RESUMO
Short and long range lithium motions in powder Li1+x Ti2-x Al x (PO4)3 (LTAP) NASICON compounds prepared by ceramic (x = 0.2 and 0.4) and sol-gel (x = 0.3 and 0.4) routes are discussed. ND diffraction and MAS-NMR spectroscopy were previously used to investigate structural features of these compounds. In particular, Fourier map differences showed that the amount of Li atoms allocated at M3 increases at the expense of M1 sites when the Li content increases. In this work, PFG-NMR results show that diffusion coefficients rise with the amount of lithium and temperature. The restricted diffusion inside NASICON particles is compared with "free" diffusion processes. At 300 K, diffusion coefficients D PFG â¼ 5 × 10-12 m2 s-1 have been deduced in ceramic x = 0.2 and 0.4 samples, decreasing with diffusion time Δ used in PFG experiments. In sol-gel samples, diffusion coefficients are near those of ceramic samples, but decrease faster with diffusion Δ times, as a consequence of the Li confinement inside sub-micrometric crystallites. The NMR spin-echo signal displays minima at specific q(γgδ) values that are related to the crystallite size. From R dif â¼ q m -1 distances, calculated from the position of minima, and from diffusion coefficients deduced for high Δ values, the mean crystallite size was estimated. Finally, from the temperature dependence of conductivity and diffusion coefficients, the activation energy and charge carriers concentrations were determined.
RESUMO
The Li1/2-2x Sr x La1/2TiO3 series (0 ≤ x ≤ 0.25) is investigated with X-ray diffraction, nuclear magnetic resonance, and impedance spectroscopy techniques. The substitution of two Li+ by one Sr2+ in Li1/2La1/2TiO3 perovskite generates cation vacancies that, when ordered in alternating planes along the c-axis, confer a two-dimensional character to Li mobility. In previous works, it was shown that Li+ ions partially occupy the center of the six faces of the cubic perovskite, resulting in the associated A-sites to participate like a vacancy in the definition of the percolation vacancy threshold. The results obtained in the Li1/2-2x Sr x La1/2TiO3 series are compared with those obtained in the Li3x La2/3-x TiO3 series, and other Sr-doped solid solutions (Li1/2-x Sr2x La1/2-x TiO3 and Li x Sr x La2/3-x TiO3), to highlight the importance of the effective vacancies with respect to the nominal ones in conductivity. The analysis of four series, belonging to the ternary SrTiO3-La2/3TiO3-Li2TiO3 phase diagram, permits a better understanding of the ionic conduction mechanism in perovskites. The results show that the vacancy percolation model is more adequate to explain Li conductivity than the conventional hopping probability model. In the analyzed series, Li conductivity is maximum when a small amount of Sr is incorporated into the pseudo-cubic La1/2Li1/2TiO3 end member, while it decreases as the amount of strontium increases.
RESUMO
Universal dielectric response (UDR) and nearly constant loss (NCL) dispersive regimes have been investigated in fast ion conductors with perovskite and NASICON structure by using NMR and impedance spectroscopy (IS). In this study, the electrical behavior of La(0.5)Li(0.5)TiO3 (LLTO-05) perovskite and Li(1.2)Ti(1.8)Al(0.2)(PO4)3 (LTAP0-02) NASICON compounds was investigated. In both systems a three-dimensional network of conduction paths is present. In the Li-rich LLTO-05 sample, lithium and La are randomly distributed on A-sites of perovskites, but in LTAP0-02 Li and cation vacancies are preferentially disposed at M1 and M2 sites. In perovskite compounds, local motions produced inside unit cells are responsible for the large "near constant loss" regime detected at low temperatures, however, in the case of NASICON compounds, local motions not participating in long-range charge transport were not detected. In both analyzed systems long-range correlated motions are responsible for dc-conductivity values of ceramic grains near 10(-3) S cm(-1) at room temperature, indicating that low-temperature local motions, producing large NCL contribution, are not required to achieve the highest ionic conductivities.
RESUMO
The crystal structure and dielectric properties of slowly cooled A-site-deficient perovskites Li(x)Sr(x)La(2/3-x)â¡(1/3-x)TiO(3) (0.04 ≤ x ≤ 0.33) have been investigated by powder X-ray diffraction (XRD), impedance spectroscopy, and (7)Li NMR techniques. In this series, nominal vacancies decrease with Li content, but the total amount of A-site vacancies, n(t) = Li + â¡, participating in conduction processes remains basically constant. Rietveld analysis of the XRD patterns showed a change of symmetry from orthorhombic to tetragonal when the lithium and strontium contents increased above x = 0.08 and from tetragonal to cubic above x = 0.16. Structural modifications are mainly due to the cation vacancy ordering along the c axis, which disappear gradually when the lithium content increases. In agreement with the structural information, two lithium signals with different quadrupole constants are detected in (7)Li NMR spectra of orthorhombic/tetragonal phases, which have been associated with lithium in two crystallographic z/c = 0 and 1/2 planes of perovskites. In cubic samples, only a single narrow component, indicative of mobile species, was detected. Lithium motion was thermally activated, with activation energies going from 0.35 to 0.38 eV. Evolution of the bulk dc-conductivity preexponential factors along the series showed a maximum that has been first related to the dependence of lithium hopping on the lithium and vacancy concentrations. Finally, changes in the vacancy ordering, produced along the series, affect the dimensionality of the conductivity, indicating that not only the amount of vacancies but also its distribution are relevant.