Effects of microstructural ordering and mechanical stress on ionic conductivity of Li0.29La0.57TiO3

The effects of microstructural ordering and uniaxial compressive stress on the Li-ion conductivity of Li 0.29 La 0.57 TiO 3 (LLTO) were investigated. Samples with various degrees of microstructural ordering ( R ) were prepared through controlled thermal treatments. The disordered phase exhibited the highest bulk and grain-boundary conductivities, whereas the highly ordered tetragonal phase showed the lowest. As the degree of ordering increased, the bulk conductivity decreased significantly in the low-ordering regime, which was attributed to cell volume reduction. In the high-ordering regime, however, the conductivity showed a gradual decline, accompanied by the formation of numerous domain boundaries. These microstructural changes within grain interiors appeared to significantly affect the state of the grain boundaries, leading to a notable variation in grain-boundary conductivity. The stress dependence of ionic conductivity was strongly influenced by the degree of ordering. The ordered tetragonal phase exhibited linear elastic behaviour, with both bulk and grain-boundary conductivities decreasing slightly under compression due to volume reduction and reversible changes in local geometry at grain boundaries. In contrast, the disordered cubic phase showed nonlinear mechanical behaviour with distinct hysteresis at lower stress levels. This was accompanied by a stress-induced preferential orientation of (110) planes, leading to a remarkable increase in bulk conductivity and a simultaneous irreversible decrease in grain-boundary conductivity. These findings suggest that uniaxial compression alters the microstructural alignment within grain interiors, which in turn modifies the state of the grain boundaries. Whilst the disordered phase exhibits the highest conductivity, it could be further enhanced through fine microstructural alignment, indicating that LLTO possesses an even higher intrinsic diffusivity than previously realised.