Structural investigation of the 'tripled-tetragonal-tungsten-bronze' phases Sr2M10−O27− (M = Nb, Ta)

Sr2Nb10O27 and Sr2Ta10−xO27−y exhibit the tripled tetragonal-tungsten-bronze (TTTB)-related superstructures. In contrast to Sr2Ta10−xO27−y, where an extended solid solution was found, Sr2Nb10O27 only forms a narrow range of solid solution, Sr1−xNb10O27−z (0 ≤ x ≤ 0.04). Serial X-ray and neutron diffraction studies show that both compounds can be modelled in the orthorhombic Pba2 space group. Lattice parameters obtained from powder X-ray refinements are a = 12.3887(14) A, b = 37.103(4) A, c = 3.866(2) A, for Sr2Ta10−xO27−y, and a = 12.3888(2) A, b = 36.9839(5) A, and c = 3.94027(3) A for Sr2Nb10O27. Neutron Rietveld refinements of both structures revealed the possible reason for the differing solid solution behavior. For Sr2Nb10O27, all Nb sites are fully occupied, whereas in Sr2Ta10−xO27−y, three out of the nine crystallographically unique Ta sites were shown to be partially occupied. Charge adjustment is therefore possible by increasing the Sr concentration, leading to a solid solution series and smaller Ta/Sr ratio as compared to Sr2Nb10O27. In the TTTB-related structures, Nb and Ta ions both occupy octahedral and pentagonal bipyramidal coordination sites. Layers of interconnected corner-shared octahedra were found parallel to the ab plane, with 3-, 4- and 5-fold tunnels found in the structure running parallel to the c-axis. Sr ions partially fill the 4- and 5-fold tunnels.