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

The chiral magnetoelectric compound ${\mathrm{Ni}}_{3}{\mathrm{TeO}}_{6}$ 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 antiferromagnetic and the magnetoelectric phases. The x-ray absorption near edge spectra and x-ray photoelectron spectra show that the sample consists of both ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Ni}}^{3+}$ 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 ${\mathrm{Te}}^{4+}$ state is found to coexist with the majority ${\mathrm{Te}}^{6+}$ state, which may arise from the charge disproportionation between Ni and Te ions (${\mathrm{Ni}}^{2+} + {\mathrm{Te}}^{6+}\ensuremath{\Leftrightarrow}\phantom{\rule{4pt}{0ex}}{\mathrm{Ni}}^{3+} + {\mathrm{Te}}^{4+}$). The observed mixed valency of Ni is also confirmed by the total paramagnetic moment (${\ensuremath{\mu}}_{\text{eff}}$) 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.