Polymorphism and sodium-ion conductivity of NaTa2PO8 synthesized via the Li+/Na+ ion-exchange reaction of LiTa2PO8

• Published in: Ceramics international 2022-07, Vol.48 (14), p.20712-20720
• Main Authors: Kim, Jaegyeom, Ha, Junho, Khefif, Fouzia, Kim, Jong-Young, Pee, Jae-Hwan, Kim, Seung-Joo
• Format: Article
• Language: English
• Subjects: Ion-exchange; Ionic conductivity; Polymorphism; X-ray diffraction
• ISSN: 0272-8842; 1873-3956
• DOI: 10.1016/j.ceramint.2022.04.052

Ion exchange reaction is a promising method to explore metastable compounds that could not be synthesized by conventional high-temperature solid-phase reactions. Herein, a new sodium tantalum phosphate, NaTa2PO8 was synthesized via the Li+/Na+ ion-exchange reaction of the parent compound, LiTa2PO8 in molten NaNO3 medium. NaTa2PO8 underwent an irreversible phase transition from the low- (LT-NaTa2PO8) to the high-temperature polymorph (HT-NaTa2PO8) at approximately 1000 °C. The crystal structures were solved using an ab initio structural determination method based on synchrotron X-ray powder diffraction data. The LT-NaTa2PO8 presented an orthorhombic structure, closely related to that of the parent LiTa2PO8 structure. In contrast, the HT-NaTa2PO8 was found to adopt a monoclinic structure, belonging to a family of monophosphate tungsten bronzes with pentagonal tunnels. The ionic conductivities of LT-NaTa2PO8 (σ = 5 × 10−5 S/cm at 309 °C) and HT-NaTa2PO8 (σ = 2 × 10−7 S/cm at 300 °C) exhibited Arrhenius behavior with activation energies of 0.49 and 0.79 eV, respectively. Bond valence energy landscape (BVEL) calculations indicated that a three-dimensional (3D) conduction pathway is formed in LT-NaTa2PO8 structure, while the conduction pathway in HT-NaTa2PO8 shows a two-dimensional (2D) character.