Ionic conductivity of Li3TaO4 depending on polymorphism and non-stoichiometric defects

Abstract

Ionic conductivity of complex rock-salt Li3TaO4 was studied using low-temperature (LT; disordered) and high-temperature (HT; ordered) polymorphs modified with Li/Ta non-stoichiometry. For each of LT and HT derivatives, samples were prepared with four different nominal compositions of Li3.05Ta0.99O4, Li3TaO4, Li2.99Ta1.002O4, and Li2.98Ta1.004O4. The synchrotron X-ray diffraction analysis showed that the ordered HT phases have larger lattice volume than the disordered LT phases by ≈0.5%. Among both LT and HT sample groups, the lattice volume was smallest in the stoichiometric phases Li3TaO4. The X-ray diffraction line broadening analysis implied that the disordered LT phases contain local distortions that resemble the ordered HT phases. The solid-state magic-angle-spinning 7Li nuclear magnetic resonance spectroscopy revealed that LT phases have the larger LiO6 octahedra and the shorter Li–Li distances than HT phases. According to the ac impedance measurement and equivalent circuit analysis, the disordered LT phases had higher ionic conductivities (σi) than the ordered HT phases, by more than two orders of magnitudes. Particularly, the σi of Li2.98Ta1.004O4 (at ≈390 °C) was evaluated to be 10−2.4 S/cm and 10−4.8 S/cm for LT and HT phases, respectively. Meanwhile, the Li/Ta non-stoichiometry contributed to increase the σi of Li3TaO4, by more than an order.