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

Abstract

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, Ta2PO7.5, 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.