Low-temperature behaviour of (NH4)4H2(SeO4)3 and (ND4)4D2(SeO4)3 superionic conductors

The structure of (NH4)4H2(SeO4)3 and of its deuterated analogue was re-determined at room temperature and at ∼140 K. In the examined temperature range, the same centrosymmetric space group P1̄ was ascribed to both crystals. A continuous temperature variation of lattice parameters was still apparent there. Splitting of the Raman band at 867 cm−1, arising from ν1 vibrations of two-proton donor selenate tetrahedra in (ND4)4D2(SeO4)3, was observed below 215 K. The low-temperature dependence of dielectric permittivity of (NH4)4H2(SeO4)3 exhibits a broad maximum which we relate to a glass-like transition and not to a ferroelectric–paraelectric one.