Furthering the understanding of silicate-substitution in α-tricalcium phosphate: An X-ray diffraction, X-ray fluorescence and solid-state nuclear magnetic resonance study

Abstract

High-purity (SupT) and reagent-grade (ST), stoichiometric and silicate-containing α-tricalcium phosphate (α-TCP: ST0/SupT0 and Si-TCP x=0.10: ST10/SupT10) were prepared by solid-state reaction based on the substitution mechanism Ca3(PO4)(2-x)(SiO4)x. Samples were determined to be phase pure by X-ray diffraction (XRD), and Rietveld analysis performed on the XRD data confirmed inclusion of Si in the α-TCP structure as determined by increases in unit cell parameters; particularly marked increases in the b-axis and β-angle were observed. X-ray fluorescence (XRF) confirmed the presence of expected levels of Si in Si-TCP compositions as well as significant levels of impurities (Mg, Al and Fe) present in all ST samples; SupT samples showed both expected levels of Si and a high degree of purity. Phosphorus ((31)P) magic-angle-spinning solid-state nuclear magnetic resonance (MAS NMR) measurements revealed that the high-purity reagents used in the synthesis of SupT0 can resolve the 12 expected peaks in the (31)P spectrum of α-TCP compared to the low-purity ST0 that showed significant spectral line broadening; line broadening was also observed with the inclusion of Si which is indicative of induced structural disorder. Silicon ((29)Si) MAS NMR was also performed on both Si-TCP samples which revealed Q(0) species of Si with additional Si Q(1)/Q(2) species that may indicate a potential charge-balancing mechanism involving the inclusion of disilicate groups; additional Q(4) Si species were also observed, but only for ST10. Heating and cooling rates were briefly investigated by (31)P MAS NMR which showed no significant line broadening other than that associated with the emergence of β-TCP which was only realised with the reagent-grade sample ST0. This study provides an insight into the structural effects of Si-substitution in α-TCP and could provide a basis for understanding how substitution affects the physicochemical properties of the material.

Structural changes during crystallization of apatite and wollastonite in the eutectic glass of Ca3(PO4)2‐CaSiO3system

“…The 31 P NMR spectrum of the sample annealed at 1375°C/6 min was formed by a broad complex peak with maxima at 2.7 and 1.4 ppm, attributed to α‐Ca 3 (PO 4 ) 2 . The 29 Si MAS‐NMR spectrum of this sample showed peaks at −72 ppm (Q 0 units) and characteristics peaks of two CaSiO 3 (Q 1 and Q 2 units) polymorphs.…”

Solid‐State NMR Study of Mn2+ for Ca2+ Substitution in Thermally Processed Hydroxyapatites

“…The first one can be assigned to the highly disordered PO 4 3− groups present in oxyapatite [Ca 10 (PO 4 ) 6 O], not accompanied by the structural OH − ions as those in HA. The second line, much less intensive, can be ascribed to TTCP and α‐TCP [Ca 4 (PO 4 )O and α‐Ca 3 (PO 4 ) 2 , respectively] . The oxyapatite signal accounts for 2.5% of whole phosphorus in the HAs sample, while the α‐TCP and TTCP signals correspond in total to ~0.5% of this element.…”

Ionic Substitutions in Non-Apatitic Calcium Phosphates

“…Ionic substitution in both α and βTCP is currently being investigated. A solid-state reaction, as well as the heat treatment of already precipitated powders, are commonly applied to synthesize these materials [ 97 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 , 108 , 109 , 110 , 111 , 112 , 113 , 114 , 115 , 116 , 117 , 118 , 119 , 120 , 121 , 122 , 123 , 124 , 125 , 126 , 127 , 128 , 129 , 130 , 131 , 132 , 133 , 134 , 135 , 136 , 137 , 138 , 139 , 140 , 141 , 142 , 143 , 144 , 145 , 146 ]. It is worth underlining that there are two main directions in ionic modification of α and βTCP.…”

“…αSiTCP samples can be obtained using various wet and solid-state methods [ 98 , 99 , 100 , 101 , 102 , 103 ]. Reid et al [ 98 ] prepared Si-substituted αTCP using one of the most ubiquitous ways of synthesis, i.e., by sintering previously precipitated precursor, which was Si-enriched CaP.…”

“…To synthesize the final material, the sol-gel technique combined with the appropriate heat treatment was applied. Surprisingly, it turned out that an Mg content of barely 250 to the chemical structure of αSiTCP was investigated by Duncan et al [ 99 ], who obtained high-purity αSiTCP through a solid-state reaction in a furnace at 1300 °C. Detailed physicochemical analysis (X-ray diffraction with Rietveld analysis and solid-state nuclear magnetic resonance) allowed to confirm crystal structure and silicon incorporation.…”