Rietveld structure refinements of calcium hydroxylapatite containing magnesium

Abstract

The crystal structures of four hydroxylapatite (HA) samples prepared from solutions in the presence of 10, 15, 25 and 30 Mg-atom-% have been investigated by X-ray powder pattern fitting. The total magnesium content of the solid samples, as determined by chemical analysis, was 4.9, 14.1, 20.4 and 30.6 Mg-atom-%, respectively. Rietveld analysis was performed using the computer program PREFIN implemented with routines which allow the refinements of the average crystallite sizes. Different refinement procedures were carried out in order to evaluate the effect of the amorphous and background profiles on the occupancy factor data. For comparison, magnesium-free hydroxylapatite was refined with the same strategies. The results of the different approaches indicate that the degree of magnesium substitution for calcium in the HA structure can be at most ,-~ 10 atom-%. Magnesium substitutes calcium preferentially at the 6(h) site. The broadening of the diffraction peaks increases on increasing the total magnesium content in the solid phase, which is always significantly higher than the amount incorporated into the HA structure. The excess is probably located in the amorphous phase and/or on the crystallite surface.

Magnesium incorporation into hydroxyapatite

“…Such discrepancies might be due to differences in synthetic procedures (which could perhaps alter the final site for incorporated Mg), and/or to the low quality of the XRD patterns used in the Rietveld refinements. Indeed, it is noteworthy that some of the refined distances are surprising: for a sample with ~ 30% Mg, an average M•••O bond distance was found that was almost identical to the non-substituted compound, [20] which seems contradictory with the fact that average Recently, alternative approaches have started to emerge to investigate in more depth the mode of incorporation of substituting divalent cations in HA phases. In particular, it has been shown that much information could be accessed experimentally using spectroscopic techniques which are sensitive to the local structure around particular cations, such as X-ray absorption spectroscopy and solid state NMR.…”

“…[15,21,22] The main difficulty in answering this question in the case of Mg-HA arises from the absence of high resolution spectroscopic data over an important range of composition, and the significant loss of crystallinity of Mg-HA compounds above ~ 20% Mg. Thus, in contrast with Sr-or Ba-substituted HA, [23,24] Rietveld analyses based on X-ray powder patterns are not conclusive for Mg-HA, and both the Ca(I) [15] and the Ca(II) [20] sites have been suggested to be the preferential site of incorporation of Mg. Such discrepancies might be due to differences in synthetic procedures (which could perhaps alter the final site for incorporated Mg), and/or to the low quality of the XRD patterns used in the Rietveld refinements.…”

“…[18,19] Numerous spectroscopic techniques have been used to learn more about the substitution of magnesium in HA. Nevertheless, although X-ray diffraction, [20] X-Ray Photoelectron Spectroscopy, [14] and cathodoluminescence spectroscopy [21] clearly indicate that Mg enters the HA lattice, several key structural characteristics remain unsolved, and little is known about how exactly Mg-incorporation affects the bulk structure of apatites. In particular, the position of Mg in the HA lattice is still an open question: does it occupy one or both of the two crystallographic calcium sites, referred to as Ca(I) and Ca(II), which present different local environments, as depicted in Figures 1 and 2?…”

Diffraction techniques and vibrational spectroscopy opportunities to characterise bones

“…Among them, let us quote Mg [56], Si [57], Cd [58], Sr [59] and Pb [60]. More precisely, Zhu et al [61] founded that the metal ions of Pb 2+ , Sr 2+ and Cd 2+ preferentially occupied Ca[II] sites in the apatite structure (two occupation sites exist as gathered in Table 1) .…”

Some features of thermo‐activated structural transformation of biogenic and synthetic Mg‐containing apatite with β‐tricalcium‐magnesium phosphate formation

“…It is well illustrated by temperature unstability of Mg-containing apatite [10,12]. The increase of Mg content in synthetic HA reduces temperature of phase decomposition with formation of magnesium substituted β-TCP (or β-TCMP) (Ca,Mg) 3 (PO 4 ) 2 , and the extent of HA conversion into β-TCMP on heat treatment appears strongly related to Mg amount of the apatitic solid phase [12][13][14]. In work [14] the conclusion is made, that whole Mg from nonequilibrium apatitic phase of the different HA samples enters (at Т=900 °C) the crystal structure of β-TCMP.…”