Crystal structure of Kuzelʹs salt 3CaO·Al2O3·1/2CaSO4·1/2CaCl2·11H2O determined by synchrotron powder diffraction

The crystal structure of Kuzelʹs salt has been successfully determined by synchrotron powder diffraction. It crystallizes in the rhombohedral R3̅ symmetry with a = 5.7508 (2) Å, c = 50.418 (3) Å, V = 1444.04 (11) Å3. Joint Rietveld refinement was realized using three X-ray powder patterns recorded with a unique wavelength and three different sample-to-detector distances. Kuzelʹs salt is the chloro-sulfoaluminate AFm phase and belongs to the layered double hydroxide (LDH) large family. Its structure is composed of positively charged main layer [Ca2Al(OH)6]+ and negatively charged interlayer [Cl0.50·(SO4)0.25·2.5H2O]−. Chloride and sulfate anions are ordered into two independent crystallographic sites and fill successive interlayer leading to the formation of a second-stage compound. The two kinds of interlayer have the compositions [Cl·2H2O]− and [(SO4)0.5·3H2O]−. The crystal structure explains why chloride and sulfate anions are not substituted and why the formation of extended solid solution in the chloro-sulfate AFm system does not occur.