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

Abstract

We report the discovery of a new superprotonic compound, Cs 7 (H 4 PO 4 )(H 2 PO 4 ) 8 , or CPP, which forms at elevated temperatures from the reaction of CsH 2 PO 4 and CsH 5 (PO 4 ) 2 . The structure, solved using high-temperature single-crystal X-ray diffraction and confirmed by high-temperature 31 P NMR spectroscopy, crystallizes in space group Pm3̅ n and has a lattice constant of 20.1994(9) Å at 130 °C. The unit cell resembles a 4 × 4 × 4 superstructure of superprotonic CsH 2 PO 4 , but features an extraordinary chemical moiety, rotationally disordered H 4 PO 4 + 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 CsH 2 PO 4 , 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 7 (H 4 PO 4 )(H 2 PO 4 ) 8 is moderate, 5.8 × 10 −4 S cm −1 at 140 °C, but appears nevertheless facilitated by polyanion (H 2 PO 4 − ) group reorientation.

Proton-conducting membranes based on CsH2PO4 and copolymer of tetrafluoroethylene with vinylidene fluoride

“…Solid Acid Fuel Cell (SAFC) is a promising new type of fuel cells with a CsH2PO4 acid salt as a membrane [1,2]. Interest in solid acid compounds, such as alkali metal dihydrogenphosphates and dihydrogensulfates, is constantly growing, a number of salts of this family is increasing; as a result, new compounds with Cs3(H1.5PO4)2 [3][4][5] and Cs7(H4PO4)(H2PO4)8 [6] compositions have been recently discovered. However, CsH2PO4 remains the salt with the highest value of proton conductivity.…”

Coexistence of Cationic and Anionic Phosphate Moieties in Solids: Unusual but Not Impossible

“…1−5 Recently, a new solid acid heptacesium tetra-hydroxyphosphonium octa-dihydrogen phosphate Cs 7 (H 4 PO 4 )(H 2 PO 4 ) 8 (CPP) has been synthesized, which is formally obtained from CsH 2 PO 4 (CDP) by replacing one out of eight Cs ions with the tetra/ hydroxyphosphonium polycation H 4 PO 4 + . 6 Wang et al confirmed (based on 31 P NMR and high-temperature singlecrystal X-ray diffraction experiments) that both anionic H 2 PO 4 − and cationic H 4 PO 4 + coexist in CPP. 6 The synthesis and investigation of CPP can be seen as a step toward creating an unconventional protonation pattern in solids, which may lead to higher proton mobilities.…”

“…6 Wang et al confirmed (based on 31 P NMR and high-temperature singlecrystal X-ray diffraction experiments) that both anionic H 2 PO 4 − and cationic H 4 PO 4 + coexist in CPP. 6 The synthesis and investigation of CPP can be seen as a step toward creating an unconventional protonation pattern in solids, which may lead to higher proton mobilities. Both CPP and CDP belong to the solid acid family.…”

“…The existence of a cationic phosphate tetrahedron such as the tetra-hydroxyphosphonium cation H 4 PO 4 + is highly unusual especially in the absence of external stabilization factors such as low pH conditions. So far, the H 4 PO 4 + cation has been reported only five times in the literature. − Recently, a new solid acid heptacesium tetra-hydroxyphosphonium octa-dihydrogen phosphate Cs 7 (H 4 PO 4 )­(H 2 PO 4 ) 8 (CPP) has been synthesized, which is formally obtained from CsH 2 PO 4 (CDP) by replacing one out of eight Cs ions with the tetra/hydroxyphosphonium polycation H 4 PO 4 + . Wang et al confirmed (based on 31 P NMR and high-temperature single-crystal X-ray diffraction experiments) that both anionic H 2 PO 4 – and cationic H 4 PO 4 + coexist in CPP …”

“…This strategy has already been tested by doping CDP with phosphoric acid, but this direct approach for incorporating excess protons did not significantly change the properties of the superconducting phase of CDP . From mixtures of CDP and CsH 5 (PO 4 ) 2 , Wang et al synthesized new proton conducting phases (in particular the compound CPP) with increased proton concentrations compared to CDP. , In addition to increasing the proton concentration, the partial replacement of Cs ions with H 4 PO 4 + also significantly affects the topology of the hydrogen bond network in CDP. A fluctuating and strong hydrogen bond network is a fundamental prerequisite for efficient Grotthuss-like proton transfer. − Therefore, this type of chemical modification of CDP represents a promising approach for fine-tuning the proton conductivity in this compound.…”