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

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

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

Z. Kristallogr. 220 (2005) 231–244 231 # by Oldenbourg Wissenschaftsverlag, München The real structure of Na3BiO4 by electron microscopy, HR-XRD and PDF analysis Sascha Vensky I , Lorenz Kienle I , Robert E. Dinnebier I , Ahmad S. Masadeh II , Simon J. L. Billinge II and Martin Jansen*,I I Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany II Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA Dedicated to Professor Dr. Hans-Jörg Deiseroth on the occasion of his 60 th birthday Received July 9, 2004; accepted September 16, 2004 Electrocrystallization / High resolution transmission electron microscopy / Pair distribution function / Sodium bismuthate / Powder diffraction structure analysis / X-ray diffraction Abstract. The real structure of a new crystalline high temperature phase, metastable at room temperature, in the system sodium – bismuth – oxygen, b-Na3BiO4, was determinated using high resolution X-ray powder diffraction, pair distribution function analysis, and high resolution transmission electron microscopy. b-Na3BiO4 was synthesized by anodic oxidation of bismuth(III)-oxide in a sodium hydroxide – lithium hydroxide melt. The average crystal structure of b-Na3BiO4 at ambient conditions (R3m, a ¼ 3.32141(9) A, c ¼ 16.4852(5) A) is structurally related to a-NaFeO2 with metal layers almost statistically occupied in a Na : Bi ratio of 3 : 1. Analysis of the longrange 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. Introduction The rock salt arrangement is among the fundamental building principles in three-dimensional space. Besides the vast families of chemically different AB compounds, it is realized in salts containing complex anionic and/or cationic constituents, even including extended cluster ions. Examples are calcite CaCO3, [1] sodium nitrate NaNO3, [2] sodium ozonide NaO3, [3] calcium carbide CaC2, [4] sodium azide NaN3, [5] and fulleride compounds of the * Correspondence author (e-mail: m.jansen@fkf.mpg.de) [M(NH3)6] C60 6NH3 type, M ¼ (Cd, Co, Mn, Zn) [6, 7]. Substitution variants, with either the cationic or anionic sublattices occupied by different species in an ordered manner, represent another class of rock salt derivatives. Here various ternary alkali metal oxides of general formula types ABO2, A2BO3, A3BO4, A4BO5, ... (A ¼ alkali metal, B ¼ metal or nonmetal) need to be included. Some of the latter show order-disorder transitions within their cationic sublattices, and are reluctant to fully order, during the synthesis along the solid state route. In the past, this phenomenon has caused some confusion with respect to the correct indexing of the powder patterns of e.g. Li2SnO3, [8, 9] Li2MnO3, [10–12] or Na2RuO3 [13, 14]. The room temperature modification of Na3BiO4, referred to as a-Na3BiO4, hereafter, is a fully ordered rock salt substitution variant with monoclinic symmetry [15]. Here we report on a heavily disordered high temperature modification of Na3BiO4, i.e. b-Na3BiO4, grown electrochemically from a NaOH/Bi2O3 melt. The oxidation state of +V for bismuth in oxides is generally rare. However, it has been realized in a number of alkali bismuthates: ABiO3 and A3BiO4 with A ¼ Li, Na, K, Li5BiO5, and Li7BiO6 [15 – 25]. Out of these, the only one accessible through electrocrystallization from a melt, besides solid state routes, was KBiO3 [20 – 22]. Experimental Syntheses and analyses Crystalline material of b-Na3BiO4 was obtained by electrocrystallization from alkali hydroxide melts containing bismuth(III) oxide Bi2O3. The components of the melt, 1 g Bi2O3 (Riedel-de Haen, 10305), 12 g NaOH (Merck, 106498), 3.4 g LiOH (Merck, 105691), and 0.4 g ZnO (Chempur, 008417), were used without pre-treatment. Figure 1 shows a schematic drawing of the electrolysis cell used. A nickel crucible containing the components of the melts was placed into a closed glass reaction vessel and heated during three hours starting from a temperature of T ¼ 200 C up to a temperature slightly above the electro-

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