Apatite Crystallization in an Aluminosilicate Glass Matrix: Mechanistic Studies by X-ray Powder Diffraction, Thermal Analysis, and Multinuclear Solid-State NMR Spectroscopy

Abstract

The crystallization of fluoroapatite, Ca5(PO4)3F (FAP), from a phase-separated glass of composition (mol %) 61.5 SiO2−9.4 Al2O3−9.2 Na2O−7.7 K2O−6.0 CaO−1.9 P2O5−2.5 F, with minor additional components of CeO2, ZrO2, TiO2, Li2O, and B2O3, was monitored by X-ray powder diffraction (XRD), differential thermal analysis (DTA), and advanced high-resolution solid-state NMR spectroscopic techniques on two series of bulk and powder samples subjected to different annealing protocols. The chemical changes occurring as a function of heating time and temperature are sensitively reflected in the 31P and 19F magic-angle spinning (MAS) NMR spectroscopic results. Furthermore, important information concerning the elemental distribution over the two microphases and the chemical identity of crystallization intermediates can be revealed by various 31P ↔ 19F, 19F ↔ 27Al, 31P ↔ 23Na, 31P ↔ 27Al, and 23Na ↔ 19F double-resonance NMR experiments. The glassy material separates into an aluminosilicate-type matrix glass and a droplet phase containing Ca, Na, and the entire phosphorus component. Fluorine dissolves in both of these phases, and the corresponding local environments are differentiable on the basis of their 19F NMR chemical shifts and 19F−31P and 19F−27Al dipole−dipole coupling strengths. During the course of the crystallization, 31P ↔ 23Na and 31P ↔ 19F double-resonance NMR results confirm conclusions based on X-ray powder diffraction that the initial stages of crystallization involve the formation of fluorine-free precursor phases such as NaCaPO4 and a sodium calcium phosphate silicate. However, the comparison between the two series indicates that the crystallization mechanism is also influenced by sample thermal history. Crystallization of fluoroapatite sets in at 700 °C as clearly revealed by XRD and double-resonance NMR. The results illustrate the utility of advanced solid-state NMR techniques for the study of crystallization processes in compositionally complex ceramics.