Beyond the Structure-Property Relationship Paradigm: Influence of the Crystal Structure and Microstructure on the Li+ Conductivity of La2/3LixTi1−xAlxO3 Oxides

The crystal structures of several oxides of the La2/3LixTi1−xAlxO3 system have been studied by selected‐area electron diffraction, high‐resolution transmission electron microscopy, and powder neutron diffraction, and their lithium conductivity has been by complex impedance spectroscopy. The compounds have a perovskite‐related structure with a unit cell √2 ap×2 ap×√2 ap (ap=perovskite lattice parameter) due to the tilting of the (Ti/Al)O6 octahedra and the ordering of lanthanum and lithium ions and vacancies along the 2 ap axis. The Li+ ions present a distorted square‐planar coordination and are located in interstitial positions of the structure, which could explain the very high ionic conductivity of this type of material. The lithium conductivity depends on the oxide composition and its crystal microstructure, which varies with the thermal treatment of the sample. The microstructure of these titanates is complex due to formation of domains of ordering and other defects such as strains and compositional fluctuations. Complementary methods: To properly determine the structure of complex materials such as the perovskite‐like La2/3−xLixTiO3 family, averaging (PXRD and PND) and local techniques (such as SAED and HRTEM) must be combined. Only by doing so can a complete and adequate structural model be developed.