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The output power (30–150 W) and the periodicity (20–100%) of ultrasound emission were varied in a wide range to regulate and improve the crystallization process in the commonly used co-precipitation technique of chloride-intercalated CaFe-layered double hydroxides. The influence of ultrasound irradiation on the as-prepared materials was studied by X-ray diffractometry, dynamic light scattering, UV–Vis–NIR diffuse reflectance spectroscopy, specific surface area measurement, pore size analysis, ion-selective electrode potentiometric investigations and thermogravimetry. Additionally, structural alterations due to heat treatment at various temperatures were followed in detail by Fourier-transform infrared and X-ray absorption spectroscopies as well as scanning electron microscopy. The ultrasonic treatment was capable of controlling the sizes of primarily formed (from 19 nm to 30 nm) as well as the aggregated (secondary) particles (between 450 nm and 700 nm), and thus modifying their textural parameters and enhancing the incorporation of chloride anions into the interlamellar space. For the first time, the optical energy gap of CaFe-LDH was reported here depending on the nature of applied stirring (4.18–4.34 eV). The heat-treatment investigations revealed that the layered structure was stabile until 200 °C, even at the atomic level.

An ultrasonically-enhanced mechanochemical method was developed to synthesize CaFe-layered double hydroxides (LDHs) with various interlayer anions (CO32−, NO3−, ClO4−, N3−, F−, Cl−, Br− and I−). The duration of pre-milling and ultrasonic irradiation and the variation of synthesis temperature in the wet chemical step were investigated to obtain the optimal parameters of preparation. The main method to characterize the products was X-ray diffractometry, but infrared and synchrotron-based X-ray absorption spectroscopies as well as thermogravimetric measurements were also used to learn about fine structural details. The synthesis method afforded successful intercalation of the anions, among others the azide anion, a rarely used counter ion providing a system, which enables safe handling the otherwise highly reactive anion. The X-ray absorption spectroscopic measurements revealed that the quality of the interlayered anions could modulate the spatial arrangement of the calcium ions around the iron(III) ions, but only in the second coordination sphere.

The combination of mechanochemical and ultrasonic treatment was applied to synthesize CaAlFe-layered triple hydroxides with carbonate or chloride anions in the interlamellar space. The optimal parameters of the preparation were explored by altering the initial ratio of the metal ions and the temperature of ultrasonic irradiation. The resulting triple hydroxides were characterized by X-ray diffractometry, infrared and X-ray absorption spectroscopies, thermogravimetric analysis and scanning electron microscopy. The products were close-to-phase-pure CaAlFe-layered triple hydroxides. Elevation of the temperature transformed the CaAlFe-Cl(-)-layered triple hydroxide to rare oxyhalides (Ca2FeO3Cl and Ca12Al14O32Cl2).

An ultrasonically-enhanced mechanochemical method was developed to synthesize CaFe-layered double hydroxides (LDHs) with various interlayer anions (CO