In recent years all-solid-state batteries have gained increased there has been an increasing interest in lithium ion conducting solid electrolytes for use in lithium batteries, due to safety concerns with the flammable liquid electrolyte system. A new phosphate LiTa₂PO₈ (LTPO) has been discovered with Li-ion conductivity in the order of 10^-4 S cm^-1 at room temperature.^[1] The crystal structure of LTPO has been identified by Kim et al. in space group C2/c with cell dimensions a = 9.716 Å, b = 11.536 Å, c = 10.697 Å and b = 90.04°. The structure contains a framework of corner sharing TaO₆ and PO₄ polyhedra, generating various voids with Li⁺ ions located over three crystallographically distinct sites.^[1] Based on results from a recent neutron diffraction experiment, we have identified two additional sites for Li+ ions in the structure. Using a two-step synthesis method for LTPO, we have managed to reduce the grain boundary resistance, yielding a total conductivity of 1 × 10^-3 S cm^-1 at room temperature, with an activation energy of 0.35 eV.

Interestingly, although the crystallographic analysis indicates that there is only one single crystallographic position for phosphorous in the average structure, the ³¹P MAS-NMR data indicates multiple resonances, suggesting a number of different local structure motifs.^[2] 2D-NMR and neutron total scattering experiments were conducted to study the local structure. ³¹P-⁶Li dipolar heteronuclear multiple-quantum coherence (D-HMQC) experiments reveal a broad ⁶Li peak made up of at least two Li species that couple with two ³¹P species, one is coupled to both P species while other Li species is coupled to only one phosphorous. This appears to correlate with our neutron diffraction results which reveal Li⁺ ions are disordered over 4 crystallographic sites; one of these sites has relatively short contacts to two neighbouring phosphate tetrahedra, while the other 3 sites neighbour only a single phosphate tetrahedron.

Our analysis of total neutron scattering data on the LTPO system has demonstrated the ability to accurately determine the distribution and local structure of Li+ ions. Reverse Monte Carlo (RMC) calculations of the pair distribution function reveal the local atom-atom distributions. Specifically, the unique Li-ion distribution observed in the sample may provide insights into the transport mechanism within LTPO.