Structure and Properties of Cs₇(H₄PO₄)(H₂PO₄)₈: A New Superprotonic Solid Acid Featuring the Unusual Polycation...

We report the discovery of a new superprotonic compound, Cs₇(H₄PO₄)(H₂PO₄)₈, or CPP, which forms at elevated temperatures from the reaction of CsH₂PO₄ and CsH₅(PO₄)₂. The structure, solved using high-temperature single-crystal X-ray diffraction and confirmed by high-temperature ³¹P NMR spectroscopy, crystallizes in space group Pm3̅n and has a lattice constant of 20.1994(9) Å at 130 °C. The unit cell resembles a 4 × 4 × 4 superstructure of superprotonic CsH₂PO₄, but features an extraordinary chemical moiety, rotationally disordered H₄PO₄⁺ cations, which periodically occupy one of every eight cation sites. The influence of this remarkable cation on the structure, thermodynamics, and proton transport properties of the CPP phase is discussed. Notably, CPP forms at a temperature of 90 °C, much lower than the superprotonic transition temperature of 228 °C of CsH₂PO₄, and the compound does not appear to have an ordered, low-temperature form. Under nominally dry conditions, the material is stable against dehydration to ∼151 °C, and this results in a particularly wide region of stability of a superprotonic material in the absence of active humidification. The conductivity of Cs₇(H₄PO₄)(H₂PO₄)₈ is moderate, 5.8 × 10^-4 S cm^-1 at 140 °C, but appears nevertheless facilitated by polyanion (H₂PO₄^-) group reorientation.