Improvement of electric insulation in dielectric layered perovskite nickelate films via fluorination

Abstract

Layered perovskite nickelates have recently emerged as materials with colossal dielectric permittivity. However, they exhibit relatively high values of loss tangent (tan δ) owing to insufficient electric insulation; thus, lowering of tan δ is crucial for their use in practical applications. Herein, we demonstrate that fluorine doping is an effective way to improve the electrical insulation. Epitaxial thin films of La_3/2Sr_1/2NiO_xF_y were prepared via low-temperature topotactic fluorination of oxide precursors. The fluorine content (y) was controllable over a wide range of 0.4–3. The film with y ∼ 0.4 exhibited 10⁴ times lower leakage current than the precursor oxide film, leading to a low tan δ of 0.02–0.03 at 1–10 kHz. First-principles calculations showed that high electric insulation is a consequence of suppressed hopping of holes in the Ni 3d orbitals owing to random distortion of Ni–O–Ni and Ni–F–Ni bonds. Fluorine doping can provide large and random bond distortions, unlike conventional cation doping. In addition, the dielectric constant of the film with y ∼ 0.4 was maintained at a high value of 9.4 × 10² at 1 kHz, which can be rationalized by assuming that holes were located at the Ni 3d orbital in less-tilted octahedrons.