Transmission electron microscopy of topochemical conversion interface between La2Ti2O7 reactive template and perovskite product Li0.16La0.62TiO3 electrolyte

The topochemical conversion interface between a layered perovskite La2Ti2O7 and an ordered regular perovskite Li0.16La0.62TiO3, (LLTO) electrolyte was investigated using high-resolution transmission electron microscopy and scanning-transmission electron microscopy (STEM) for platelike La2Ti2O7 particles semi-reacted (at 1273 K) with the complementary reactants Li4Ti5O12 and TiO2. High-temperature X-ray diffraction analysis confirmed that regular perovskite-type product was formed in situ at 1173 K and above without the formation of any intermediate phases. The crystallographic relationship between the product and the template at the interface was determined by selected area electron diffraction to be orthorhombic LLTO [001](110) || monoclinic La2Ti2O7 [001](100). STEM revealed that the alternating La-rich and La-poor planes in the LLTO phase were aligned perpendicular to the interface, which was parallel to the interlayers, i.e., the (200) planes of the La2Ti2O7 template. At the interface, a unit-cell-thick La2Ti2O7 or a block of four-layered TiO6 octahedra between two interlayers is a basic unit converted into a regular perovskite-type product by shifting the block with La along the c-axis. This microscopic observation of the interface suggests the possibility of designing domain structures with improved ionic conductivity.