Structure and conductivity of Nd6MoO12-based potential electron-proton conductors under dry and wet redox conditions


Nominal compositions Nd6MoO12 (3:1) and Nd10Mo2O21 (5:2) were prepared by high-temperature synthesis in air from a mechanically activated mixture of Nd2O3 and MoO3. Phase-pure Nd6MoO12- and Nd6MoO12- have been obtained at 1600 and 1650 degrees C, respectively. They are both slightly rhombohedrally distorted derivatives from the cubic fluorite structure. Nd10Mo2O21 with a lower content of Nd2O3 was shown to be a more complex phase based on the rhombohedral phase (R3) in the 1600-1650 degrees C temperature range. As a result of the formation of a more complex rhombohedral phase, the conductivity of Nd10Mo2O21 changes dramatically in comparison with Nd6MoO12-. In wet air Nd6MoO12- is a p-type electronic conductor, whereas proton conductivity dominates in Nd10Mo2O21 over the entire temperature range studied. The electrical conductivity dependence of Nd6MoO12- on the oxygen partial pressure shows a V-type behaviour typical of a transition from a p-type to n-type conductivity mechanism at 800 T 1000 degrees C. There is no p-type conductivity contribution in Nd10Mo2O21 in the same temperature range. The prevalence of electronic conductivity in the samples with nominal composition Nd6MoO12 in a wide temperature range is due to the fact that Nd and Mo in the fluorite materials are readily reduced. Predominantly Nd3+ and Mo6+ forms exist in more complex rhombohedral phase Nd10Mo2O21 and it has proton conductivity approximate to 8.5 x 10(-3) S cm(-1) at 800 degrees C. Thus, the loss of dimensional stability is more characteristic of fluorites and rhombohedral phases with small rhombohedral distortion (Nd5.4Zr0.6MoO12.3, Nd6MoO12-) than more complex rhombohedral phases based on (R3) (Nd10Mo2O21). A comparative high-temperature in situ neutron diffraction study under high vacuum of rhombohedral Nd6MoO12- and cubic fluorite Nd5.4Zr0.6MoO12.3 showed that the former transforms to the high-temperature cubic fluorite type above approximate to 1140 degrees C while the latter retains its cubic structure in the studied range up to 1350 degrees C.



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Shlyakhtina, AV; Avdeev, M; Abrantes, JCC; Gomes, E; Lyskov, NV; Kharitonova, EP; Kolbanev, IV; Shcherbakova, LG

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This work was supported by the Russian Foundation for Basic Research (grants no. 16-03-00143, 19-03-00358) and supported into frameworks of the state task for the ICP RAS no. 0082-2014-0011. State registration number AAAA-A17-111711600093-8.

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