Ultraslow Lithium Diffusion in Li3NbO4 Probed by Li Stimulated Echo NMR Spectroscopy

While huge research activities are being directed towards fast lithium ion conductors as potential elec-trolytes in batteries, it should be noted that also materials with low lithium diffusivity have many applications in modern technology. They are used, e.g., in semiconductor fabrication as non-doping sub-strate or are considered as tritium breeding blankets in fusion reactors offering both a slow lithium release and a fast release of tritium after neutron irridiation. Many of the materials investigated and discussed for the latter applications are lithium metal oxides. A well known example is single or mi-crocrystalline lithium niobate, LiNbO 3 , which is not only interesting due to its ferroelectricity, but also due to its very slow lithium diffusion [1,2]. Here, we report on ultraslow lithium diffusion in lithium orthoniobate, Li 3 NbO 4 , a cubic (space group I-43m, see [3]) lithium rich phase in the lithium niobium oxide system. The correlation time of the hopping lithium ions was accessed via one-particle two-times correlation functions recorded using the 7 Li spin alignment echo nuclear magnetic resonance (SAE NMR) technique [4-6]. Microcrystalline Li 3 NbO 4 was prepared in a solid state reaction of equimolar amounts of LiNbO 3 (99.9%, Cerac/Pure Inc.) and Li 2 CO 3 (99.95%, Alfa). After heat treatment in a tube furnace (20 h at 973 K followed by 48 h at 1123 K) the crystallinity and phase purity of the product was verified by x-ray diffraction (XRD) using a Philips PW 1800 powder diffractometer operating at 40 kV. For the NMR measurements the sample material was dried under vacuum (373 K) and sealed into quartz tubes. 7 Li NMR spectroscopic measurements were carried out by means of a Bruker MSL 400 NMR spectrometer operating at ω 0 /2π = 155 MHz and a commercial probe (Bruker). 7 Li NMR spin–lattice relaxation rates were obtained using a saturation recovery pulse sequence and by subsequent fitting of the data with a stretched exponential function. In order to study ultraslow hopping processes of Li + , mixing time (t m) dependent 7 Li stimulated NMR echoes were recorded. Therefore, the pulse sequence (π/2) φ1 – t p – (π/4) φ2 – t m – (π/4) φ3 – t introduced by Jeener and Broekaert was applied [7]. The first two pulses generate an alignment state and are separated by the preparation time t p. After the third pulse a stimulated spin echo is …