Physical properties and conductivity relaxation processes in sodium sulfo-borophosphate glasses

Many physical properties of a sodium-borophosphate glass system (BPSx) having the starting composition (30H₃BO₃  +  xNa₂SO₄  +  (60  -  2x) NaH₂PO₄  +  10ZnO) were detremined. It was found that the glass density ρ, the transition temperature T _g, and the optical gap energy decrease whereas the molar volume and Urbach ▵E energy increases with the substitution of Na₂O by Na₂SO₄. The analysis of the evolution of the complex dielectric permittivity ε ^* and the complex electric modulus M ^* with temperature and composition of the glass indicates that the dielectric loss is dominated by ionic conduction, and the relaxation processes are non-Debye type. The evolution of the Kohlrausch-Williams-Watts (KWW) exponent β, estimated from the imaginary part of the electric modulus indicates that the investigated glass behaves as an intermediate elastic system as compared to flexible and stressed rigid phase. The obtained values of the decoupling index are of the same order of magnitude as observed for fast-ion conducting glasses. From the analysis of the experimental data, we may suggest that the glass sample BPS4 is the more thermally stable and has the highest ionic conductivity since it has the highest dielectric constant favorable for the dissociation of sodium salt.