Ionic Disorder-Mediated Exfoliation and Optical Birefringence in a Non-van der Waals Oxide

TL;DR

This study demonstrates that Na2Zn2TeO6, a non-vdW oxide, can be exfoliated into thin layers due to ionic disorder, exhibiting strong optical birefringence and potential for iono-photonic applications combining ionic transport and optical properties.

Contribution

We introduce NZTO as a novel exfoliable non-vdW oxide with inherent ionic disorder that enables mechanical cleavage and exhibits significant optical birefringence, expanding the 2D photonics material library.

Findings

NZTO can be exfoliated to few-nanometer thicknesses.

It exhibits a wide-bandgap dielectric with strong optical birefringence.

Lattice dynamics show decoupled ionic mobility and framework rigidity.

Abstract

The landscape of two-dimensional photonics has been dominated by van der Waals (vdW) materials. Expanding this library to include non-vdW layered systems promises enhanced environmental robustness and access to novel functionalities, such as strong ionic conductivity, yet their exfoliation remains challenging. Here, we establish Na2Zn2TeO6 (NZTO), a P2-type superionic conductor, as an exfoliable non-vdW optical material. We demonstrate that the highly disordered, mobile Na+ interlayer inherently facilitates mechanical cleavage down to few-nanometer thicknesses (about 4 nm). Optical interrogation via spectroscopic ellipsometry reveals NZTO as a wide-bandgap dielectric with pronounced optical birefringence (Delta_n about 0.25) across the visible and near-infrared spectrum. The lattice dynamics, probed by temperature-resolved Raman spectroscopy, underscore the rigidity of the [Zn2TeO6]2- framework, which remains largely decoupled from the high ionic mobility. These results identify NZTO as a compelling platform for robust, anisotropic dielectric photonics, simultaneously opening a pathway toward the convergence of ionic transport and optical control - an emerging paradigm we term iono-photonics.