Structural transition in orthorhombic Li5−xHxLa3Nb2O12 garnets induced by a concerted lithium and proton diffusion mechanism

Abstract

Proton-exchanged Li-conducting garnets present novel properties compared with their parent compounds, among which proton conductivity may have potential applications. Here we study the structural and dynamical properties of Li_5−xH_xLa₃Nb₂O₁₂ garnets with an intermediate proton content (x ≈ 2.8) using neutron and X-ray diffraction, Raman and NMR techniques. At RT the structure is noncentrosymmetric and orthorhombic but transforms reversibly at ∼150 °C into a cubic, still noncentrosymmetric phase. In the low temperature phase Li is split among six different tetrahedral sites and protons form triplet-wise hydroxyl entities around two of such tetrahedra. In the high temperature phase Li occupies two tetrahedral sites, protons being dynamically disordered around half of the tetrahedra. The occupancy of octahedral Li sites is very low. Spectroscopic techniques evidence that lithium exchange between tetrahedral and octahedral sites and proton reorientation among oxygen sites occur already in the low temperature phase but these processes are restricted at a local scale due to the blocking effect of protons for lithium diffusion and vice versa. We suggest that the phase transition is the consequence of a long range Li and proton redistribution through a concerted hopping mechanism. The relationship between the compound stoichiometry, structure and proton mobility is discussed. The occurrence of proton mobility below 200 °C for this specific composition may be of practical interest.