Chirality of Sodium Ion and its transport in layered Na2Ni2TeO6
Sunil K. Karna1*, R. Sankar1, Y. Zhao2, P.-C. Tseng3, M. Avdeev4, C. W. Wang5, K. Matan6, G.-Y. Guo3, F. C. Chou1
1Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
2NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD, USA
33Department of Physics, National Taiwan University, Taipei, Taiwan
4Bragg Institute, Australian Nuclear Science and Technology (ANSTO), Sydney, Australia
5Neutron Group, National Synchrotron Radiation Research Center, Hsinchu, Taiwan
6Department of Physics and Faculty of Science, Mahidol University, Bangkok, Thailand
* presenting author:Sunil K. Karna,
The crystal structure of P2-type Na2Ni2TeO6 has been identified as edge-shared NiO6 and TeO6 octahedra in layers of hexagonal symmetry with intercalated Na atoms in between. Although the crystal symmetry has been identified with a space group of P6/mcm (No. 193) satisfactorily, we find combined real space nuclear and electronic mapping for the title compound shows additional fine-structure feature and chiral symmetry breaking for the Na sites which cannot be described fully by the original space group. Combined high-resolution neutron and synchrotron x-ray powder diffraction experiments have been applied and the real space nuclear and electronic mappings are obtained via Fourier transform. It is indicated that Na ions occupied at three different crystallographic sites of various occupancies showing fine-structure of additional chiral symmetry breaking, which is proposed to be closely related to the Na ion diffusion and the site-specific potential well. The Na+ ion occupancy distribution and Ni2+ spin ordering have been examined in detail by temperature dependence of lattice size, electron density mapping, neutron magnetic diffraction, and spin susceptibility measurements to demonstrate the impact of Na+ ion diffusion and spin-phonon coupling in the system. Experimental Na ion distribution from Fourier transform of neutron powder diffraction is shown consistent with the Na diffusion pathway simulated with the difference valence bond sum (DVBS) map. Theoretical calculations using density functional theory (DFT) with generalized gradient approximation (GGA) plus on-site Coulomb interaction (U) have been applied to extract the in-plane Ni-O-Ni superexchange FM coupling J1 = 0.14 meV, Ni-O-Te-O-Ni super-superexchange (SSE) AFM coupling J2 = -0.16 meV, and inter-layer coupling J3 = 0.08 meV. These calculated couplings have been shown to provide the observed 3D AF ordering of TN = 27 K consistently.

Keywords: Na-ion conductor, diffusion, magnetic ordering