Electrical conduction in undoped and yttrium-doped Na2SO4

The electrical conductivity of undoped and yttrium-doped Na₂SO₄ has been measured by the ac two-terminal method at temperatures between 540 and 1075 K in air or in SO₂O₂SO₃ atmospheres. At high temperatures such as 1075 K, doping of 1.7 at% yttrium (Y) decreases the conductivity, which indicates that the Frenkel defect of sodium ions is pre-dominant in Na₂SO₄ and that the mobility of the interstitial ions is higher than that of the vacancies. However, the vacancy concentration is high enough to increase the conductivity in the samples doped with 2.0–3.8 at% Y. A further addition of yttrium decreases the electrical conductivity again, suggesting the appeareance of defect interactions such as ordering or clustering of the defects. The electrical conductivity of the undoped and the 4 at% Y-doped Na₂SO₄ is independent of the SO₃ partial pressure. The complex impedance analysis shows that the grain boundary resistance increases with sintering time. This is probably caused by a decrease in the contact area between grains during sintering.