Synthesis, characterization, structure refinement and dehydration behavior of sulfate-intercalated hydrocalumite-like layered double hydroxide

The chloride-intercalated layered double hydroxide of Ca²⁺ and Al³⁺, better known as Friedel's salt, comprises positively charged metal hydroxide layers having the composition [Ca₂Al(OH)₆]⁺. The chloride ion is intercalated within all the available sites in the interlayer gallery. The perchlorate-intercalated phase is isotypic with Friedel's salt, and the perchlorate ions occupy all the anionic sites in the interlayer gallery. The sulfate ion has the same size as that of the perchlorate ion, but carries twice the charge. Thereby, the sulfate ions are expected to occupy only half the number of interlayer sites, leaving the rest vacant. With so much void space in the interlayer, the crystal is not expected to be stable. However, this work shows that a sulfate-intercalated layered double hydroxide of Ca²⁺ and Al³⁺ could be synthesized. Structure refinement shows that the sulfate ion is intercalated with one of its S–O bonds parallel to the c-axis of the crystal, which is also the stacking direction. The resultant coordination symmetry is C_3v. The crystal is stabilized by the incorporation of water molecules in voids not occupied by the sulfate ions. This additional water is hydrogen bonded to the sulfate ions and is lost at a higher temperature (160 °C) than the water molecules coordinated to the Ca²⁺ ion (100 °C). The former is structural water and its loss leads to the breakdown of the layered structure.