NIR Raman studies of superionic phase transition in (NH4)(4)H-2(SeO4)(3)

Near-infrared Fourier transform Raman spectroscopy has been used to study the molecular group dynamics in (NH4)(4)H-2(SeO4)(3) single crystal in the vicinity of its superionic phase transition (T-s similar or equal to 378 K). A change in the dynamics of SeO4 and SeO4H entities begins about 15 K below T-s and ends at T-s. The results of the Raman studies of (NH4)(4)H-2(SeO4)(3) gave evidence that the orientational disorder of SeO4 tetrahedra is responsible for the fast protonic conductivity. A very large temperature hysteresis (similar to 30 K) for the superionic phase transition in (NH4)(4)H-2(SeO4)(3) was found. (C) 1997 Elsevier Science B.V.