Sr4Al14O25: Formation, Stability, and 27Al High-Resolution NMR Characterization

Abstract

Sr4Al14O25 was synthesized from an aqueous solution of aluminum and strontium nitrates by using a spray-drying process. Its formation was studied by differential scanning calorimetry (DSC) at 5 °C/min and X-ray diffraction (XRD). The amorphous precursor crystallized first, through a sharp exothermic peak at 923 °C, into a mixture of two solid solutions: γ-Al2O3 stabilized by strontium, and SrAl2O4 in its hexagonal form stabilized with excess aluminum atoms. These two solid solutions reacted together through a second exothermic peak at 1134 °C to yield the pure Sr4Al14O25 phase. This compound was found to be stable up to 1500 °C, and decomposed at higher temperatures into SrAl2O4 and SrAl4O7, and then into SrAl2O4 and SrAl12O19. The Sr4Al14O25 local structure was characterized using 27Al magic angle spinning (MAS) and multiple quantum magic angle spinning (MQ-MAS) NMR techniques.