Structural, thermal behavior, vibrational study and superprotonic behavior of a new rubidium dihydrogen phosphate-arsenate: RbH2(PO4)0.65(AsO4)0.35

The structural characteristics of the crystals (RbH2(PO4)0.65(AsO4)0.35) were analyzed by means of X-ray diffraction, which revealed that the new title compound is nearly isomorphous with the tetragonal phase of RbH2AsO4 (RDA). The following results have been obtained: space group I− 42d, a = 7.6837 (8) Å, c = 7.3824 (19) Å, V = 435.85(13) Å3 and Z = 4. The phosphorus and the arsenic atom as well as the rubidium ions lie on points with site symmetry S4, the oxygen atoms lie in general positions about the phosphorus-arsenic atoms, in a tetrahedral arrangement. The (P/As)O4 tetrahedra are connected by OH…O hydrogen bonds laying essentially in the a,b plane. Differential scanning calorimetric traces show three phase transitions at 229, 390 and 580 K in this material. Moreover, protonic conduction of the mixed compound determined by impedance and modulus spectroscopy has been studied in the temperature range 150-450 K in order to determine the nature of the phase transition.