A single crystal neutron diffraction study on mixed crystal (K)0.25(NH4)0.75H2PO4: tuning of short strong hydrogen bonds by ionic interactions

One of the most intriguing facts about hydrogen bonds is that bonds formed between the same couple of donor and acceptor atoms can have large variation in the geometry as well as energies; this is mainly because hydrogen bonds are highly sensitive to their environment. We report here a comparative study of neutron structures of mixed crystals of potassium-ammonium dihydrogen phosphate. Basic structural framework of crystals belonging to potassium dihydrogen phosphate family is constructed by a network of zigzag O-H-O hydrogen bonded chains of PO2(OH)(2) anions, with cations occupying the interspaces of the zigzagged anionic chains. These crystals having simple crystal structure primarily determined by the O-H-O hydrogen bonds connecting PO2(OH)(2) anions provide us with a unique opportunity to tune the hydrogen bond geometry by varying the cationic occupancy and study the effect of this change on the average structure of the crystal. The cation-anion interaction is found to have a direct influence on the O-H-O hydrogen bonds of the mixed crystals.