The real structure of Na3BiO4 by electron ... - Columbia University

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238 S. Vensky, L. Kienle, R. E. Dinnebier et al. Applying this model to our problem, the structure of b- Na3BiO4 would be considered to contain centered ONaxBi6 x octahedra (i.e. clusters) which represent the smallest ordered building units of the structure. Following generalized electrostatic valence rules [61, 62], one would expect a that the compositions of the clusters and the stoichiometry of the sample are preferably identical. Hence, b-Na3BiO4 would require ONa5Bi- and ONa4Bi2-octahedra at a ratio of 1 : 1. A related ratio of clusters (VC5 & and VC4 & 2) has been reported for short-range ordered V4C3 [63]. Assum- b c d e f Fig. 9. Surface of the diffuse intensity. (a) 3D model, (b)–(f) comparison of experimental SAED patterns and sections of the surface. (b) h100icub, (c) h110icub, (d) h111icub, (e) h012icub, (f) h013icub.
Real structure of Na3BiO4 ing these two types of clusters for b-Na3BiO4 seems reasonable since these are realized in the experimentally observed (ordered) structure III, i.e. a-Na3BiO4. This simple model of the real structure holds for constant diffuse intensity within the surface. However, characteristic deviations were observed by SAED recorded in selected regions of one crystal. The most common fluctuation of the diffuse intensity is connected with partial order of the real structure in one of the {111}cub layers, as described for the (001)trig layers of structure II. Consequently, the concentrations of the diffuse intensities are observed at the positions of superstructure reflections hkltrig (cf. structure II). These concentrations occur in different extent within one crystal, as depicted in Fig. 10a and c (left and center) for the zone axes h110icub and h013icub, respectively. For the h110icub pattern, the concen- Fig. 10. SAED patterns of b-Na3BiO4. (a) SAED patterns recorded on shortrange ordered domains (left) and partially ordered domains (right) of the same crystal. (b) Twinning of partially ordered domains h100itrig. Left and right: h100itrig, center: superposition. (c) SAED patterns recorded on differently ordered domains of the same crystal. Left: short-range ordered domain h013icub, center: partially ordered domain [141]trig, right: ordered domain [212]mon. (d) SAED patterns of partially ordered ðh100itrigÞ and ordered domains ([101]mon). a b c d 239 trations (Fig. 10a, right) are exclusively observed on positions 1 =2 hhhcub or 1 =2 hhhcub. SAED patterns with simultaneous concentrations on both positions (Fig. 10b, center) were produced by twinning, as indicated by recording the diffraction patterns of the single domains sequentially, see Fig. 10b, left and right. The twinning can be rationalized by the symmetry relations between the aristotype of Na3BiO4 (i.e. structure I) and structure II. In a first step the symmetry of the aristotype is reduced to space group R3m which is a maximal t subgroup (index 4) of Fm3m (structure I). The c-axis of this trigonal unit cell is half of that assumed for structure II. Due to this symmetry reduction, a maximum number of four distinguishable orientations is expected for domains with average trigonal symmetry. In a second step (i2), the symmetry is reduced by doubling the c-axis. The coexistence of tilted domains
Page 1 and 2: Z. Kristallogr. 220 (2005) 231-244
Page 3 and 4: Real structure of Na3BiO4 Table 2.
Page 5 and 6: Real structure of Na3BiO4 coordinat
Page 7: Real structure of Na3BiO4 a b Fig.
Page 11 and 12: Real structure of Na3BiO4 Fig. 11.
Page 13 and 14: Real structure of Na3BiO4 try reduc

The real structure of Na3BiO4 by electron ... - Columbia University

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