Insights into Crystal Structure and Diffusion of Biphasic Na₂Zn₂TeO₆

The layered oxide Na₂Zn₂TeO₆ is a fast Na⁺ ion conductor and a suitable candidate for application as a solid-state electrolyte. We present a detailed study on how synthesis temperature and Na-content affect the crystal structure and thus the Na⁺ ion conductivity of Na₂Zn₂TeO₆. Furthermore, we report for the first time an O'3-type phase for Na₂Zn₂TeO₆. At a synthesis temperature of 900 °C, we obtain a pure P2-type phase, providing peak performance in Na⁺ ion conductivity. Synthesis temperatures lower than 900 °C produce a series of mixed P2 and O'3-type phases. The O'3 structure can only be obtained as a pure phase by substituting Li on the Zn-sites to increase the Na-content. Thorough analysis of synchrotron data combined with computational modeling indicates that Li enters the Zn sites and, consequently, the amount of Na in the structure increases to balance the charge according to the formula Na_2+xZn_2-xLi_xTeO₆ (x = 0.2-0.5). Impedance spectroscopy and computational modeling confirm that reducing the amount of the O'3-type phase enhances the Na⁺ ion mobility.