We address the controversial issue of the structural stability of Li 7 La 3 Zr 2 O 12 garnets, focusing on the mechanisms that result in the transformation from tetragonal to cubic symmetry. We show that undoped tetragonal Li 7 La 3 Zr 2 O 12 not exposed to humidity at any moment undergoes a reversible phase transition to cubic symmetry at T c x 645 C that we ascribe to lithium dynamic effects. On the other hand, a close correlation has been found between the appearance of a cubic phase between 100 and 200 C in X-ray diffractograms and the presence of water, either in the atmosphere in which experiments are performed or already in the starting material. The natures of the high and lowtemperature cubic garnets are totally different: the one found above the phase transition does not involve any change in the stoichiometry, whereas the cubic phase formed at low temperature is a hydrated, lithium defective phase, due to the combined effect of water insertion into the garnet structure and the H + /Li + exchange mechanism. Differences in the actual compositions of the samples depending on their thermal history are corroborated by TG-MS experiments. Chemical reactions and phases formed along the thermal evolution are elucidated with the help of Raman spectroscopy.

In this review article, new systems being investigated for application in solid oxide fuel cells are discussed. For the electrode materials, materials with the perovskite or related structures continue to dominate the field, due to the need for high electronic conductivity. Research in this field is being directed toward compositions allowing high ionic conductivity in addition to their electronic contribution. In contrast, research on new electrolyte materials has shown a diverse range of structure-types, with an apparent tendency toward structures containing cations in lower coordination environments, particularly tetrahedral. In both the electrode and electrolyte area, materials allowing the incorporation of oxygen excess into interstitial sites have shown promising results, warranting further investigations of materials that will allow this type of defect chemistry.

29Si NMR data have been recorded for the apatite series La8+xSr2-x(SiO4)6O2+x/2 (0 < or = x < or = 1.0). For x = 0, a single NMR peak is observed at a chemical shift of approximately -77 ppm, while as the La : Sr ratio and hence interstitial oxygen content is increased, a second peak at a chemical shift of approximately -80 ppm is observed, which has been attributed to silicate groups neighbouring interstitial oxide ions. An increase in the intensity of this second peak is observed with increasing x, consistent with an increase in interstitial oxide ion content, and the data are used to estimate the level of interstitial oxide ions, and hence Frenkel-type disorder in these materials. The increase in second 29Si NMR peak intensity/interstitial oxide ion content is also shown to correlate with an increase in conductivity. The effect of interstitial oxygen content can also be studied by means of Raman spectroscopy, with a new mode at 360 cm(-1) appearing for samples with x > 0 in the symmetric bending mode energy region of the SiO4 group. The intensity of this mode increases with increasing oxygen content, yielding results comparable to those from the NMR studies, showing the complementarities of the two techniques.

In this paper we report the successful incorporation of phosphate and sulphate groups into the ionic conductor, Ba 2 In 2 O 5 , with the samples analysed through a combination of X-ray diffraction, NMR, TGA, Raman spectroscopy and conductivity measurements. The results show that such oxyanion incorporation leads to a conversion from an ordered brownmillerite-type structure to a disordered perovskite-type, and hence increases the conductivity at temperatures < 800 ○ C. In wet atmospheres, there is evidence for a significant enhancement of the conductivity through a protonic contribution.2

Germanium‐based apatite compounds are fast oxide‐ion conductors for potential use in fuel cells. A combination of solid‐state 17O NMR spectroscopy, atomistic modeling, and DFT techniques help to elucidate oxygen defect sites and novel cooperative mechanisms of ion conduction. The picture shows oxygen diffusion in the studied apatite compound from molecular dynamics simulations.