Hydrothermal synthesis and 121Sb Mössbauer characterization of perovskite-type oxides: Ba2SbLnO6 (Ln = Pr, Nd, Sm, Eu)

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Abstract

A mild hydrothermal process to prepare Ba2SbLnO6 (Ln = Pr, Nd, Sm, Eu) perovskite-type oxides are presented. These perovskites were characterized on the basis of X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), inductively-coupled plasma spectra (ICP) techniques. Primary structure was confirmed using Rietveld method based on XRD data shows that the likely space groups of Ba2SbLnO6 are R-3 for Ln = Pr and Nd and Fm-3m for Ln = Sm and Eu, respectively. The measurement of Mössbauer effect of the 37.2 keV γ transition of 121Sb indicates that the isomer shift of these perovskites falls in the region of the Sb5+ and reflects some hybridized-orbital behavior in Sb–O bonds.

Introduction

The compounds of the formula Ba2SbLnO6 (Ln = lanthanides) have long been known since Blasse has firstly reported Ba2GdSbO6 to be double cubic with 1:1 ordering of Gd3+ and SbV ions [1]. Latterly, some perovskites of this family have been reported [2]. Recently, these compounds have shown to be suitable as substrates for YBa2CuO7  δ thin films [3]. As one promising applications of high Tc superconducting thin films is the field of passive microwave devices including filters, resonators, delay and transmission lines, and antenna arrays, it seems recommendable to characterize more completely these possible substrates.

On the other hand, Karunadasa et al. [4] recently have reported that rare-earth-based double perovskites A2SbLnO6 (A = Sr, Ba), with edge-shared tetrahedra of magnetic lanthanides, are of particular interest for the direct comparison of their magnetic properties to those of the equivalent lanthanide pyrochlores, in which the magnetic lattice consists of corner-sharing tetrahedra. The rare-earth ions in total spin and electron configuration exist in a systematic manner along with the growth of atomic number; therefore, rare-earth-based double perovskites seem to be a promising prototype family for making contact with theoretical models to investigate the behavior of spins of different types on cubic double perovskite.

Hydrothermal route to synthesis perovskites-type oxides have attracted a great deal of interest for a number of years [5]; however, only a few papers demonstrated the synthesis of A2BB′O6 perovskites [6]. In the synthesis of double perovskite-type oxides, the size difference and variety of the B-cations are not only responsible for the applied range of hydrothermal process but also play an important role in controlling the B-cations ordering degree. It is well-known that the physical and chemical features are strongly associated with the structure characters; therefore, it is significative to investigate the structures of Sb–O and Ln–O octahedron array, especially the ordered distribution and the electronic structure of B-cations. For Ba2SbLnO6, although the valence state of antimony in some perovskite-type oxides are proposed, direct evidences have not been given. Measurement of the Mössbauer effect is an efficient tool for investigating the microstructure of the materials because the Mössbauer isomer shift (I. S.) probes the electronic configuration of antimony because the electronic charge density at the Sb nucleus depends on the number of 5s and 5p electrons. A change in oxidation state implies a change in the nominal antimony oxidation states are Sb3+ and Sb5+ with electron configurations [Kr]4d105s25p0 and [Kr]4d105s05p0, respectively. Thus, in the ionic limit the Sb5+ has an electron density which is reduced by the contribution from the two 5s electrons relative to Sb3+. Therefore, a large change of the isomer shift is expected when the valency changes from Sb3+ to Sb5+. Numerous values of Mössbauer isomer shifts of antimonides have been collected and the latest summary was reported by Lippens [7].

In this paper, we report a facile hydrothermal synthesis of Ba2SbLnO6 (Ln = Pr, Nd, Sm, Eu) and determine their structures by the Rietveld method. Moreover, we aim at confirming the valency of B-cations and electronic structure of antimony in the octahedron array through Mössbauer measurement and other technologies.

Section snippets

Experimental

In this work, the Pr, Nd, Sm, and Eu analoge of Ba2LnSbO6 were synthesized from the starting materials Ba(OH)2 8H2O, Sb2O3, PrO2, Nd2O3, Sm2O3, and Eu2O3. The typical synthesis is described as follows: First, a 0.2 mol L 1 Ba(OH)2 solution was prepared in deionized water; Second, the other reactants were added to the solution in a sequence of lanthanide oxides, Sb2O3, H2O2 and KOH to obtain a slurry, an intimate composition of Ba:Ln:Sb:H2O2:KOH in 2:1.05:1:0.5:10; Third, after stirring strongly

Structures of Ba2SbLnO6 (Ln = Pr, Nd, Sm, Eu)

X-ray diffraction patterns of Ba2SbLnO6 at room temperature are shown in Fig. 1. It is found that these XRD patterns show a typical perovskite feature, and some impurities are identified as KSbO3 (JCPDS CARD No. 30-129), Ba2Sb3+Sb5+O6 [10] BaSb2O6 (JCPDS CARD No. 49-1065) and Ln(OH)3 (JCPDS CARD No. 6-601, 6-117, 18-504). The elemental analyses indicate that these perovskite-type oxides had a metal composition of Ba:Sb:Ln ≈ 2:1:1, which agrees with the double perovskite formula A2BB′O6.

As shown

Conclusion

In summary, Ba2SbLnO6 (Ln = Pr, Nd, Sm, Eu) perovskite-type oxides have been successfully synthesized by a mild hydrothermal process. Mössbauer spectra give direct evidence that all antimony ions centered in the octahedrons are pentavalent. The electronic configuration 5snpm of Sb5+ in the Ba2SbLnO6 (Ln = Pr, Nd, Sm, Eu) perovskites were estimated. The results reflect some hybridized-orbital behavior in Sb–O bond.

Acknowledgement

This work was supported by National Natural of Science Foundation of China (Grant No. 20571044 and 20971070).

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