Chessboard/diamond nanostructures and the a -site deficient, Li _1/2-3x Nd_1/2+xTiO₃, defect perovskite solid solution

The crystal chemical origin of nanoscale chessboard/diamond ordering in perovskite-related solid solutions of composition Li_0.5-3xNd _0.5+xTiO₃ (LNT, x ∼ 0.02-0.12) is investigated. Experimental and simulated scanning transmission electron microscopy (STEM) images are found to be consistent with the compositional modulation model proposed by previous authors. However, these earlier models do not satisfactorily explain other features observed in high-resolution STEM and TEM images, such as the two-dimensional {100} lattice fringes with the same periodicity, √2a_p × √2a_p, as the local LNT unit cell viewed along the [001] direction (where a_p is the parent perovskite unit cell parameter). Based on bond valence sum calculations, we propose a new set of crystal structures for LNT in which Li ions are primarily bonded to only two O ions, and order one-dimensionally with √2a_p periodicity. Bright-field STEM image simulations performed for this new model reproduced the experimentally observed √2a_p lattice fringes, thus strongly suggesting that the finer features of the high-resolution (S)TEM images are the result of Li ion ordering and associated local structural relaxation. In this new model, the LNT chessboard supercell then results from the ordered combinations of two sublattices: the Li ion sublattice and its translational variants on the one hand, and the Nd _0.5TiO₃ sublattice and its oxygen octahedral tilt twin variants on the other. Dielectric measurements indicate the presence of long-lived polar clusters that are easily activated under an applied electric field. This suggests that these clusters consist of spatially correlated Li ions.