Powder diffraction studies of synthetic calcium and lead apatites

Abstract

The crystal structures of M10(PO4)6X2, where M = Ca or Pb and X = OH¯, F¯, Cl¯ or Br¯, have been determined by Rietveld refinement of powder synchrotron X-ray and neutron diffraction data. All the compounds are hexagonal with space group P 63/m. For the calcium compounds, the lattice parameters are a = 9.4302(5), 9.3475(3), 9.5902(6), 9.6482(6) and c = 6.8911(2), 6.8646(1), 6.7666(2), 6.7788(2) Å, for X = OH¯, F¯, Cl¯, Br¯, respectively. For the lead compounds, the corresponding lattice parameters are a = 9.8612(4), 9.7547(5), 9.9767(4), 10.0618(3) and c = 7.4242(2), 7.2832(2), 7.3255(1), 7.3592(1) Å. In these compounds there are two cation sites, a channel of M(I) atoms and a triangle of M(II) atoms. The anion interacts most strongly with the M(II) atoms with the interaction dictating the position of the anion relative to the M(II) triangle. In Ca10(PO4)6X2, the F¯ ion sits within the triangle planes, while the larger OH¯ and Cl¯ anions are disordered above and below the M(II) triangles. The even larger Br¯ is midway between two triangles at (0, 0, ). Despite the larger size of the isostructural lead compounds, no anions are found in the triangles. The F¯, Cl¯ and Br¯ ions are at (0, 0, ) and the OH¯ ion is disordered at (0, 0, z). This difference in behaviour is possibly related to the lead 6s electrons. In this paper, the experimental results are presented and possible reasons for the observed differences are discussed.

Semiempirical electronic structure calculation on Ca and Pb apatites

“…It has been argued that polarization due to sp hybridization in Pb2 could explain dislocations of halogen ions in the X channel [20]. However, in agreement with DFT calculations [5,8], we have found no significant 6s−6p hybridization in the lead apatites.…”

FTIR, powder XRD, TEM and dielectric studies of pure and zinc doped nano‐hydroxyapatite

“…Moreover, its properties like osteoconductivity, nontoxicity, noninflammatory behaviour, and non-immunogenicity makes it applicable as bone implant, in drug dlivery system and in catalysis. Apart from this, HAP is also reported having two crystalline structures, i. e., hexagonal and monoclinic [4,5]. Nowadays, the influence of the inclusion of trace elements or dopants on the physical and/or chemical properties of the bio-ceramics is of interest in the nano and bulk forms.…”

Chemical Functionalization of a Hydroxyapatite Based Bone Replacement Material for the Immobilization of Proteins

“…Based on the known structural composition of hydroxyapatite, similarities to glass and other ceramics we therefore reasoned that a reaction with silanes should be possible. Previously we had shown that the amount of 125 I-ubiquitin nonspecifically bound to the porous hydroxyapatite ceramic Endobon [7] (Merck AG Darmstadt, density 1.289 g/cm 3 , $100 % HAP [1]) can be increased from 6 lg/ g to 24-30 lg/g by covalently linking it via a reaction with 3-aminopropyltriethoxysilane (APS) followed by carbonyldiimidazole (CDI) [8]. A similar reaction scheme has been employed by our group for several years for coupling rhBMP-2 to titanium surfaces [8,9].…”

“…Bone is composed to $ 60 % of hydroxyapatite (HAP, see [1]), which in its crystalline form has the chemical composition Ca 10 (PO 4 ) 6 (OH) 2 (see [2][3][4]). Hydroxyapatite has a very low solubility product of 2.34 x 10 -59 with a calculated dissociation constant of $0.44 lM [5].…”