Hydrogen-Induced Topotactic Phase Transformations of Cobaltite Thin Films

Abstract

Manipulating physical properties through ion migration in complex oxide thin films is an emerging research direction to achieve tunable materials for advanced applications. While the reduction of complex oxides has been widely reported, few reports exist on the modulation of physical properties through a direct hydrogenation process. Here, we report an unusual mechanism for hydrogen-induced topotactic phase transitions in perovskite La_0.7Sr_0.3CoO₃ thin films. Hydrogenation is performed upon annealing in a pure hydrogen gas environment, offering a direct understanding of the role that hydrogen plays at the atomic scale in these transitions. Topotactic phase transformations from the perovskite (P) to hydrogenated-brownmillerite (H-BM) phase can be induced at temperatures as low as 220 °C, while at higher hydrogenation temperatures (320-400 °C), the progression toward more reduced phases is hindered. Density functional theory calculations suggest that hydroxyl bonds are formed with the introduction of hydrogen ions, which lower the formation energy of oxygen vacancies of the neighboring oxygen, enabling the transition from the P to H-BM phase at low temperatures. Furthermore, the impact on the magnetic and electronic properties of the hydrogenation temperature is investigated. Our research provides a potential pathway for utilizing hydrogen as a basis for low-temperature modulation of complex oxide thin films, with potential applications in neuromorphic computing.

Figure 1

(a) XRD patterns of AG-P and hydrogenated-LSCO thin films on LSAT substrates upon annealing for 1 h. The hydrogenation temperature increases from bottom to top. (b) Crystal structure diagrams of the P and H-BM phases. La/Sr, Co, and H ions are shown in gray, blue, red, and green, respectively. The structures are oriented along the [100]-pseudocubic axes of the P phase. (c) STEM-HAADF image of the H-BM phase after annealing at 220 °C. The yellow arrow marks the horizontal dark stripe (oxygen-deficient layer). (d) Phase fraction of P and H-BM phases for hydrogenated samples as a function of the hydrogenation temperature. The shaded region denotes the range of error bars.

Figure 2

Co L-edge XA of AG-P and hydrogenated-LSCO thin films measured in (a) the TEY mode and (b) the LY mode. Solid curves are experimental data, and symbols are fitting results. Co L-edge XA reference spectra (CoFe₂O₄, LaCoO₃) are plotted. XA spectra are normalized from 0 to 1 and vertically shifted for clarity. Co L-edge XMCD measured in (c) TEY and (d) LY mode. The inset diagram in (c) represents the measurement geometry and the probing depth of TEY and LY detection modes. (e) Co-ion valence state fitting coefficients as a function of hydrogenation temperature. (f) O K-edge XA spectra taken at 300 K using the TEY mode. The shaded region in green represents the energy range associated with hybridization between O 2p and Co 3d orbitals.

Figure 3

(a) Hydrogen configuration with the lowest formation energy (ΔE_fH), where the H₂ molecules decompose and form two neighboring −OH bonds. The inset figure shows details of the −OH bonding structure, with the Co–OH bond angle marked in the figure. (b) Oxygen vacancy sites with negative oxygen vacancy formation energies (ΔE_fO), which are close to −OH bonds. (c) Oxygen vacancy sites with positive oxygen vacancy formation energy ΔE_fO), which are bonded to hydrogen.

Figure 4

(a) Magnetization as a function of the temperature for AG-P and hydrogenated-LSCO thin films. The magnetization is normalized to the thin film volume (left y-axis) and number of Co ions (right y-axis). A magnetic field of 0.08 T was applied along the in-plane [100] substrate direction during the measurements. The vertical dashed gray lines mark T_c values. (b) Resistivity as a function of temperature.