The activation energy involved in the motion of Li+ ions along the conduction channels of the NASICON framework has been determined from electrical conductivity measurements in samples of composition LiM2(PO4)3 and LiMM‘(PO4)3, where M and M‘ are Ge, Ti, Sn, and Hf, all compounds belonging to space group R3̄c. Two lithium sites, M1 and M2, inside the channels, can be distinguished. The sites are connected through triangular bottlenecks of oxygen atoms, and the size of the bottleneck has been estimated from refined and simulated structures for each composition. The plot of activation energy vs bottleneck size shows two regimes:  for sizes up to 2.04 Å the activation energy decreases steeply, but above 2.04 Å the activation energy is almost constant. These regimes are discussed on the basis of the effective Li+ ionic radius for the compounds analyzed.

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A reversible first-order phase transition has been found around 0 °C in the compound LiHf2(PO4)3, which has been followed by DSC, XRD, NMR (31P and 7Li), and impedance techniques. In the low-temperature phase, a triclinic distortion of the NASICON framework has been detected for the first time in the LiM2(PO4)3 family. In this phase location of lithium ions at low-symmetry sites (probably M2 sites) has been deduced from 7Li NMR data. Above 0 °C, the usual NASICON symmetry (rhombohedral R3̄c) is recovered and an abrupt increase of the ionic mobility is observed:  activation energy obtained from bulk dc conductivity decreases from 1.39 to 0.33 eV. From NMR data, a strong delocalization of lithium ions over M1 and M2 sites is detected when going from the triclinic to the rhombohedral phase.

Lithium mobility in has been followed by NMR and impedance spectroscopies. From this analysis, three stages have been distinguished. (1) Below 230 K, lithium occupies preferentially the octahedral sites of the NASICON structure. (2) Between 230 and 330 K, a low correlated and thermally activated lithium motion, with , is operating, as deduced from both techniques. (3) Above 330 K, the activation energy decreases and a more extended lithium ionic motion is established. As the lithium mobility increases, progressive occupation of the eight-coordinated sites is deduced from NMR data.

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Este trabalho reporta medidas de condutividade iônica realizadas para o LiHf 2(PO 4)3 calcinado a 1100 °C. As respostas devidas aos grãos -in te rior e junção -puderam ser identificadas tanto nas curvas de impedância, como na parte real das curvas de condutividade vs. freqüência. A energia de ativação, associada ao movi men to dos íons Li + no in te rior dos grãos, é 0,33 eV, enquanto que aquela associada à condutividade to tal cc, está na faixa de 0,36-0,47 eV. os resultados desta última dependem da contribuição relativa devida ao in te rior e à junção de grão. A possível aplicação do LiHf 2(PO 4)3 como eletrólito foi testada para a pilha Li/LiHf 2(PO 4)3/LiMn 2O4. Observou-se que o potencial de equilíbrio aumenta de 0,076 V a 2,217 V, quando a temperatura varia de 28 a 148 °C.The ionic con duc tiv ity of LiHf 2(PO 4)3 cal cined at 1100 °C has been mea sured. Grain in te rior and grain bound ary re sponses can be dis tin guished in the im ped ance plots as well as in the real part of con duc tiv ity vs fre quency plots. The ac ti va tion en ergy as so ci ated with the mo tion of Li + ions in side the grains is 0.33 eV while the ac ti va tion en ergy cor re sponding to the to tal dc con duc tiv ity changes from 0.36 to 0.47 eV, de pend ing on the rel a tive con tri bu tion of grain in te rior and grain bound ary. The pos si ble ap pli ca tion of LiHf 2(PO 4)3 as an elec tro lyte has been tested in the Li/LiHf 2 (PO 4 ) 3 /LiMn 2 O 4 cell. The equi lib rium poten tial in creases from 0.076 V to 2.217 V when the tem per a ture is raised from 28 to 148 ºC.Key words: NASICON, ionic con duc tiv ity, lith ium haf nium phos phate, solid state lithium bat ter ies Ar ti cle

A composite - 25% vol Teflon was prepared to prevent breakages in powder pellets due to the phase transition of and, thus, to study the effect of the phase transition on conduction. The composite microstructure, as followed by scanning electron microscopy, shows aggregated particles surrounded by Teflon regions which operate as a skeleton. The imaginary part of the electric modulus shows two peaks at high and low frequency which are ascribed to grain-interior and grain-boundary response, respectively. In the two phases the ionic conductivity inside the grains is about three orders of magnitude larger than that found for the grain boundary. The phase transition affects the activation energy in different ways. It decreases for the grain-interior response from 0.68 eV for the low-temperature phase to 0.34 eV for the high-temperature phase, while it increases from 0.45 to 0.56 eV for the movement through the grain boundaries. An anomalous increase in activation energy for the grain-interior response in the high-temperature phase before it is transformed into the low-temperature phase has been found.

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