Crystal structure, thermal behavior, and proton conductivity of HTa2PO8 synthesized via ion-exchange from LiTa2PO8

A protonated/deuterated compound, HTa₂PO₈/DTa₂PO₈, was synthesized via an ion-exchange reaction of LiTa₂PO₈ in aqueous HCl/DCl solution. The structure was analyzed to clarify the location of the proton (or deuterium) in the lattice, using synchrotron X-ray powder diffraction (SXPD) and neutron powder diffraction (NPD). The crystal structure of DTa₂PO₈ belongs to the monoclinic space group, C2/c, identical to that of parent LiTa₂PO₈. The deuterium atoms were found to partially occupy two distinct sites around oxygen atoms, located at the shared corners of two connected TaO₆ octahedra. The two distinctive H⁺ sites were confirmed by the solid-state ¹H nuclear magnetic resonance (NMR) spectrum of HTa₂PO₈. HTa₂PO₈ is thermally stable up to 400 °C, and as the temperature increases, it transforms into an intermediate compound, Ta₂PO_7.5, while maintaining its original framework structure. Above 1050 °C, it subsequently decomposes to Ta₂O₅ and TaPO₅. Impedance spectroscopy measurements, performed in both dry and wet air, showed that HTa₂PO₈ exhibited an ion conductivity ranging from 10⁻⁵ to 10⁻⁸ S/cm in the temperature range of 200 °C∼400 °C, in which protons are the primary charge carriers.