Structural and vibrational study and superprotonic behavior of the new solid acid: K0.47(NH4)0.53H2(PO4)0.52(AsO4)0.48

Abstract

The new compound K0.47(NH4)0.53H2(PO4)0.52(AsO4)0.48 crystallizes in the tetragonal system I4¯2d with lattice parameters a = 7.606(5) Å and c = 7.401(5) Å. This material has a unit cell volume of 428.16 Å3 and four formula units per cell. The main feature of the structure is the coexistence of two groups with a motive (NH4/K)+-H2(P/As)O4−. In this structure, there are two types of hydrogen bonding (OH⋯O and NH⋯O) which contributes to their stability.The infrared spectra of K0.47(NH4)0.53H2(PO4)0.52(AsO4)0.48 recorded at room temperature in the frequency range 4000–400 cm−1 confirm the presence of two different anions (AsO43− and PO43−) in the same crystal. A calorimetric study of the title compound shows two distinct endothermal peaks which are detected at 248 and 490 K. Samples were examined by impedance and modulus spectroscopy techniques. The first transition (248 K) is attributed to a antiferroelectric–paraelectric type. A high temperature phase transition (490 K) leading to a superionic–protonic phase was found, characterized by an unusual high conductivity. The conductivity relaxation parameters associated with the high-disorder protonic conduction have been determined from analysis of the M″/M″max spectrum measured in a wide temperature range. Transport properties of this material appear to be due to the proton hopping mechanism.