Crystal structure of La24Li20Ti5O56:: A pseudo-close-packed, columnar intergrowth stucture

The crystal structure of La24Li20Ti5O56 has been solved by a combination of selected area electron diffraction (SAED) and powder X-ray and neutron diffraction. The structure has a strong tetragonal subcell a = 13.244(1) Angstrom, c = 7.463(1) Angstrom, space group P4/mbm (127), by SAED, a weak supercell with doubled c is observed. The subcell structure contains alternating columns of (i) perovskite-like structure, with Li, Ti in alternating octahedral sites, and (ii) a more complex structure that is distantly related to fluorite; one half of the tetrahedral sites in fluorite is occupied in pairs by Li, but the other half of the pairs of tetrahedral sites collapse to contain either a single octahedral site occupied by Ti, or a single tetrahedral site occupied by Li. The Li sites that form the edge-sharing pairs are not fully occupied. Possible ordering of Li/vacancies within these sites may be responsible for the superstructures seen by SAED. The structure is not close-packed, cp, overall but the cp arrangements of 0 and La within the columns give the structure a high packing density. (C) 2001 Elsevier Science.