Vacancy induced mixed valence state in nickel tellurate Ni3TeO6

Recently the compound Ni3TeO6 has attracted a lot of attention because of its multiferroic and magnetoelectric behaviors. The compound shows large electric polarization at the antiferromagnetic ordering temperature of 52 K In addition, the polarization shows exotic features as a function of the applied magnetic field. The charge state of Ni and Te in this system is utmost important to understand the electronic behavior of the material. However, a detailed study regarding the charge state of Ni and Te is still lacking. In this work, we studied the compound using powder x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), transmission electrons microscopy (TEM-EDS) to understand the valency of cations. In order to establish our result, we prepared the sample of Ni3TeO6 using two preparation routes with TeO2 (Te in 4+ state) and TeO3 (Te in 6+ state) as staring materials. Both the samples show similar crystal structure with almost similar crystallographic parameters. The dielectric measurements on both the sample indicate anomaly at around 52 K, which corroborates the previous results. Most importantly, our XRD, XPS and bond- valence-sum calculation indicate that Te lies primarily in the 4+ state as opposed to previous predictions. The work also indicates that Ni is primarily in the 2+ state with the presence of minority 3+ ions. Our combined study by XRD and XPS reveal that regardless of the synthesis method, the system always shows some vacancy at the oxygen site. Such vacancy is primarily responsible for the certain degree of mixed valency in the cations, which gives rise to Te4+(majority) and Te6+(minority) as well as Ni2+ (majority) and Ni3+(minority). Further investigation by TEM-EDS confirms that the mixed valency is not due to off-stoichiometry of the cations and it further strengthens the scenario of oxygen vacancy. Copyright (C) 2022 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Symposium on Materials of the Millennium: Emerging Trends and Future Prospects.

Automated summary

The compound Ni3TeO6 has recently been studied due to its multiferroic and magnetoelectric behaviors. By using various techniques, researchers have investigated the charge state and crystal structure of the compound and found that Te primarily exists in the 4+ state, while Ni primarily exists in the 2+ state. The presence of oxygen vacancies plays a significant role in the formation of the mixed valency observed.