Origin of dielectric relaxations in Na2SO4 in the intermediate temperature regime

The dielectric permittivity of Na2SO4 was measured in the frequency range 5 Hz-13 MHz. It shows a distinct relaxation at low frequencies at several isotherms (T≤377 °C). For example, the relaxation frequency, fmax, is around 1 MHz at 247 °C, which shifts to higher frequencies (∼4.5 MHz) as the temperature increases. The fmax vs. reciprocal T shows an activated relaxation process over the temperature range 247-377 °C with activation energy Ea=0.38 eV, which is close to that derived from the dc conductivity, Eσ=0.44 eV. We suggest that the observed dielectric relaxation might be attributed to polarization induced by the sodium-ions hops and the sulfate-groups reorientations. The displacement of mobile Na+ ions accompanied by SO42− tetrahedra reorientations create structural distortion in both sublattices that induce localized dipole-like moments.