Thermodynamic and Structural Properties of Bulk and Thin-Film CeVO4 and LaVO4

The thermodynamic and structural properties of 1 nm thick films of CeVO4 and LaVO4 supported on γ-Al2O3 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 CeVO4 and LaVO4 uniformly covered the γ-Al2O3 in the form of two-dimensional crystallites. Thin-film LaVO4 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(O2) at which equilibrium is established between these two phases was 7 orders of magnitude higher for the thin-film material. Thin-film CeVO4 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 CeVO4. Multiple techniques showed that the oxidation states of both Ce and V in the thin-film CeVO4 were +4, which is also different from bulk CeVO4. Reasons for why the properties of the thin-film, rare-earth vanadates differ from their bulk counterparts are discussed.