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

Abstract Sr 2 Nb 10 O 27 and Sr 2 Ta 10-x O 27-y exhibit the tripled tetragonal-tungsten-bronze (TTTB)-related superstructures. In contrast to Sr 2 Ta 10-x O 27-y , where an extended solid solution was found, Sr 2 Nb 10 O 27 only forms a narrow range of solid solution, Sr 1-x Nb 10 O 27-z (0 ≤ x ≤ 0.04). Serial X-ray and neutron diffraction studies show that both compounds can be modelled in the orthorhombic Pba 2 space group. Lattice parameters obtained from powder X-ray refinements are a = 12.4269(2) A, b = 37.194(7) A, c = 3.87897(4) A, for Sr 2 Ta 10-x O 27-y , and a = 12.3281(3) A, b = 37.1561(11) A, and c = 3.93971(7) A for Sr 2 Nb 10 O 27 . Neutron Rietveld refinements of both structures revealed the possible reason for the differing solid solution behavior. For Sr 2 Nb 10 O 27 , all Nb sites are fully occupied, whereas in Sr 2 Ta 10-x O 27-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 Sr 2 Nb 10 O 27 . 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.