NMR Study of Li⁺ Ion Dynamics in the Perovskite Li_3xLa_1/3-xNbO₃

Abstract

(7)Li and (6)Li nuclear magnetic resonance (NMR) experiments are carried out on the perovskite Li(3x)La(1/3-x)NbO(3). The results are compared to those obtained on the titanate Li(3x)La(2/3-x)TiO3 (LLTO) in order to investigate the effect, on the lithium ion dynamics, of the total substitution of Nb(5+) for Ti(4+) in the B-site of the ABO(3) perovskites. The XRD patterns analysis reveals that this substitution leads to a change in the distribution of the La(3+) ions in the structure. La(3+) ions distribution is very important, in regard to ionic conductivity, because these immobile ions can be considered as obstacles for the long-range Li+ motion. If compared to the titanates, the compounds of the niobate solid solution have a bigger unit cell volume, a smaller number of La(3+) ions, and a higher number of vacancies. These should favor the motion of the mobile ions into the structure. This is not experimentally observed. Therefore, the interactions between the mobile species and their environment greatly influence their mobility. (7)Li and (6)Li NMR relaxation time experiments reveal that the Li relaxation mechanism is not dominated by quadrupolar interaction. (7)Li NMR spectra reveal the presence of different Li+ ion sites. Some Li+ ions reside in an isotropic environment with no distortion, some others reside in weakly distorted environments. T(1), T(1)(rho), and T(2) experiments allow us to evidence two motions of Li+. As in LLTO, T(1) probes a fast motion of the Li+ ions inside the A-cage of the perovskite structure and T(1)(rho) a slow motion of these ions from A-cage to A-cage. At variance with what has been observed in LLTO, these different Li+ ions can be differentiated through the spin-lattice relaxation times, T(1) and T(1)(rho), as well as through the transverse relaxation time, T(2).

Cation ordering in A-site-deficient Li-ion conducting perovskites La(1−x)/3LixNbO3

“…No La atoms are discernible in the A2 layers, consistent with earlier XRD results 27. 3a and bshow Z-contrast HAADF micrographs taken along the [110] p zone axes of LNO and LLNO4, respectively.The micrographs were taken from thin specimen regions.…”

“…27 However, this is not what is experimentally observed. Compared to La (2Àx)/3 Li x TiO 3 (LLTO), LLNO perovskites have a larger unit cell volume, a smaller concentration of La 3+ ions and a higher concentration of intrinsic vacancies; the latter are expected to favour higher Li-ion mobility and hence higher ionic conductivity.…”

“…Compared to La (2Àx)/3 Li x TiO 3 (LLTO), LLNO perovskites have a larger unit cell volume, a smaller concentration of La 3+ ions and a higher concentration of intrinsic vacancies; the latter are expected to favour higher Li-ion mobility and hence higher ionic conductivity. 27 However, this is not what is experimentally observed. Electrochemical measurements show that LLNO compounds have a maximum bulk ionic conductivity at room temperature of about 10 À4 S cm À1 when x ¼ 0.195, 9 which is an order of magnitude smaller than that of LLTO for its optimum composition of x ¼ 0.29.…”

Evidence for changes on the lithium conduction dimensionality of Li0.5−yNayLa0.5Nb2O6 (0 ≤ y ≤ 0.5) perovskites

“…27 The study of the temperature dependence of NMR relaxation T 1 and T 2 times showed the presence of two Li motions that were assigned to fast motions inside A-cavities and slow motions between contiguous A-cavities. 28 In our work, 7 Li NMR spectra show the presence of two types of lithium that display different mobility (Fig. 4b).…”

Defect structure and property consequence when small Li+ ions meet BaTiO3

“…Due to its small size, the doped Li ion can affect the structural stability, homogeneity, local environment, and even dynamic behavior of the host materials for achieving optimal performance and properties. However, the Li-ion dynamics, which determine the desired functional properties in applications, are not completely understood and are the subject of ongoing research [8][9][10]. The probable position of Li ion in a compound is often diverse and thus remains controversial in many cases [11][12][13].…”