Formation of defect-fluorite structured NdNiO_xH_y epitaxial thin films via a soft chemical route from NdNiO₃ precursors

Abstract

A new phase of oxyhydride NdNiO_xH_y with a defect-fluorite structure was obtained by a soft chemical reaction of NdNiO₃ epitaxial thin films on a substrate of SrTiO₃ (100) with CaH₂. The epitaxial relationship of this phase relative to SrTiO₃ could be controlled by changing the reaction temperature. At 240 °C, NdNiO_xH_y grew with a [001] orientation, forming a thin layer of infinite-layer NdNiO₂ at the interface between the NdNiO_xH_y and the substrate. Meanwhile, a high-temperature reaction at 400 °C formed [110]-oriented NdNiO_xH_y without NdNiO₂.

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Introduction

The topotactic reaction of transition-metal oxides with metal hydrides is utilized as a facile and efficient synthesis approach for novel or mixed-anion oxides.^1,2 In this class of reaction, metal hydrides can act as either strong reducing reagents for removing oxide anions or incorporating hydride anions. Reactions with the former role yield transition-metal oxide phases with exceptionally low oxidization states (Ni⁺, Ru²⁺, Co⁺),^3–7 unusual coordination networks (square-planar Fe²⁺),⁸ or both, while those with the latter allow the preparation of oxyhydrides^9–11 such as LaSrCoO₃H_0.7, BaTiO_3−xH_x, and SrVO₂H. Topotactic reactions can be performed at low reaction temperatures; for example, a typical reaction temperature during oxygen deintercalation or hydride ion incorporation is below 500 °C,^3–10 which is considerably lower than those of conventional “hard” solid-state reactions (∼1000 °C). Reactions at these low temperatures, classified as “soft” chemical reactions, enable the synthesis of metastable compounds, such as oxyhydrides, that can decompose easily at elevated temperatures.

Although topotactic reactions are typically used to produce powdered samples, several groups have applied the metal-hydride reaction to thin films, reporting fabrications of LaNiO₂,^12,13 SrFeO₂,¹⁴ Sr_1−xEu_xFeO₂,¹⁵ATiO_3−xH_x ¹⁶ (A = Ca, Ba, or Sr), SrCoO_xH_y,¹⁷ and LaSrCoO_4−xH_x ¹⁸ as thin films. The topotactic reaction of epitaxial thin films allows the preparation of single-crystalline films, which can be used to measure the intrinsic physical properties of materials. In addition, the concentration of the introduced heteroatoms can be much higher in films than in bulk samples because of the higher reactivity of thin films. Katayama et al.¹⁹ recently reported that a SrFeO₂ thin film, obtained by a topotactic reaction between SrFeO_2.5 and CaH₂, contained a significant amount of hydrogen (∼3 × 10²¹ atoms per cm³). The hydrogen could serve as an electron acceptor and generate metallic conductivity, in contrast to bulk SrFeO₂.

Here, we investigate the topotactic reaction of perovskite NdNiO₃ epitaxial thin films with CaH₂. Perovskite NdNiO₃ shows a metal–insulator transition at 200 K (ref. 20) associated with charge ordering and a structural transition.²¹ The reduction of NdNiO₃ powder using metal hydrides was reported to yield infinite-layer NdNiO_2+x,^3,4 although the presence of hydrogen in the reduced product was not confirmed. In this study, we found that the CaH₂ treatment of NdNiO₃ epitaxial thin films completely altered the cation framework, producing defect-fluorite-structured NdNiO_xH_y. Moreover, the growth orientation of the defect-fluorite phase changed as a function of the reaction temperature with CaH₂.

