Evidence of exchange striction and charge disproportionation in the magnetoelectric material Ni3TeO6

The chiral magnetoelectric compound Ni3TeO6 is investigated through temperature-dependent synchrotron-based powder x-ray diffraction and x-ray absorption spectroscopy between 15 and 300 K. Our work provides direct evidence for the exchange-striction in the material around the concomitant onset point of collinear anti-ferromagnetic and the magnetoelectric phases. The x-ray absorption near edge spectra and x-ray photoelectron spectra show that the sample consists of both Ni2+ and Ni3+ ions in the lattice. The ionic state of Ni is found to be quite robust, and it is largely independent of the preparation route. Additionally, a minority Te4+ state is found to coexist with the majority Te6+ state, which may arise from the charge disproportionation between Ni and Te ions (Ni2+ + Te6+ double left right arrow Ni3+ + Te4+). The observed mixed valency of Ni is also confirmed by the total paramagnetic moment (mu(eff)) per Ni atom in the system. This mixed valency in the metal ions and the exchange-striction may be attributed to the observed magnetoelectric effect in the system.