Proton distribution visualization in perovskite nickelate devices utilizing nanofocused x rays

We use a 30-nm x-ray beam to study the spatially resolved properties of a ${\mathrm{SmNiO}}_3$-based nanodevice that is doped with protons. The x-ray absorption spectra supported by density-functional theory simulations show partial reduction of nickel valence in the region with high proton concentration, which leads to the insulating behavior. Concurrently, x-ray diffraction reveals only a small lattice distortion in the doped regions. Together, our results directly show that the knob which proton doping modifies is the electronic valency and not the crystal lattice. The studies are relevant to ongoing efforts to disentangle structural and electronic effects across metal-insulator phase transitions in correlated oxides.

Figure 1

(a) Orthorhombic crystal structure of SNO. The breathing mode is shown by color: Expanded ${\text{NiO}}_6$ octahedra are yellow and contracted are orange. (b) Schematic representation of the ${\mathrm{NiO}}_6$ octahedra rotations (tilt pattern). (c) Breathing mode. (d) Displacement of the rare-earth cation from the centrosymmetric position inside the cavity between ${\mathrm{NiO}}_6$ octahedra. (e) Scheme of the nanofocused x-ray experiment and the SNO-based device. The focused beam is used for raster scanning of the device where the diffraction signal is recorded by a 2D detector in reflection geometry, and the fluoresce signal is collected by a point energy-resolving detector oriented perpendicular to the sample surface. The central beamstop and order-sorting aperture for the zone plate are not shown.

Figure 2

(a) Spatially resolved map of the fluorescence signal at $E=8345$ eV. The white dashed lines outline the Pd and Au electrodes. Bright areas next to the Pd electrode correspond to the reduced valence of nickel ions due to the presence of ${\mathrm{H}}^{+}$. (b) Experimentally measured normalized XAS spectra at K-edge of Ni in the pristine SNO away from the Pd electrode and the doped H-SNO under the Pd electrode (solid lines) and FEFF-simulated XAS spectra for the doped H-SNO and undoped SNO (dashed lines). The simulated spectra were additionally convoluted with a Gaussian function to match the same energy resolution as in the experiment. The experimentally observed shift of the XAS spectrum was reproduced in the DFT simulations at the 1H:1Ni doping level. (c) Dependence of the absorption peak shift $\mathrm{\Delta }E$ on the concentration of H dopant for the simulated XAS spectra. Horizontal dashed line marks the experimentally observed value of $\mathrm{\Delta }E=1.2$ eV.

Figure 3

(a) Normalized intensity map of the (101) reflection. The white dashed lines outline Pd and Au electrodes. The dark region between the electrodes corresponds to the area where ${\mathrm{H}}^{+}$ doping results in structural changes in the film. (b) Spatially resolved map of the $Q$-position (${\AA }^{-1}$) of the (202) reflection. (c) Normalized intensity map of the (202) reflection.

Figure 4

(a)–(c) Simulated intensity the (101) reflection and (d)–(f) intensity of the (202) reflection as a function of ${\mathrm{NiO}}_6$ octahedra tilt angles $\alpha$, $\beta$, $\gamma$ (a), (d); the displacement of the ${\mathrm{Sm}}^{3+}$ cation $d_x,d_y,d_z$ (b), (e); and the breathing distortion $B$ (c), (f). The intensities $I_{101}$ and $I_{202}$ are normalized to the value in the pristine SNO.

Figure 5

(a), (b) Dependence of the (101) peak intensity on the root-mean-square displacement (a) and mean position of the ${\mathrm{Sm}}^{3+}$ ions (b). The intensity was evaluated by fixing the positions of all ions in the SNO unit cell and allowing only ${\mathrm{Sm}}^{3+}$ to move. For each value of the parameters the result was statistically averaged over ${10}^4$ realizations. (c) Mean shift of the ${\mathrm{Sm}}^{3+}$ ions and the corresponding change of intensity along the $x=0$ line in the spatially resolved map in Fig. 3 using Eq. (2). Vertical lines mark Au and Pd electrodes.