The study of the Ba/La order in the Ba6-x-yLa8+2/3x+yTi18-y AlyO54 solid-solution was performed by X-ray diffraction on powders and single-crystals. The crystal structure of these phases belongs to the tetragonal tungsten bronze type, build on the basis of (3x3) TiO6 octahedron more than (2X2) (space group Pnma, a≈22.4Å, b≈7.7Å, c≈12.2Å). Two structural models were built, corresponding to two different orders within the structure. The model I, of formula [Ba4] A2 [Ba2-z La2x/3+y+q q’ ]A1'[La8-q' q] A1Ti18-y AlyO54, corresponds to a model associated to infinite perovskite rows parallels to Oy axis, build on the basis of (3X3) TiO6 octahedron. The model II, with formula [Ba6-z z-2] A2 [La8+2x/3+y 2-(2x/3)-y] A1’A1Ti18-yAlyO54, can be described on the basis of three perovskite octahedron layers parallels to xOz plane.The study of the Ba/La order in the Ba6-x-yLa8+2/3x+yTi18-y AlyO54 solid-solution was performed by X-ray diffraction on powders and single-crystals. The crystal structure of these phases belongs to the tetragonal tungsten bronze type, build on the basis of (3x3) TiO6 octahedron more than (2X2) (space group Pnma, a≈22.4Å, b≈7.7Å, c≈12.2Å). Two structural models were built, corresponding to two different orders within the structure. The model I, of formula [Ba4] A2 [Ba2-z La2x/3+y+q q’ ]A1'[La8-q' q] A1Ti18-y AlyO54, corresponds to a model associated to infinite perovskite rows parallels to Oy axis, build on the basis of (3X3) TiO6 octahedron. The model II, with formula [Ba6-z z-2] A2 [La8+2x/3+y 2-(2x/3)-y] A1’A1Ti18-yAlyO54, can be described on the basis of three perovskite octahedron layers parallels to xOz plane
