Self-diffusion in polycrystalline Li1+xTi2-xAlx(PO4)3 (0.2 ≤ x ≤ 0.4) samples followed by 7Li PFG (Pulse Field Gradient) NMR spectroscopy

Isabel Sobrados, Virginia Diez-Gómez, Cristina Ruiz-Santaquiteria, Wilmer Bucheli,

Ricardo Jiménez, Jesús Sanz.

Short and long range lithium motions are discussed in powder Li1+xTi2-xAlx(PO4)3 (LTAP) NASICON compounds prepared by ceramic (x= 0.2 and 0.4) and sol-gel (x= 0.3 and 0.4) routes.

Self-diffusion coefficients were determined with the PFG (pulse field gradient) technique. In these experiments, the stimulated echo π/2-t1-π/2-t2-π/2 sequence was used, in which two field gradient pulses of δ width and g intensity were applied between the two first π/2 pulses and after the third π/2 radiofrequency pulse. From the echo-signal attenuation, induced by the increment of exponent parameters, self-diffusion coefficients (DPFG) were deduced, using the Stejskal and Tanner expression [1]

A(2t1+t2)/A0(2t1+t2)=exp[-ɣ2.g2.δ2.(Δ-δ/3)·DPFG]=exp(-b·DPFG)

where A and A0 stand for echo signal intensity at (2t1+t2) with and without field gradient pulses, ɣ is the nuclear gyro-magnetic ratio, and Δ is the diffusion time used in experiments.

In PFG experiments, time scales differ considerably from those involved in 1/T2 and 1/T1 measurements, however, DPFG values deduced for short Δ times are similar to those deduced by NMR relaxometry. In all analyzed samples, diffusion coefficients measured at short Δ times, are between 5x10-12 and 1x10-11 m2 s-1. At increasing Δ, diffusion coefficients decrease due to restriction. In ceramic LTAP02-C sample, Li diffusion is less restricted than in LTAP04-C sample, where DPFG values increase and the particle size decreases. The analysis of DPFG coefficients in sol-gel LTAP03-SG and LTAP04-SG samples, show strong restriction effects that considerably reduce DPFG values when Δ increases, suggesting that Li diffusion is strongly restricted when the LTAP particles are smaller than 1µm. The restricted diffusion inside NASICON particles is compared to "free" diffusion processes.

From the temperature dependence of conductivity and diffusion coefficients, the activation energy and charge carrier concentrations were determined. In this work, PFG-NMR results show that diffusion coefficients rise with the amount of lithium and temperature.

To reduce restriction effects, denser samples should be prepared. In future works, the PFG technique will permit a better optimization of transport properties in fast ion conductors prepared with different conditions.

1. J.E. Tanner, E.O. Stejskal. J Chemical Physics. 49, 1768- 1777 (1968).