Program

O.6 -- Oral, NCTP-41

Date: Monday, 1 August 2016

Time: 14h20 - 14h40

First-Principles Study on Electronic Structures and Oxygen Vacancy Formation in Strained LaNiO3 Structures

Huy Duy Nguyen, Duy Ba Pham, and Bach Thanh Cong

VNU University of Science

The LaNiO3 is a rare perovskite oxide that exhibits paramagnetic metallic behavior, and is frequently utilized in the cathode materials for solid oxide fuel cells (SOFCs) [1]. The usage of LaNiO3 in SOFCs stems from the ability to catalyze the oxygen reduction reaction (ORR), as well as the high-temperature stability and acceptable thermal expansion properties. In the ORR process, oxygen vacancies are important as the vacancies can strongly couple to the oxygen dissociation, transport, and incorporation. Previous theoretical work based on first-principles calculations have predicted that bulk LaNiO3 has a much stronger preference for oxygen vacancy formation compared to SrTiO3 [2]. However, as LaNiO3 is always deposited on a substrate, the effect of interfacial strain on the formation of oxygen vacancy is important, and to the best of our knowledge, number of studies on such matter is limited. In this paper, we study the effects of interfacial strain on the electronic structures and the oxygen vacancy formation in LaNiO3 structures. The first-principles calculations are carried out within the framework of density functional theory using a plane wave basis set, as implemented in the quantum espresso software package. The electron-ion interaction is described using Vanderbilt ultrasoft pseudopotentials, and the exchange-correlation interaction is treated by the local density approximation. The interfacial strain is simulated by changing the lattice parameters in the \textit{xy} plane and allow the structure to relax in the z direction. We investigate the 2x2x2 LaNiO3 supercell and find that the optimized bulk structure exhibits clear local distortions of NiO6 octahedrals, i.e., the octahedrals are titled in correlation to each other. The tilting is suppressed (enhanced) in the case of compressive (tensile) strains. This octahedral tilting results in the reduction of the hybridization between O 2p and Ni 3d states. By plotting the local densities of states, we confirm that O pz- and Ni e2g-derived states become more localized as the Ni-O-Ni angle along the z direction decreases. The oxygen vacancy is investigated by removing one oxygen atom in the LaO or NiO2 layer, and the dependence of oxygen vacancy formation energy (EVO) on the in-plane strain for a missing oxygen in LaO and NiO2 layer, relative to the EVO in the unstrained structure, is studied. It is found that EVO for a missing oxygen in the NiO2 layer decreases by applying both compressive or tensile strains. On the contrary, the EVO for a missing oxygen in the LaO-layer increases (decreases) as the compressive (tensile) strain increases. This behavior is attributed to the reduction in the hybridization between O pz and Ni e2g states in the case of tensile strain leading to the decrease in EVO. Our results indicate that interfacial strains can be utilized to modify the oxygen vacancy concentration in the bulk LaNiO3. References [1] S. B. Adler, Chem. Rev. (Washington, D.C.) 104, 4791 (2004). [2] A. Malashevich and S. Ismail-Beigi, Phys. Rev. B 92, 144102 (2015).

Presenter: Nguyen Duy Huy