A MAS-NMR and Combined Rietveldt Study of Mixed Calcium/Strontium Fluorapatite Glass-Ceramics

Abstract

Strontium is one of the most common substituents in apatite crystals. The presence and behavior of Sr in apatite-group phases are of considerable significance in biology. The present paper investigates the substitution of strontium for calcium in a glass-ceramic of the following composition 4.5SiO23Al2O31.5P2O54CaO1CaF2. The glasses were characterized using Differential Thermal Analysis (DTA), X-ray powder diffraction (XRD), neutron diffraction (ND) and 19F Resonance Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR). The all calcium glass crystallized to calcium fluorapatite (Ca5(PO4)3F). Substituting strontium partially for calcium resulted in the formation of a mixed strontium/calcium fluorapatites. In contrast complete substitution resulted in the formation of strontium fluorapatite. MAS-NMR showed the the F to be present as F-Ca(3) representing a fluoride ion surrounded by three Ca2+ ions in the all calcium glass and was present as F-Sr(3) in the all strontium glass. In the mixed glasses fluorine was present as FCa( 3), F-Ca(2)Sr, F-CaSr(2) and F-Sr(3). Ca had a higher tendency to occupy the F-M(3) sites than Sr which may reflect the higher charge to size ratio of Ca2+ relative to Sr2+ and its greater affinity for F- ions.

The early stages of nucleation and crystallisation of an apatite glass-ceramic: Evidence for nano-scale crystallisation

“…The MAS NMR results for the glass and the heat-treated samples and, in particular, 19 F MAS NMR data, are largely in agreement with previous studies of both fluorine containing glass-ceramics and FAp glass-ceramics [13,18,29,31,32]. From the 19 F and 31 P NMR chemical shifts there is some evidence of Mg substituting for Ca in the Ca(II) positions surrounding the fluoride ion in the FAp lattice.…”

“…Given the absence of any nucleating agent it is thought likely that the nucleation step probably involves APS of the glass, which is known to occur in closely related glass compositions containing no MgO [13,18,31]. However Tulyganov et al [17] found no evidence of APS in their study of the ternary FAp-anorthite-diopside system and the SANS results show no sign of APS in glass either (Fig.…”

“…6. Sometimes the broadening is so severe that the only evidence for nanometre sized crystals is from the line width of 19 F MAS NMR in FAp [13,18,36]. In these previously studied compositions, the FAp crystals generally grow rapidly giving rise to sharp XRD patterns and large acicular FAp crystals.…”

“…Surprisingly, despite the commercial use of single crystal lasers based on apatites and the fact that apatite lattices are also very attractive for accommodating rare earth ions, there is only one publication on nano-crystalline fluoropatite (FAp) glass-ceramics [2]. There is however a considerable number of published papers on calcium apatite glass-ceramics [10][11][12][13][14][15][16][17] and papers on mixed calcium-strontium FAp [13,18] glass-ceramics for medical and dental applications. Most of these papers are concerned with obtaining large elongated FAp in needle like form that imparts high strength and toughness to the glass-ceramic.…”

Apatite Glass-Ceramics: A Review

“…In another study, Hill et al (2006) elucidated Ca and Sr sites in mixed FAp through 19 F MAS-NMR. Results showed the F to be present as F-Ca(3) in the all calcium glass and as F-Sr(3) in the all strontium glass.…”