Structure and Properties of Cs(HPO)(HPO): A New Superprotonic Solid Acid Featuring the Unusual Polycation (HPO).

We report the discovery of a new superprotonic compound, Cs(HPO)(HPO), or CPP, which forms at elevated temperatures from the reaction of CsHPO and CsH(PO). The structure, solved using high-temperature single-crystal X-ray diffraction and confirmed by high-temperature P NMR spectroscopy, crystallizes in space group 3̅ and has a lattice constant of 20.1994(9) Å at 130 °C. The unit cell resembles a 4 × 4 × 4 superstructure of superprotonic CsHPO, but features an extraordinary chemical moiety, rotationally disordered HPO 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 CsHPO, 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 Cs(HPO)(HPO) is moderate, 5.8 × 10 S cm at 140 °C, but appears nevertheless facilitated by polyanion (HPO) group reorientation.