The real structure of Na3BiO4 by electron microscopy, HR-XRD and PDF analysis

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Abstract
The real stucture of a new crystalline high temperature phase, metastable at room temperature, in the system sodium – bismuth – oxygen, β-Na3BiO4, was determinated using high resolution X-ray powder diffraction, pair distribution function analysis, and high resolution transmission electron microscopy. β-Na3BiO4 was synthesized by anodic oxidation of bismuth(III)-oxide in a sodium hydroxide – lithium hydroxide melt. The average crystal structure of β-Na3BiO4 at ambient conditions (R3m, a = 3.32141(9) Å, c = 16.4852(5) Å) is structurally related to α-NaFeO2 with metal layers almost statistically occupied in a Na:Bi ratio of 3:1. Analysis of the long-range order on the bulk material by Rietveld refinement led to approximately Na:Bi ratios of 2:1 and 4:1, in consecutive metal layers, while a detailed analysis of the local order by means of the pair distribution function revealed the existence of almost pure sodium layers and mixed 1:1 – sodium:bismuth layers. Complementary studies on single crystallites using high resolution transmission electron miscroscopy exhibited a complex domain structure with short-range ordered, partially ordered, and long-range ordered domains.

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