Neutron diffraction study of the atomic structure of cubic sodium-tungsten bronze (Na 0.69 WO 3 ) single crystal
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CTURE OF INORGANIC COMPOUNDS
Neutron Diffraction Study of the Atomic Structure of Cubic Sodium–Tungsten Bronze (Na0.69WO3) Single Crystal I. V. Isakova, A. I. Kalyukanova, V. L. Volkovb†, R. P. Ozerova, and L. E. Fykina a
Karpov Institute of Physical Chemistry (Obninsk Branch), Obninsk, Russia email: [email protected] b Institute of SolidState Chemistry, Ural Branch, Russsia Received September 22, 2010
Abstract—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 (WWRc reactor at the Karpov Institute of Physical Chemis try, Obninsk Branch; λ = 1.168 Å; λ/2 contribution < 0.8%; sinθ/λ ≤ 0.810; T = 288 K; crystal sphere ∅ = 4.4 mm; cubic unit cell with a = 7.672 Å, 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 (Rw = 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 posi tions (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. DOI: 10.1134/S1063774511030096
INTRODUCTION The Na0.69WO3 compound belongs to oxide bronzes with complex physical (electrochromic, superconducting, and superionic) properties [1–4], which may depend on the bronze structural features; many of them have perovskite structure. The atomic structure of the other representatives of this structural class—ferroelectrics and piezoelec trics—has been intensively investigated for a long time. The difference in their physical properties is related to certain distortions of the ideal perovskite structure. There are three possible ways to distortion from the ideal perovskite structure: the displacement of cations from ideal positions, which results in ferro electricity; the distortion of the oxygen octahedron; and rotations of the oxygen octahedron around one or several fourfold axes. Glazer [5] proposed a classifica tion for the rotations of oxygen octahedra in perovs kites and showed how these rotations are related to the unitcell symmetry. All possible rotations of octahedra lead to 23 spatial groups. Twenty two of them result from rotations around three axes of the [100] type (14 groups), two rotational axes (6 groups), and one rotational axis (2 groups); one group (without rotations) corresponds to ideal perovskite. Since there are diverse versions of spatial groups, the study of perovskite structures should be emphasized on searching for and recording †Deceased.
weak difference reflections in the halfinteger planes of the reciprocal cell. An extensive list of compounds identified as ideal perovskites by the Xra
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