Structural and transport properties of doped bismuth titanates and niobates
Introduction
In the last decades compounds with the pyrochlore structure based on bismuth oxide –mixed titanates or niobates [1], [2] attracted increased attention due to new possibilities of their application. Thus, zinc-containing bismuth niobates with a high dielectric constant ε ~ 154 [3] are promising for producing multi-layer capacitors; manganese and cobalt-containing bismuth niobates were investigated with the purpose of detection of multiferroic ordering in them [4], [5]. Doped bismuth titanates are interesting from the point of view of their luminescent [6], [7], photocatalytic [8], [9] and dielectric [1], [10], [11] properties.
Bismuth titanate, Bi2Ti2O7, is interesting as a material for capacitors owing to its high dielectric constant, low dielectric losses and a low temperature coefficient of capacitance. However, its practical application is limited by the decomposition at T > 650°С [1], [12] and co-existence of Bi4Ti3O12 [13] and Bi2Ti4O11 [14] phases. In previous studies it was shown that doped bismuth titanate pyrochlores Bi1.6MxTi2O7 − δ (M – Cr, Cu, Mn, Fe, Mg, Zn, Sc) are stable up to the melting temperature [15], [16], [17], [18].
For A2B2O6O′ pyrochlore structure with the space group, A-sublattice sites located in 16c → 96h, 96g Wyckoff positions can be occupied by Bi as well as dopant cations. B-sublattice sites located in 16d Wyckoff positions can be occupied by Ti or Nb as well as dopant cations. The oxygen sublattice is nonuniform and divided into O (48f positions) and O′ (8a → 32e positions) sublattices. The dopant atoms at х < 0.3 are predominantly located in the bismuth sites of the Bi1.6MxTi2O7 − δ structure. At х > 0.3 they are partially distributed over the titanium sites [16], [17], [18]. In chromium-containing Bi1.6CrxTi2O7 − δ system the homogeneity region is narrow, x varies from 0.04 to 0.12 and chromium atoms in this system are located in the Bi sites. A mixed ionic-electronic conductivity was found in Bi1.6MxTi2O7 − δ (M – Cr, Fe, Cu, Mn) systems [15], [16], [17], [18]. When 3d element dopants are distributed over B-sites, dominating electronic type of conductivity dominates and antiferromagnetic superexchange for dopants was observed [16]. So, the presence of substituting cation can be also crucial for the relaxation process in doped bismuth titanates and niobates [19]. In a recent study of oxygen diffusion by the oxygen isotope heteroexchange in bismuth titanium pyrochlores doped with magnesium, scandium and copper we have shown that already at 350 °C a fast oxygen transport occurs [20].
The aim of this work is elucidating effect of transition metal nature and cation distribution on composition, structure and transport characteristics of bismuth based pyrochlores. Doped Bi niobates/titanates solid solutions were prepared and their synthesis procedures were optimized to obtain fine powders and functional ceramics possessing required phase stability in certain conditions (temperature, pressure, content of dopants, etc.). Distribution of dopants between two types of cation positions in the pyrochlore structure was studied. The magnetic properties of materials obtained were investigated and factors affecting their magnetic behavior and clustering of paramagnetic atoms were elucidated. Conductivity of materials was estimated depending on the temperature, dopant content and the oxygen partial pressure. Oxygen transport properties were studied using oxygen isotope heteroexchange with C18O2.
Section snippets
Materials and methods
Co- and Zn-doped bismuth titanates Bi1.6Zn0.2Ti2O7 − δ, Bi1.6CoxTi2O7 − δ (0.08 ≤ x ≤ 0.40) and mixed Mg-Cu-containing bismuth niobate Bi1.6Mg0.4Cu0.4Nb1.6O7 − δ were synthesized by the method of organic-inorganic precursors combustion. The pyrochlore structure A2B2O6O′ is formed when relation of ionic radii of all atoms in A sites to ionic radii of all atoms in B sites is in the range of 1.46–1.78 [1]. This factor is a main reason for synthesis of bismuth deficient titanates in our work. For the
Structure and morphology features
Single phase Bi1.6Zn0.2Ti2O7 − δ, Bi1.6Co0.23Ti2O7 − δ and Bi1.6Mg0.4Cu0.4Nb1.6O7 − δ samples with the pyrochlore type structure were obtained (Fig. 1). All X-ray diffraction patterns exhibit only characteristic peaks of the pyrochlore phase [25]. The homogenous ranges of Bi1.6CoxTi2O7 − δ and Bi1.6ZnxTi2O7 − δ are 0.08 ≤ х(Co) ≤ 0.23 and 0.1 ≤ х(Zn) ≤ 0.5, respectively [26]. As can also be seen in the SEM images of polished surface (Fig. 2) any admixtures in obtained ceramics are absent. The composition of
Conclusions
Co or Zn-doped bismuth titanates and Mg + Cu-doped bismuth niobate with the pyrochlore type structure stable up to the melting points were synthesized for the first time. According to the results of full-profile analysis of the XRD patterns and pycnometric density estimation, zinc and cobalt cations are mainly located in Bi sites, Mg cations-only in Nb sites, while Cu cations are equally distributed between Bi and Nb sites. According to magnetochemical analysis and NEXAFS spectroscopy Co is in 2 +
Acknowledgements
The support of transport properties studies by the Russian Science Foundation (Project 16-13-00112) and Russian Fund of Basic Research (project No. 15-03-09173 A) is gratefully acknowledged. Authors would also like to appreciate Solid State Ionics – 21st International Conference Organization Committee.
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