Experimental methods

Epitaxial NdNiO₃ thin films were deposited as precursors onto SrTiO₃ (STO) (100) substrates (Shinkosha Co., Ltd) by pulsed laser deposition (PLD) at a substrate temperature of 650 °C, an oxygen partial pressure of 13 Pa, a laser (KrF excimer, 248 nm wavelength) fluence of 2 J cm⁻², and a repetition rate of 5 Hz. The obtained precursor films were then reacted with CaH₂ powder (Wako Pure Chemical Industries, Ltd) in a Pyrex tube evacuated with a rotary pump. The reaction temperature and reaction time were varied in the ranges of 240–400 °C and 1–24 h, respectively. After the reaction, the film was ultrasonically washed with 2-butanone to remove residual powders from the surface. The crystal structures of the samples were characterized by X-ray diffraction (XRD, Bruker AXS D8 Discover) with Cu Kα radiation, as well as by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM, Hitachi High-Technologies Co., HD-2700). The amount of hydrogen within the films was measured by dynamic secondary-ion mass spectrometry (SIMS, ULVAC-PHI PHI-ADEPT1010, primary ion: Cs⁺, secondary ion: H⁻, acceleration voltage: 1.0 kV, detection limit: 1 × 10¹⁹ cm⁻³, depth resolution: ∼3 nm).

Results and discussion

Fig. 1a shows the out-of-plane 2θ–θ XRD patterns of the precursor NdNiO₃ film on the STO (100) substrate and the film after reaction with CaH₂ at 240 °C for 12 h. The pattern of the precursor films shows the 002 diffraction peak of perovskite NdNiO₃ at 2θ = 48.05°. The out-of-plane lattice constant c is calculated as 3.78 Å, in good agreement with the previously reported values.^22,23 Reciprocal space mapping (RSM) around the 103 asymmetric diffraction (Fig. 1b) reveals that the in-plane lattice of the NdNiO₃ (a-axis) is completely locked to the STO substrate. The in-plane lattice constant is larger than the out-of-plane one, indicating that the film is under in-plane tensile strain from the substrate. Meanwhile, in the 2θ–θ pattern of the film treated with CaH₂ (Fig. 1a), the peak from NdNiO₃ has completely disappeared; a new peak at 2θ ≈ 55.8° (d = 1.65 Å) has emerged, which corresponds to the 002 diffraction of infinite-layer NdNiO₂. In RSM (Fig. 1c), the NdNiO₂ 103 peak is located just above that of STO 103, demonstrating that the epitaxial relationship between the film and the substrate is maintained during the chemical reaction with CaH₂. However, the intensity of the NdNiO₂ 002 diffraction is much weaker than that in the pattern of the precursor film. Furthermore, in the RSM image, the diffraction spot is significantly elongated along the q_z direction. These changes suggest that NdNiO₂ exists as a very thin layer within the film.

Fig. 1 (a) XRD 2θ–θ patterns before (black) and after (red) the reaction with CaH₂ at 240 °C. Reciprocal space maps (b) before and (c) after the reaction.

To obtain further information on the crystal structure, we performed atomic-level HAADF-STEM imaging of the CaH₂-treated film. Fig. 2a shows a large area (∼200 nm width) HAADF image of the CaH₂-treated film, indicating almost no contrast. Since the brightness of HAADF images depends on the atomic number of the constituent elements, the lack of contrast indicates that neodymium and nickel are homogeneously distributed in the film with no cation segregation. Fig. 2b is a high-resolution image taken near a film/substrate interface. In the region near the substrate (Region ii), a rectangular lattice with (a, c) = (3.9 Å, 3.3 Å), representing c-axis oriented NdNiO₂, is observed. The thickness of this region is approximately ten monolayers; this explains the weakness and breadth of the NdNiO₂ diffraction peak in Fig. 1. Fig. 2b also indicates the presence of a thin interfacial layer measuring ∼0.5 nm in thickness (Region i). A similar interfacial layer was observed in infinite-layer Sr_0.9La_0.1CuO₂ thin films grown by molecular-beam epitaxy;²⁴ it was proposed to promote the growth of the infinite-layer phase.

