Substitution of calcium by strontium within selected calcium phosphates

Sr incorporation in the molecules of amorphous calcium phosphate, apatitic tricalcium phosphate, hydroxyapatite, octacalcium phosphate and dicalcium phosphate dihydrate was investigated. The concentration of Sr ranged from 225 to 1010 μ g / g, i.e. it overlapped with the physiological range of Sr concentrations in human bone. The leading experimental technique was extended X-ray absorption fine structure (EXAFS) at the Sr K edge. Results of these studies demonstrated the following: (1) Sr incorporation in the calcium phosphates is compound-dependent, (2) the coordination of incorporated Sr atoms in the Ca-P molecules is similar to that of Ca atoms, but interatomic distances are ≈0.015 nm larger, (3) in apatitic tricalcium phosphate, hydroxyapatite and octacalcium phosphate lattices Sr atoms may occupy selected Ca sites, which was not the case for dicalcium phosphate dihydrate, (4) in the apatite lattice Sr atoms are coordinated by 6 PO₄ tetrahedrals and (5) EXAFS spectra at the K edge of the incorporated Sr may be used to distinguish the structures of amorphous calcium phosphate, dicalcium phosphate dihydrate as well as apatite and its derivatives (apatitic tricalcium phosphate, octacalcium phosphate).     

Many body contributions to the electronic structure of nickel

The full temperature-dependence of the electronic quasiparticle properties of ferromagnetic Ni is investigated by use of a theoretical model, which takes into account all intraatomic interactions in thed-band complex. After introduction of effective spin operators the model-Hamiltonian consists of a one-particle term, an intraband-interaction of Hubbardtype, and an interband-exchange as in thes-f (ord-f model. The one-particle energies are taken from a realistic bandstructure calculation in order to incorporate approximately all those interactions, which are not directly covered by our model. The model contains two parameters, the intraband couplingU and the interband exchangeJ. ChoosingU=6 eV,J=0.4 eV and applying a Green-function technique we get results in almost quantitative agreement with the experiment:T _c=635 K,m(T=0)=0.56 [ _B , Curie-Weiss behaviour of the static susceptibility, satellite peak with temperature-dependent spinpolarization some 6 eV below the chemical potential , exchange splittings atT=0 of order 0.2–0.35 eV. The full temperature-dependencies of the electronic selfenergy, the one-particle spectral density, the quasiparticle density of states, and the quasiparticle bandstructure for two high symmetry directions are derived and discussed.     

Influence of electron correlations on Auger electron —and appearance potential — spectra of solids

Within the framework of the Hubbard-model the influence of electron correlations on AES and APS for a non-degenerate energy band is investigated. Both spectroscopies are determined by the same two-particle Green function which is solved by a diagrammatic vertex-correction method in the Matsubara-formalism. For empty (n=0) and for completely filled (n=2) bands the method turns out to be exact. The spectra are strongly influenced by the Coulomb interactionU/W and by the degreen of band-filling. Already very weakU/W are sufficient for a substantial deviation of APS and AES from the self-fold of the one particle density of states. For intermediate or even strongU/W(>1) the spectra consist of two parts, a relatively broad band-like region and a sharp satellite. As soon as the satellite splits off, it takes practically the whole spectral weight. In all cases the satellite has almost exactly the shape of the free density of states. The two-hole (electron) bound state, which causes the satellite, propagates virtually without scattering through the lattice. For fixedU/W the band occupationn must be below (above) a critical value to push away a satellite in APS (AES). The temperature-dependence of the spectra is non-negligible for partially filled bands (exceptionsn=0,2), being, however, qualitatively not very striking as long as a non-magnetic systems is considered.     

Extrapolation of real-space quantum chemical calculations from finite-size super cells to the ideal infinite system. I. Theory

A method is proposed how to calculate the correct density matrix of an infinite polymeric chain from that of a standard finite supercell calculation. The density matrix of the finite supercell is transformed into k-space for all k-values allowed by the periodic boundary conditions. The k-dependent matrices are then unitarily transformed, with each unitary matrix being represented by a set of complex rotation matrices. It is shown that the corresponding angles can be interpolated and extrapolated toward the zone boundaries in a straghtforward manner and that this extrapolation can be done from any finite supercell with reasonable accuracy. This gives rise to an infinite system density matrix for which all fundamental properties are guaranteed by construction. This infinite system density matrix may be used to construct a corrected density matrix for the finite supercell calculation. © 1994 John Wiley & Sons, Inc.     

Magnon spectrum and curie temperature of doped ferromagnetic semiconductors

The influence of a partially filled conduction band on the magnetic properties of ferromagnetic semiconductors is studied within the framework of thes-f model. Allowing for magnon scattering to arbitrary order in the form of virtual electron- holecreation the magnon spectral density is derived, from which one gets magnon energies which are substantially renormalized by the presence of conduction electrons. In particular it is shown how the quasiparticle structure of the electronic excitation energies [4, 5] leads to scattering corrections in the magnon spectrum. These corrections are always negative and reduce the positive mean field part, which is proportional to the electron densityn. The calculated magnon spectrum is then used to determine the strikingn-dependence of the Curie-temperatureT _c of Gd-doped EuO and EuS.     

Synthesis of the C6-C21 segment of amphidinolide E

[reaction: see text] A convergent approach to a C6-C21 segment of the polyketide amphidinolide E has been developed through combination of three subunits by allenylindium bromide-aldehyde addition and Suzuki sp2-sp3 coupling.     

Characterization of Electron Transfer Dissociation in the Orbitrap Velos HCD Cell

Electron transfer dissociation (ETD) is commonly employed in ion traps utilizing rf fields that facilitate efficient electron transfer reactions. Here, we explore performing ETD in the HCD collision cell on an Orbitrap Velos instrument by applying a static DC gradient axially to the rods. This gradient enables simultaneous three dimensional, charge sign independent, trapping of cations and anions, initiating electron transfer reactions in the center of the HCD cell where oppositely charged ions clouds overlap. Here, we evaluate this mode of operation for a number of tryptic peptide populations and the top-down sequence analysis of ubiquitin. Our preliminary data show that performing ETD in the HCD cell provides similar fragmentation as ion trap-ETD but requires further optimization to match performance of ion trap-ETD.      

A quantitative interpretation of the magnetic field effect on hyperfine-coupling-induced triplet fromation from radical ion pairs

Experimentally determined B_1/2 values characterizing the magnetic field dependence of molecular triplet production from radical ion pairs originating from photoinduced electron transfer are compared with semi-empirical values obtained according to B_1/2(hfi) = 2(B₁² + B₂²)/(B₁ + B₂) from the root-mean-square values for the hyperfine coupling of the two radicals, B₁ and B₂. The very good agreement is discussed.