Atomic-resolution STEM-EDS studies of cation ordering in Ti-Nb oxide crystals

Sumio Iijima,Ichiro Ohnishi,Zheng Liu

doi:10.1038/s41598-021-97244-0

Abstract

Ternary metal oxide compounds, such as Ti-Nb and Nb-W oxides, have renewed research interest in energy storage materials because these oxides contain multivalent metal ions that may be able to control the ion transport in solid lithium batteries. One of these oxides is Ti2Nb10O29, which is composed of metal–oxygen octahedra connected through corner-sharing and edge-sharing to form “block structures”. In the early 1970s Von Dreele and Cheetham proposed a metal-atoms ordering in this oxide crystal using Rietveld refined neutron powder diffraction method. Most recent studies on these oxides, however, have not considered cation ordering in evaluating the battery electrode materials. In this paper, by utilizing the latest scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy imaging technology, the cation chemical ordering in those oxide crystals was directly revealed at atomic resolution.

Highlights

It is important to know the real crystal structures of any material in order to understand their physical and chemical properties as well as their practical uses
The first atomic resolution scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy (EDS) images were reported by Watanabe et al.[25] and the usefulness was demonstrated in our previous publication, in which we have reported the cation ordering in Nb-W ternary oxide compounds with the tetragonal tungsten bronze (TTB)-type structure[26]
In our new analyses of the cation ordering in Ti2Nb10O29 crystal using STEM-EDS, besides a similar result to that obtained by the earlier Rietveld powder neutron diffraction[20], we found some discrepancies in the detailed metal ion occupancies

Summary

Introduction

It is important to know the real crystal structures of any material in order to understand their physical and chemical properties as well as their practical uses. The structural entity of the crystal is only MO6 octahedron This implies that the charge states of the metal ions of the ternary oxide examined in this work will vary, at the CS planes where reduction will take place; a cation ordering can be expected. Von Dreele and Cheetham[20] examined the crystal utilizing an earlier Rietveld neutron powder diffraction which has an advantage over the X-ray diffraction method for oxygen ion identification because of its much larger scattering factor than that of the X-ray They concluded the Ti ions were located preferentially near the CS planes, while the Nb ions were located near the center of the R eO3-type blocks. The first atomic resolution STEM-EDS images were reported by Watanabe et al.[25] and the usefulness was demonstrated in our previous publication, in which we have reported the cation ordering in Nb-W ternary oxide compounds with the tetragonal tungsten bronze (TTB)-type structure[26]. The quantification of the atomic resolution EDS-STEM imaging has not been established well because of fundamental problems with EDS emissions from a solid crystal such as electron channeling effect, we demonstrate here what we can do for atomic resolution EDS-STEM imaging in such a limited circumstance

Methods

Results

Conclusion