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

Sr 4 Al 14 O 25 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-Al 2 O 3 stabilized by strontium, and SrAl 2 O 4 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 Sr 4 Al 14 O 25 phase. This compound was found to be stable up to 1500 °C, and decomposed at higher temperatures into SrAl 2 O 4 and SrAl 4 O 7, and then into SrAl 2 O 4 and SrAl 12 O 19 . The Sr 4 Al 14 O 25 local structure was characterized using 27 Al magic angle spinning (MAS) and multiple quantum magic angle spinning (MQ-MAS) NMR techniques.