Structural Polymorphism in Na₄Zn(PO₄)₂ Driven by Rotational Order-Disorder Transitions and the Impact of Heterovalent...

Solid electrolytes have regained tremendous interest recently in light of the exposed vulnerability of current rechargeable battery technologies. While designing solid electrolytes, most efforts concentrated on creating structural disorder (vacancies, interstitials, etc.) in a cationic Li/Na sublattice to increase ionic conductivity. In phosphates, the ionic conductivity can also be increased by rotational disorder in the anionic sublattice, via a paddle-wheel mechanism. Herein, we report on Na₄Zn(PO₄)₂ which is designed from Na₃PO₄, replacing Na⁺ with Zn²⁺ and introducing a vacancy for charge balance. We show that Na₄Zn(PO₄)₂ undergoes a series of structural transitions under temperature, which are associated with an increase in ionic conductivity by several orders of magnitude. Our detailed crystallographic study, combining electron, neutron, and X-ray powder diffraction, reveals that the room-temperature form, α-Na₄Zn(PO₄)₂, contains orientationally ordered PO₄ groups, which undergo partial and full rotational disorder in the high-temperature β- and γ-polymorphs, respectively. We furthermore showed that the highly conducting γ-polymorph could be stabilized at room temperature by ball-milling, whereas the β-polymorph can be stabilized by partial substitution of Zn²⁺ with Ga³⁺ and Al³⁺. These findings emphasize the role of rotational disorder as an extra parameter to design new solid electrolytes.