17O NMR in room temperature phase of La2Mo2O9 fast oxide ionic conductor

Abstract

A room temperature (17)O NMR study of La(2)Mo(2)O(9), a fast oxide ionic conductor exhibiting a phase transition at 580 degrees C between a low-temperature alpha-phase and a high-temperature beta-phase, is presented. Four partly overlapping quasi-continuous distributions of oxygen sites are evidenced from 1D magic angle spinning (MAS) and 2D triple quantum MAS NMR experiments. They can be correlated with the three oxygen sites O1, O2 and O3 of the high-temperature crystal structure. The low-temperature phase is characterized by two distributed sites of type O1, which proves that the symmetry is lower than in the cubic high-temperature phase. Two-dimensional experiments show that there is no dynamic exchange process, on the NMR time-scale, between the different oxygen sites at room temperature, which agrees well with conductivity results.

Solid oxide fuel cells—a challenge for materials chemists?

“…From these data it was clear that the crystal chemistry of the b phase lamox materials contributes to the ionic conductivity, a feature later confirmed through the use of 17 O NMR spectroscopy. 174 Finally, in discussing these possible electrolytes the compatibility with existing electrodes has to be considered and it is clear that, whilst the lamox material is stable at low pO 2 it is also reactive with LSM and LSCF cathode materials. 44 However there appears to be little reactivity with CGO, and therefore there is the possibility of using CGO as an interlayer.…”

On the Flexibility of the Structural Framework of Cubic LAMOX Compounds, in Relationship with Their Anionic Conduction Properties

“…These anti-tetrahedral units are supposed to be the most stable part of the structure and O1 the less mobile oxide ion since it is surrounded by four cations. In comparison, O2 and O3 are surrounded by three cations only and are therefore less trapped than O1. Moreover, O2 and O3 sites are partially occupied and have larger and more anisotropic Debye−Waller thermal factor than O1 .…”

Characterization of Oxygen Dynamics in ZrW2O8

“…Crystalline ionic materials are well suited to 17 O studies, since the broadenings due to quadrupolar coupling are modest, allowing relatively small chemical shift differences to be resolved − In the case of zirconium tungstate, 17 O NMR shows unambiguously that the disorder is dynamic: all oxygen sites are in exchange. Moreover, as shown in Figure d, it clearly shows evidence for dynamics (via peak broadening) in the low-temperature phase.…”