Crystal Structure and Li-Ion Conductivity of LiGa_1−xAl_xGeO₄Phenacite Compounds with 0 ≤x≤ 1

Abstract

Compounds of LiGa 1−x Al x GeO 4 with 0 ≤ x ≤ 1 were examined using powder X-ray diffraction measurements and electrochemical impedance spectroscopy, to clarify their structures and Li-ion conductivity (σ Li ) as a function of x, and to compare with those for a wide variety of AB 2 O 4 -type oxides. The x = 0 and 0.25 samples were found to exhibit a phenacite structure with R 3 space group, whereas the x ≥ 0.5 samples were found to exhibit an ordered phenacite structure with R3 space group due to the 1:1 cation order between Ga 3+ /Al 3+ and Ge 4+ ions. The σ Li value for the x = 0 sample was 2.81(1) × 10 −5 S•cm −1 at 973 K. Owing to the 1:1 cation order, both σ Li and activation energy (E a ) were slightly improved at x = 1. The structural details of LiGa 1−x Al x GeO 4 , possible conduction pathway of Li + ions, and correlation between crystal structure and σ Li (E a ) are presented.

Li-Ion Diffusion Correlations in LiAlGeO4: Quasielastic Neutron Scattering and Ab Initio Simulation

“…The present QENS spectrometer (resolution ∼μeV) can capture the diffusion dynamics of ∼1 × 10 −10 m 2 /s or conductivity of ∼mS cm −1 . The ionic conductivity measurements 28 show a conductivity of 2 × 10 −2 mS cm −1 at 973 K, which is well below the QENS accessible value of ∼mS cm −1 , which means that the Li hopping between different sites is extremely slow. Most interestingly, the large stochastic motion localized within the cavities is well capture by QENS, which also gives important information about the dynamics and validates the simulations.…”

“…However, Al and Ge have random distribution at two nonequivalent 9b sites with 50% occupancy of each of Al and Ge. A later study by Mukai et al 28 on LiGa 1−x Al x GeO 4 (LGAGO) (0 ≤ x ≤ 1) has shown that LAGO has a highly ordered hexagonal phenacite structure (space group, R3) with the same lattice parameters as determined by Fleet et al 38 Recently, Yang et al 48 confirmed that LAGO has the ordered hexagonal-phenacite structure (space group R3). The crystal structure of ordered hexagonal phenacite LiAlGeO 4 is shown in Figure 1.…”

“…Many studies have been performed to investigate the structure correlation with ionic conductivity. A few promising structures have been identified, such as fluorites, antiperovskites, garnets, olivine, and phenacite. − In the antifluorite-based compounds, the cations occupy the tetrahedral site, and the diffusion processes mainly occur through tetrahedral hopping; for example, the antifluorite structured compounds Li 2 X (X = O, S, Se), Cu 2 X (X = S, Se), CaF 2, etc., show a high cationic diffusion coefficient of ∼1 × 10 –5 cm 2 s –1 . ,,, Recently, some Na/Li antiperovskites were found to be very promising for solid electrolyte application. , Extensive work on antiperovskite Na 3 FSe, Na 3 HSe, and Li 3 FSe by Fujii et al identified the role of halide doping on ionic transport . Recently, we have extensively studied the Na-based antiperovskite Na 3 FY (Y = S, Se, and Te) and explained the explicit role of low-energy phonons participating in Na hopping processes .…”

“…The tetrahedral units form a corner-sharing ring-like structure. Each ring consists of four or six polyhedral units of AlO 4 and GeO 4 , as shown in Figure . ,, These ring-like structures are stacked along the c h -axis and form two types of channels parallel to the c h -axis. K Mukai et al reported the Li + conductivity in crystalline LAGO through electrochemical impedance spectroscopy measurements and estimated the conductivity (σ Li 2 × 10 –2 mS cm –1 at 973 K) and activation energy barrier ( E a 0.97 eV).…”

“…All the above-said studies establish the role of structure in enabling faster diffusion. Germanates with olivine and phenacite structure 32 Li conductivity at elevated temperatures such as LiZnVO 4 , 33 LiAlSiO 4 , 34,35 Li 2 WO 4 , 36 Li 2 MoO 4 , 37 LiAlGeO 4 , 28,38 LiGa-GeO 4 , 28,38 Li 4−3x Al x GeO 4 , 39 etc. However, few studies are available on the Li diffusion processes and the effect of host dynamics on such compounds.…”