Neutron diffraction study of the atomic structure of cubic sodium-tungsten bronze (Na0.69WO3) single crystal

The atomic structure of a single crystal of one of four Na0.69WO3 phases, which exist below 293 K, has been refined from neutron diffraction data (WWR-c reactor at the Karpov Institute of Physical Chemistry, Obninsk Branch; λ = 1.168 A; λ/2 contribution < 0.8%; sinθ/λ ≤ 0.810; T = 288 K; crystal sphere ∅ = 4.4 mm; cubic unit cell with a = 7.672 A, sp. gr. Im3, z = 8, μ = 1.9 mm−1). The Na0.69WO3 atomic structure has been refined (198 independent reflections) taking into account the anisotropy of thermal vibrations (R w = 4.0%). The stoichiometric coefficient Na(0.69) is also refined. A structural distortion is revealed, which is characterized by the displacement of oxygen atoms (0, 0.2609(2), 0.2391(2)) from the ideal perovskite positions (0, 1/4, 1/4); this displacement doubles the ideal perovskite lattice period. The oxygen displacements can be described as rotations of oxygen octahedra by 3.58° around the [111] direction. The structure remains cubic because the octahedra rotations with respect to all three perovskite cubic axes are identical.