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

Phosphoric acid is commonly known either as a neutral molecule or as an anion (phosphate). We theoretically confirm by ab initio molecular dynamics simulations (AIMD) that a cationic form H₄PO₄⁺ coexists with the anionic form H₂PO₄^- in the same salt. This paradoxical situation is achieved by partial substitution of Cs⁺ by H₄PO₄⁺ in CsH₂PO₄. Thus, HPO₄ acts simultaneously as both the positive and the negative ion of the salt. We analyze the dynamical protonation pattern within the unusual hydrogen bond network that is established between the ions. Our AIMD simulations show that a conventional assignment of protonation states of the phosphate groups is not meaningful. Instead, a better description of the protonation situation is achieved by an efficiently fractional assignment of the strongly hydrogen-bonded protons to both its nearest and next-nearest oxygen neighbors.