Crystal structure and proton conductivity of a new Cs3(H2PO4)(HPO4)·2H2O phase in the caesium di- and monohydrogen orthophosphate system.

The MxHy(AO4)z acid salts (M = Cs, Rb, K, Na, Li, NH4; A = S, Se, As, P) exhibit ferroelectric properties. The solid acids have low conductivity values and are of interest with regard to their thermal properties and proton conductivity. The crystal structure of caesium dihydrogen orthophosphate monohydrogen orthophosphate dihydrate, Cs3(H1.5PO4)2·2H2O, has been solved. The compound crystallizes in the space group Pbca and forms a structure with strong hydrogen bonds connecting phosphate tetrahedra that agrees well with the IR spectra. The dehydration of Cs3(H1.5PO4)2·2H2O with the loss of two water molecules occurs at 348-433 K. Anhydrous Cs3(H1.5PO4)2 is stable up to 548 K and is then converted completely into caesium pyrophosphate (Cs4P2O7) and CsPO3. Anhydrous Cs3(H1.5PO4)2 crystallizes in the monoclinic C2 space group, with the unit-cell parameters a = 11.1693 (4), b = 6.4682 (2), c = 7.7442 (3) Šand ß = 71.822 (2)°. The conductivities of both compounds have been measured. In contrast to crystal hydrate Cs3(H1.5PO4)2·2H2O, the dehydrated form has rather low conductivity values of ∼6 × 10-6-10-8 S cm-1 at 373-493 K, with an activation energy of 0.91 eV.