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Structural Investigation of the "Tripled-Tetragonal-Tungsten-Bronze" Phases Sr2M10-xO27-y (M = Nb, Ta)



Justin B. Felder, Winnie Wong-Ng, Rania A. Qabbani, Brian H. Toby, Julia Chan


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 modeled in the orthorhombic Pba2 space group. Lattice parameters obtained from powder X-ray refinements are a = 12.4269(2) Å, b = 37.194(7) Å, c = 3.87897(4) Å, for Sr2Ta10-xO27-y, and a = 12.3281(3) Å, b = 37.1561(11) Å, and c = 3.93971(7) Å 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 three-, four- and five-fold tunnels found in the structure running parallel to the c-axis. Sr ions partially fill the four- and five-fold tunnels.


Tripled tetragonal-tungsten-bronze (TTTB), X-ray and neutron diffraction, structure, Sr2Nb10O27 and Sr2Ta10-xO27-y


Felder, J. , Wong-Ng, W. , Qabbani, R. , Toby, B. and Chan, J. (2019), Structural Investigation of the &#147;Tripled-Tetragonal-Tungsten-Bronze&#148; Phases Sr2M10-xO27-y (M = Nb, Ta), Tetrahedron, [online],, (Accessed April 16, 2024)
Created September 14, 2019, Updated October 12, 2021