Magnesium substitution in calcium and strontium fluoro-phospho-aluminosilicate glasses by multinuclear 19F, 31P, 27Al, and 29Si MAS-NMR spectroscopy

Abstract

Two series of glasses were produced by systematically substituting magnesium for calcium and strontium in the glass compositions, 4.5SiO2. 3Al2O3. 1.5P2O5. (3-x)CaO/SrO. xMgO. (2-y)CaF2/SrF2. yMgF2, 0 ≥ x ≥ 3, 0 ≥ y ≥ 2 and these glasses were characterised by 19F, 31P, 27Al, and 29Si MAS-NMR spectroscopy. The substitution of magnesium for calcium and strontium resulted in the gradual formation of F-Mg(n) species over the F-Ca(n) and F-Sr(n) with additional loss of AlF species. The 31P and 27Al MAS-NMR spectra showed the presence of Al-O-PO33− species charge balanced by a mixture of Ca2+ and Mg2+ or Sr2+ and Mg2+. In addition to that, the 31P MAS-NMR spectrum of the fully magnesium substituted glass showed the presence of amorphous magnesium pyrophosphate, Mg2P2O7. Al(IV) was the major Al species with small amount of Al(V) and Al(VI), but the amount of Al(V) increased with increase in magnesium substitution. Moreover, the fully magnesium substituted fluoro-phospho-aluminosilicate glass resulted in the more polymerised silicate network with increased number of the bridging oxygen on SiO4 tetrahedra. Keywords: MAS-NMR, Aluminosilicate glass, Magnesium, Calcium, Strontium