Crystal structure, hydrogen bonds and thermal transformations of superprotonic conductor Cs 6 (SO 4 ) 3 (H 3 PO 4 ) 4 .

Crystals of Cs ₆ (SO ₄ ) ₃ (H ₃ PO ₄ ) ₄ belong to the family of alkali metal acid salts that show a high protonic conductivity at relatively low temperatures. Such properties make superprotonic crystals an excellent choice for the study of the influence of the hydrogen subsystem on the physicochemical properties and promising materials for energy-efficient technologies. Single crystals of Cs ₆ (SO ₄ ) ₃ (H ₃ PO ₄ ) ₄ were studied by neutron diffraction methods, optical polarization microscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Neutron diffraction studies made it possible to determine the positions of all the atoms with high accuracy, including the H atom on a hydrogen bond characterized by a single-minimum potential, to confirm the chemical composition of the Cs ₆ (SO ₄ ) ₃ (H ₃ PO ₄ ) ₄ crystals and their cubic symmetry in low- and high-temperature phases, and to draw conclusions about the three-dimensional system of hydrogen bonds, which is fundamentally different in comparison with other superprotonic compounds. Based on the experimental data obtained, crystal transformations with temperature changes are reported, and the stability of the chemical composition is shown.