Ceria is one of the most important functional rare-earth oxides with wide industrial applications. Its amazing oxygen storage/release capacity is attributed to cerium’s flexible valence conversion between 4+ and 3+. However, there still exists some debate on whether the valence conversion is due to the Ce-4f electron localization-delocalization transition or the character of Ce–O covalent bonds. In this work, a mixed valence model was established and the formation energies of oxygen vacancies and electronic charges were obtained by density functional theory calculations. Our results show that the formation energy of oxygen vacancy is affected by the valence state of its neighboring Ce atom and two oxygen vacancies around a Ce⁴⁺ in CeO₂ have a similar effect to a Ce³⁺. The electronic charge difference between Ce³⁺ and Ce⁴⁺ is only about 0.4e. Therefore, we argue that the valence conversion should be understood according to the adjustment of the ratio of covalent bond to ionic bond. We propose that the formation energy of oxygen vacancy be used as a descriptor to determine the valence state of Ce in cerium oxides.

By regulating the one-step secondary reduction process (Cu( ii ) → CuCl( i ) → Cu ₂ O( i ) → Cu(0)) is used to slow down the reaction rate and obtain high-performance ultrafine copper powder protected by methionine (Met).

Segregation at metal alloy surfaces is an important issue because many electrochemical and catalytic properties are directly correlated to the surface composition. We have performed density functional theory calculations for Mo segregation in MoNi(111) in the presence of chemisorbed atomic oxygen. In particular, the coverage dependence and possible adsorption-induced segregation phenomena are addressed by investigating segregation energies of the Mo atom in MoNi(111). The theoretical calculated results show that the Mo atom prefers to be embedded in the bulk for the clean MoNi(111), while it segregates to the top-most layer when the oxygen coverage is thicker than 1/9 monolayer (ML). Furthermore, we analyze the densities of states for the clean and oxygen-chemisorbed MoNi(111), and see a strong covalent bonding between Mo d-band states and O p-states. The present study provides valuable insight for exploring practical applications of Ni-based alloys as hydrogen evolution electrodes.

Compounds of rare earth zirconates with pyrochlore structure are candidates for the application of thermal barrier coatings of next generation. In order to modify the mechanic properties and maintain the low thermal conductivity, other trivalent rare-earth element substitution is commonly used. Presently, investigation on the evaluation of the property of thermal expansion is attracting more attention. In this paper, a feature parameter of thermal expansion coefficient at high temperature (α_∞) was proposed by combining Grüneisen’s equation and the Debye heat capacity model. Using α_∞ model, the thermal expansion property of different compounds can be easily figured out by first principles. Firstly, α_∞ of ZrO₂, HfO₂, were calculated, and results are in good agreement with the experimental data from the literature. Moreover, α_∞ of La₂Zr₂O₇, Pr₂Zr₂O₇, Gd₂Zr₂O₇, and Dy₂Zr₂O₇ were calculated, and results demonstrated that the model of α_∞ is a useful tool to predict the thermal expansion coefficient at high temperature. Finally, Gd₂Zr₂O₇ with 4 different Yb dopant concentrations (Gd_1-xYb_x)₂Zr₂O₇ (x = 0, 0.125, 0.3125, 0.5) were calculated. Comparing with the experimental data from the literature, the calculation results showed the same tendency with the increasing of Yb concentration.