Coexistence of Two Types of Lithium Motion in Monoclinic Li2HfO3: 6,7Li NMR and Ab Initio Calculation Results

Abstract

This paper presents the results of lithium dynamics studies of monoclinic β-Li2HfO3. The 6Li MAS and 7Li static NMR experiments and ab initio calculations have been performed to clarify the features of lithium motion in this layered Li-conducting oxide. It was revealed that two types of lithium motion with significantly different characteristic jump frequencies coexist at 425–900 K. One of the processes, responsible for the long-range lithium diffusion, is characterized by an activation energy of Ea = 1.00 ± 0.05 eV and an ion jump frequency of τd–1 ∼ 104 s–1, at T ∼ 750 K. The long-range lithium diffusion represents the successive ion jumps between the nonequivalent octahedral sites in Li/Hf and Li layers, occurring through the distinct type of tetrahedral interstitials (meanwhile, two other types of tetrahedra are less involved in motion). Another type of ion dynamics is the much more rapid localized motion of Li ions with Ea ∼ 0.6 eV and τd–1 ∼ 104 s–1 already at T ∼ 400 K. The mechanisms of lithium mo...