Thermodynamic and Structural Properties of Bulk and Thin-Film CeVO<sub>4</sub> and LaVO<sub>4</sub>
Kai Shen, Jian Chang, Rajeev Kumar, Ching‐Yu Wang, Eric A. Stach, Raymond J. Gorte, John M. Vohs
Abstract
The thermodynamic and structural properties of 1 nm thick films of CeVO 4 and LaVO 4 supported on γ-Al 2 O 3 were examined and compared to the corresponding bulk rare-earth vanadates. The films were prepared by atomic layer deposition (ALD) on high-surface-area powders and then examined after either oxidation or reduction at 1073 K. Both CeVO 4 and LaVO 4 uniformly covered the γ-Al 2 O 3 in the form of two-dimensional crystallites. Thin-film LaVO 4 reversibly transitioned between a perovskite structure upon reduction to a monazite structure upon oxidation, similar to what is observed for the bulk material; however, coulometric titration (CT) showed that the P(O 2 ) at which equilibrium is established between these two phases was 7 orders of magnitude higher for the thin-film material. Thin-film CeVO 4 also formed a perovskite phase upon reduction but reversibly transitioned to a fluorite phase upon oxidation rather than the zircon phase, which is formed for bulk CeVO 4 . Multiple techniques showed that the oxidation states of both Ce and V in the thin-film CeVO 4 were +4, which is also different from bulk CeVO 4 . Reasons for why the properties of the thin-film, rare-earth vanadates differ from their bulk counterparts are discussed.