamic structure of liquid Al is studied using the orbital free ab initio molecular dynamics method. Two thermodynamic states along the coexistence line are considered, namely T=943 and 1323 K, for which x-ray and neutron scattering data are available. A kinetic-energy functional which fulfills a number of physically relevant conditions is employed, along with a local first-principles pseudopotential. In addition to a comparison with experiment, we also compare our ab initio results with those obtained from conventional molecular-dynamics simulations using effective interionic pair potentials derived from second-order pseudopotential perturbation theory.

Theoretical insights into bone grafting silicon-stabilized α-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 α-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 α-TCP. The stability of the Si-substituted TCP seems to be determined b...

Orbital free ab initio molecular dynamics study of liquid Al near melting

The orbital free ab initio molecular dynamics method is applied to study the static and dynamic structure of liquid Al near the triple point. The method uses a new kinetic energy functional, along with a local pseudopotential constructed within the same kinetic energy functional. The results obtained for the dynamic structure factor are compared with recent experimental data.

Surface plasmon excitations in C60, C60K and C60H clusters

Abstract The optical photoabsorption cross section of the C 60 cluster and of the endohedral complexes C 60 K and C 60 H has been calculated using the spherically averaged pseudopotential (SAPS) model combined with the density functional formalism for the ground state and the time-dependent density functional theory (TDDFT) for the response function. We have analyzed the results for the electronic structure comparing them with more refined calculations and we conclude that the main ingredients needed to describe the collective excitations are included in our model. Describing carbon as a monovalent atom we get a surface plasmon (π plasmon) slightly below 8 eV, which is in the experimental energy range. The higher-energy surface plasmon seen in the experiments can be qualitatively described taking into account all the active electrons of the cluster, that is, considering carbon as a tetravalent atom (π and Σ electrons together). The influence of the endohedral impurities is discussed, their main effect being to increase the Landau damping of the collective resonance, more for H than for K. We have also found the highest-order multipolar plasmon the fullerenes can support to be l ≈ 7–8. A simpler jellium-shell model for pure C 60 is also presented that gives for the plasmon frequencies results similar to those of the SAPS model.

Si complexes in calcium phosphate biomaterials

Silicon complexes in silicon doped calcium phosphate bioceramics have been studied using 29Si 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 predominantly of a silicon stabilized α-tricalcium phosphate (α-TCP) phase plus a hydroxyapatite (HA) phase, a single phase Si-HA material and a single phase silicon stabilized α-TCP material. NMR results showed that in all three materials the silicon dopants formed Q1 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 α-TCP is found at low temperature after sintering.