First-principles modeling combined with experimental methods were used to study hydroxyapatite in which Sr2+ is substituted for Ca2+. Detailed analyses of cation-oxygen bond distributions, cation-cation distances, and site 1-oxygen polyhedron twist angles were made in order to provide an atomic-scale interpretation of the observed structural modifications. Density functional theory periodic band-structure calculations indicate that the Ca2+ to Sr2+ substitution induces strong local distortion on the hydroxyapatite lattice: the nearest neighbor Sr-O bond structures in both cationic sites are comparable to pure SrHA, while Sr induces more distortion at site 2 than site 1. Infrared vibrational spectroscopy (FTIR) and extended X-ray absorption fine structure (EXAFS) analysis suggest increasing lattice disorder and loss of OH with increasing Sr content. Rietveld refinement of synchrotron X-ray diffraction patterns shows a preference for the Ca1 site at Sr concentrations below 1 at.%. The ideal statistical occupancy ratio Sr2/Sr1=1.5 is achieved for approximately 5 at.%; for higher Sr concentrations occupation of the Ca2 site is progressively preferred.

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Characterization of lead substitution for calcium in hydroxyapatite (CaHA) is carried out, using experimental techniques and Density Functional theoretical (DFT) analyses. Theoretical modeling is used to obtain information of the Pb chemical environment for occupancy at either Ca(I) or Ca(II) sites of CaHA. Effects of the larger ionic radius of Pb(+2) compared to Ca(+2) are apparent in embedded cluster calculations of local chemical bonding properties. DFT periodic planewave pseudopotential studies are used to provide first-principles predictions of local structural relaxation and site preference for Pb(x)Ca(10-x)HA over the composition range x< or = 6. General characteristics of the polycrystalline material are verified by X-ray diffraction and FTIR analysis, showing the presence of a single phase of CaHA structure. A short range structure around lead is proposed in order to interpret the Pb L-edge EXAFS spectrum of the solid solution Ca(6.6)Pb(3.4)HA. In this concentration we observe that lead mainly occupies Ca(II) sites; the EXAFS fit slightly favors Pb clustering, while theory indicates the importance of Pb-Pb avoidance on site (II).

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The electronic structures of clusters representing crystalline compounds of Sn{II) and Sn(IV) were investigated, employing the first-principles discrete-variational method and local-density theory. Densities of states and related parameters were obtained and compared with experimental measurements and with results from band-structure calculations. Effects of cluster size and of cluster truncated bonds are discussed. Electric-field gradients at the Sn nucleus were calculated; results are analyzed in terms of the charge distribution and chemical bonding in the crystals.PACS number(s): 71.25.Tn, 76.80.+y knowledge, this is the first calculation of electric-field gradients in Sn crystalline compounds.Although we had a great interest in electric-field gradients, these were by no means the only focus of our calculations. In fact, the six compounds considered include bonding between Sn and anions that may be considered to vary between ionic and covalent, since Sn has both oxidation states 2+ and 4+, and the anions cover a wide range of electronegativities; accordingly, their electronic structures and charge distributions may be thus considered interesting per se, to help understand how chemical bonds occur in Sn solids.Finally, we mention that in the list of solids studied, at least three compounds have known important applications. The Sn(IV) compound Sn02 is a large-gap semiconductor, and has been widely used as radioactive source in Mossbauer experiments;technologically, its importance derives from the fact that it may be used as a transparent electrode. Sn02 also has an important application as an "inorganic exchanger" in inorganic analytical chemistry. The Sn(II) compounds SnS and SnSe are also semiconductors. Recent technological applications of Sn and Pb chalcogenides include the construction of lasers and detectors in the infrared region. The self-consistent cluster calculations were performed employing the discrete-variational method and localdensity theory for clusters representing the solids. The convergence of the results obtained with cluster size was investigated by performing calculations, for each compound, for clusters with a different number of atoms. The clusters were embedded in the charge density of many layers of external atoms; the potential thus generated was made consistent with the cluster potential. 8584