Mechanism of Mg2+-induced and ultrasound-assisted rapid synthesis of macallisterite: Raman, DFT, and morphology control

Macallisterite, a functional material with excellent properties, is widely used in industries. However, the rapid synthesis of high-purity macallisterite with both high yield and uniform morphology remains challenging due to the complex polymerization of borate species during the production process. Herein, we propose a novel magnesium ion-induced strategy for the efficient synthesis of macallisterite by combining sonochemistry assistance and additive method. The polymerization mechanism of borate species under excess magnesium salts was investigated using Raman spectroscopy and DFT. It has been shown that chloride magnesium is the optimal inducer for the macallisterite synthesis as Cl − exhibits lower electrostatic interaction and spatial steric hindrance than SO 4 2− and NO 3 − . Sonochemical assistance significantly accelerated the nucleation and crystallization of macallisterite through kinetic conditions created by cavitation effects. Accordingly, the crystallization is promoted achieving a maximum yield of 97.92%. Raman and DFT study revealed that Mg 2+ ions can effectively induce the formation of the stable neutral complex cluster [Mg(H 2 O) 4 (B 6 O 7 (OH) 6 )] in the supersaturated solution, which lowers the reaction activation energy through charge transfer and coordination. Furthermore, dense macallisterite spherulites with uniform morphology and high purity can also be achieved by using sodium dodecyl sulfonate (SDS) as additive. Based on these findings, this study not only elucidates a metal ion-induced crystallization mechanism but also establishes an efficient, green, and scalable synthetic route for high-quality macallisterite.