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Evidence of exchange-striction and charge disproportionation in the magneto-electric material Ni3TeO6

The chiral magneto-electric compound Ni3TeO6 is investigated through temperature-dependent synchrotron-based powder x-ray diffraction and x-ray absorption spectroscopy between 15 to 300 K. Our work provides direct evidence for the exchange-striction in the material around the concomitant onset point of collinear antiferromagnetic and magneto-electric 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, the 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+ --> Ni3+ + Te4+). The observed mixed valency of Ni is also confirmed by the total paramagnetic moment per Ni atom in the system. This mixed valency in the metal ions and the exchange-striction may be attributed to the observed magneto-electric effect in the system.