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

A protonated/deuterated compound, HTa 2 PO 8 /DTa 2 PO 8 , was synthesized via an ion-exchange reaction of LiTa 2 PO 8 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 DTa 2 PO 8 belongs to the monoclinic space group , C 2/ c , identical to that of parent LiTa 2 PO 8 . The deuterium atoms were found to partially occupy two distinct sites around oxygen atoms, located at the shared corners of two connected TaO 6 octahedra . The two distinctive H + sites were confirmed by the solid-state 1 H nuclear magnetic resonance (NMR) spectrum of HTa 2 PO 8 . HTa 2 PO 8 is thermally stable up to 400 °C, and as the temperature increases, it transforms into an intermediate compound, Ta 2 PO 7.5 , while maintaining its original framework structure. Above 1050 °C, it subsequently decomposes to Ta 2 O 5 and TaPO 5 . Impedance spectroscopy measurements, performed in both dry and wet air, showed that HTa 2 PO 8 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. H(D)Ta 2 PO 8 was synthesized through an ion-exchange reaction of LiTa 2 PO 8 in an acidic solution. • H(D)Ta 2 PO 8 was synthesized through an ion-exchange reaction of LiTa 2 PO 8 . • The framework [Ta 2 PO 8 ] - in H(D)Ta 2 PO 8 retains the structural features of LiTa 2 PO 8 . • The principal conduction mechanism in HTa 2 PO 8 is governed by proton conduction. • HTa 2 PO 8 decomposes in two steps, with the formation of an intermediate Ta 2 PO 7.5 phase.