ABSTRACT
There is an ongoing effort to replace rare and expensive noble-element catalysts with more abundant and less expensive transition metal oxides. With this goal in mind, the intrinsic defects of a rhombohedral perovskite-like structure of LaMnO3 and their implications on CO catalytic properties were studied. Surface thermodynamic stability as a function of pressure (P) and temperature (T) were calculated to find the most stable surface under reaction conditions (P=0.2â atm, T=323â K to 673â K). Crystallographic planes (100), (111), (110), and (211) were evaluated and it was found that (110) with MnO2 termination was the most stable under reaction conditions. Adsorption energies of O2 and CO on (110) as well as the effect of intrinsic defects such as Mn and O vacancies were also calculated. It was found that O vacancies favor the interaction of CO on the surface, whereas Mn vacancies can favor the formation of carbonate species.
ABSTRACT
In this work, we have synthesized and fabricated solar cells with the hybrid metal halide compounds with the general formula ABX3, where the A cation is methylammonium, the B cation is nickel, and the X anion is chlorine or a mixture of chlorine and iodine. We obtained experimental evidence that this material is a semiconductor with an orthorhombic crystalline structure which pertains to the space group Cmcm. The bandgap can be modulated from 1.4 eV to 1.0 eV by changing the chlorine anion to iodine. Therefore, we were able to obtain solar cells with efficiencies up to 0.16% with the CH3NH3NiCl2I composition. We have also studied by means of first-principles calculations, taking into account van der Waals dispersive forces, the ground state properties of these materials such as their crystal structure and formation and decomposition energies. We have found that these energies are lowered by the lighter mass anion, and the calculated decomposition energies show that only CH3NH3NiCl3 is stable with respect to the most probable decomposition pathway. The electronic band structure and band edge alignments have been calculated using quasiparticle effects through the GW0 approximation; these materials show an indirect bandgap with the valence band maxima at -6.93 and -5.49 eV with respect to vacuum and the conduction band minima at -5.62 and -4.60 eV with respect to vacuum for CH3NH3NiCl3 and CH3NH3NiI3, respectively. This work provides a pathway to explore new hybrid A+B2+X3--type semiconductor materials.
ABSTRACT
We identify two general types of electronic behaviors for transition-metal impurities that introduce excess electrons in oxides. (i) The dopants introduce resonant states inside the host conduction band and produce free electrons; (ii) the dopants introduce a deep gap state that carries a magnetic moment. By combining electronic structure calculations, thermodynamic simulations, and percolation theory, we quantify these behaviors for the case of column V-B dopants in anatase TiO2. Showing behavior (i), Nb and Ta dopants can convert the insulator TiO2 into a transparent conductor. Showing behavior (ii), V dopants could convert nonmagnetic TiO2 into a ferromagnet. Whether a dopant shows behavior (i) or (ii) is encoded in its atomic d orbital energy.
ABSTRACT
We performed a theoretical study of the crystal structures of cesium and rubidium under high compressions. Our results confirm the recent high-pressure experimental observations of new complex crystal structures for the Cs III and Rb III phases. The calculated transition pressures agree extremely well with the measured data. Thus, it is now certain that the famous isostructural phase transition in cesium is actually a new crystallographic phase transition. A d-orbital occupation number of about 0.52 is crucial for the occurrence of these complex structures.
