Electrical conductivity and phase diagram studies of some solid electrolytes based in LI2SO4 and Na2SO4

Abstract

Electrical conductivity of the <formula>, + <formula>, , <formula>,, - <formula>, eutectic + A103, <formula> - <formula>, and the <formula>, systems have been measured using ac impedance technique. Phase diagrams of the <formula>, and the <formula>,, systems have been determined in detail using the differential scanning calorimetry and the high tem- perature powder X-ray diffraction.

In the <formula> system, the eutectic composition which is at 17.5 mol%<formula>, shows the maximum conductivity, e.g. <formula> at 500°C, When A1,0, was added to the eutectic mixture of <formula>, , a further enhancement of conductivity could be seen and Pen.the maximum was observed for 40 mol% <formula>. The conductivity enhance- ment of the eutectic mixture is attributed to the composite effect due to the maximum grain boundary area in the eutectic mixture and the addi:c tional conductivity enhancement is attributed to the increased inter- facial area due to the presence of aera grains.

In the <formula>, system, two conductivity maxima have been observed below 500°C. They were at about 34 and 70 mol% <formula>,. The first maximum is due to the composite effect, and the second one is due to the partial replacement of woe with sof creating more inters- titial sites. The high ionic conductivity of a-<formula>, has been explained by the paddle-wheel mechanism in which the cationic mobility is enhanced by the coupled rotation of translationaly static SO), ions. Replacement of 80," by larger WO. ions has shown a conductivity drop within the solid solubility region of y-phase. This is a strong experi- mental evidance for the proposed paddel-wheel mechanism of ion trans-port in a-<formula>, .

In the <formula>,, system, two intermediate compounds have been observed at 7.7 and 33 mol% <formula>, - The solid solubility of the <formula>, (I) phase extends up to 40 mol% <formula>, at 915°C. The electrical conductivity of this solid solution increases rapidly with increasing <formula>, content and reaches a maximum at about 5 mol% <formula>), and the maximum conductivity a 300°C is 3.5x107° @ tom. In the <formula>, system, <formula>, and <formula> have been Identified as intermediate compounds. The | solid solution of the <formula> (1) phase is stable up to 35 mol% <formula>, at 680°C. The electrical conductivity Of this solid solution increases rapidly with increasing <formula>, content And reaches the maximum at 20 mol% Jem at 520°C. In <formula>,. The maximum conductivity is about 2x107* Qen” both these systems, The conductivity maxima can be explained by the for- mation of clusters And defect interactions at high vacancy concentration. In the <formula>,

system, the conductivity up to 30 mol% <formula>, has been studied and the conductivity enhancement observed is very small compared to the enhancement due to the cation substitution in <formula>. The conductivity enhancement can be attributed to the lattice expansion dué to the replacement of smaller so; by larger Se0), in the solid solubility region.

In the <formula> ternary system, the highest conductivity is observed for the composition <formula>, which has the <formula>, (I) structure, and the conductivity of this composition is 1.0x10°*a at 350°C. The conductivity enhancement observed in this systemis possibly caused by the creation of more vacancies due to <formula> subs- titution combined with high Li’ ion mobility.