  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.
