Role of Sr doping and external strain on relieving bottleneck of oxygen diffusion in La2−xSrxCuO4−δ

Abstract

In many complex oxides, the oxygen vacancy formation is a promising route to modify the material properties such as a superconductivity and an oxygen diffusivity. Cation substitutions and external strain have been utilized to control the concentration and diffusion of oxygen vacancies, but the mechanisms behind the controls are not fully understood. Using first-principles calculations, we find how Sr doping and external strain greatly enhances the diffusivity of oxygen vacancies in La2−xSrxCuO4−δ (LSCO) in the atomic level. In hole-doped case (2x > δ), the formation energy of an apical vacancy in the LaO layer is larger than its equatorial counterpart by 0.2 eV that the bottleneck of diffusion process is for oxygen vacancies to escape equatorial sites. Such an energy difference can be reduced and even reversed by either small strain (< 1.5%) or short-range attraction between Sr and oxygen vacancy, and in turn, the oxygen diffusivity is greatly enhanced. For fully compensated hole case (2x ≦ δ), the formation energy of an apical vacancy becomes too high that most oxygen vacancies cannot move but would be trapped at equatorial sites. From our electronic structure analysis, we found that the contrasting change in the formation energy by Sr doping and external strain is originated from the different localization natures of electron carrier from both types of oxygen vacancies.

Mechanism for the Enhancement of the Oxygen Diffusivity by Cation Substitution in La2–xSrxCuO4

“…Further Sr-substitution beyond n = 2, however, scarcely changes E V , indicating that more holes from Sr atoms cannot change the vacancy charge state. Interestingly, the energy difference between V a and V e significantly reduces from 1.84 eV for n = 0 to 0.20 eV for n = 2 . This means that when Sr atoms provide sufficient holes, most vacancies become the +2 charge state, letting both V a and V e play equally important roles in a diffusion process.…”

“…Interestingly, the energy difference between V a and V e significantly reduces from 1.84 eV for n = 0 to 0.20 eV for n = 2. 24 This means that when Sr atoms provide sufficient holes, most vacancies become the +2 charge state, letting both V a and V e play equally important roles in a diffusion process. To verify the effect of the Hubbard parameter U eff , we also evaluated the vacancy formation energies with U eff = 4 and compared them to those with U eff = 7 eV.…”

“…To incorporate the cation substitution and oxygen vacancies, we expanded the primitive unit cell of La 2 CuO 4 with lattice constants of a = b = 5.420 Å and c = 13.103 Å to the ( 2 2 × 2 2 ) R 45 ° × 2 supercell, which is in the low-temperature tetragonal (LTT) phase, where CuO 6 octahedra are alternately tilted along the [110] direction. It is noted that the pristine La 2 CuO 4 is more stable in the low-temperature octahedral phase where the tilting axis is the [100] direction than in the LTT phase. , It was, however, reported that their energy difference is only ∼4.4 meV per formula unit, , and moreover, a small amount of Sr-substitution reverses such small energy difference, resulting in the LTT to be more stable . The Brillouin zone of the supercell was sampled with a 4 × 4 × 1 k -point mesh.…”

“…22,23 It was, however, reported that their energy difference is only ∼4.4 meV per formula unit, 22,23 and moreover, a small amount of Sr-substitution reverses such small energy difference, resulting in the LTT to be more stable. 24 The Brillouin zone of the supercell was sampled with a 4 × 4 × 1 k-point mesh. We described the electron correlation associated with the Cu 3d orbital by selecting its on-site Coulomb repulsion 25 to be U eff ≡ U − J = 7.0 eV, which reproduces the measured Cu magnetic moment (0.50− 0.63 μ B ) 26,27 and band gap (1.80−2.00 eV) 28,29 with 0.603 μ B and 1.82 eV, respectively.…”