31P and 29Si NMR study of sol–gel-synthesized phosphate ceramics

Abstract

³¹ P and ²⁹Si solid-state magic angle spinning (MAS) and liquid-state nuclear magnetic resonance (NMR) techniques have been applied to the study of sol–gel-synthesized phosphate ceramic composites. This study emphasizes the chemistry and structural properties for both sols and gels prepared by directly reacting P₂O₅ with tetraethoxysilane (TEOS). This new method of formation of both the sols and gels is a drastic improvement in the retention of phosphorus during the formation of the phosphosilicate gels, and it provides a lower temperature route to six-coordinate silicon. Both³¹P and ²⁹Si liquid-state NMR spectra for sols prepared without water showed the presence of P–O–Si bonds. The ³¹P NMR spectra for sols prepared with water resembled those for the PO(OH)_x(OR)_3–xprecursors, while²⁹Si NMR spectra indicated that TEOS had undergone partial hydrolysis and condensation. Hexacoordinated silicon was observed for the first time in gels prepared at low temperature (70 °C). P–Si gels prepared directly by using P₂O₅ were compared with those using other molecular precursors.