Thermo-mechanical behaviours of Li3La1/3-MO3 (M = Ta and Nb)

The present study investigated thermo-mechanical behaviour of A-site deficient perovskites Li3xLa1/3-xTaO3 (LLTaO) and Li3xLa1/3-xNbO3 (LLNbO) with x = 0.00 to 0.06 as a function of temperature by means of thermo-mechanical analysis. LLTaO for all compositions exhibits an orthorhombic-tetragonal phase transition around 200 K, above which it has a tetragonal phase. As the temperature increases from 200 K to 400 K, the linear expansion coefficient shows a slight decrease for all the compositions, followed by a constant value of ~3 × 10−6 K−1 above 400 K regardless of x. On the other hand, LLNbO has an orthorhombic phase for x ≤ 0.02 and the tetragonal for x ≥ 0.04 at room temperature. The linear expansion coefficient of LLNbO with x ≤ 0.02 shows a peak around 470 K, which can be attributed to an orthorhombic-tetragonal phase transition. In addition, the expansion coefficient of LLNbO with x ≥ 0.02 significantly increases as the temperature rises from 200 K to 400 K, followed by a plateau and a relatively abrupt decrease after reaching a temperature above 600 K. Additional ionic conductivity measurement of LLNbO suggested that the inconstant expansion behaviour could be attributed to the lithium-ion motion.