Fig. 2 (a) Wide and (b) magnified view of HAADF-STEM images after the reaction at 240 °C for 12 h. Inset: Fourier transform of the fluorite phase region (iii). Green and red circles show the positions of neodymium and nickel atoms, respectively.

Meanwhile, the atomic arrangement in Region iii, located above Region ii and occupying most of the film, is completely different from that of NdNiO₂: each atom in Region iii is imaged with a contrast brighter than that of nickel but darker than that of neodymium in Region ii. This suggests that the metals are randomly distributed in Region iii. The lattice image in Region iii can be understood by assuming a face-centered cubic (fcc) lattice viewed along the [110] direction, as depicted by the green and red points in the figure. From the Fourier transform of the HAADF image (inset in Fig. 2b), the lattice constant is estimated as 5.5 Å. The observed atomic arrangement and lattice constant are near those of a fluorite-structured oxyhydride, NdHO²⁵ (a = 5.61 Å in pseudocubic notation). Therefore, we tentatively conclude that the reaction of NdNiO₃ with CaH₂ yields a fluorite-structured NdHO-related phase as a major component of the film; hereafter, this is called the fluorite phase.

In order to determine the chemical composition of this NdHO-related fluorite film, we performed dynamic SIMS and EDS measurements. Fig. 3a shows a hydrogen depth profile measured by SIMS for the sample treated with CaH₂ at 240 °C, clearly indicating that hydrogen is homogeneously distributed in the film. The hydrogen density is evaluated as ∼9 × 10²¹ atoms per cm³ (0.7 per formula unit, fu), although this has some uncertainty because hydrogen-implanted STO was used as a standard reference. The hydrogen density in the substrate was more than one order of magnitude smaller than that in the film. Fig. 3b plots the differential secondary-ion intensities of strontium, titanium, neodymium, nickel, and hydrogen. As observed, the hydrogen density abruptly drops at a position several nanometers shallower than the film–substrate interface. This indicates that hydrogen is mainly incorporated into the fluorite phase, rather than the infinite-layer NdNiO₂.

Fig. 3 (a) Depth dependence of atomic hydrogen density (n_H) after the reaction at 240 °C. (b) Differential secondary-ion intensity (dI/dx) of each element. (c) EDS spectra of the films before (black) and after the reaction (blue, red), measured at an acceleration voltage of 2.5 kV.

Fig. 3c compares the EDS spectra of the film before and after the reaction with CaH₂. The nickel/neodymium ratio remains constant, whereas the intensity of the O Kα peak is suppressed by 20% after the reaction. Assuming that the peak intensity of each element is proportional to its concentration, the oxygen density is estimated as 2.3/fu. From these results, the chemical formula of the fluorite phase is estimated as NdNiO_xH_y with (x, y) ≈ (2.3, 0.7). The total anion (oxygen + hydrogen)/cation (neodymium + nickel) ratio is approximately 3/2, smaller than the ideal value of 2, which suggests that the obtained oxyhydride phase has a defect-fluorite structure.

Finally, we describe the selective synthesis of the oxyhydride phase via a solid-phase reaction with CaH₂ at higher temperature. Fig. 4a and b show the out-of-plane (χ = 90°) and asymmetric (χ = 35°) XRD patterns of the film prepared at 400 °C. Notably, the former indicates no peaks assignable to the infinite-layer NdNiO₂ phase, whereas a clear peak from the fluorite oxyhydride phase is observed in the latter, demonstrating the formation of a phase-pure oxyhydride phase without infinite-layer NdNiO₂. The peak at 2θ = 28° in Fig. 4b is assignable to the 111 diffraction of the fluorite phase. This implies that the oxyhydride film heat-treated at 400 °C is [110]-oriented, in contrast to the film prepared at 240 °C with the [001]-orientation. A HAADF-STEM measurement also confirms this orientation change. Most of the film consisted of the [110]-oriented crystals (as in Fig. 4c), while the crystals adjacent to the substrate–film interface (2–3 nm thick, Fig. 4d) remained [001]-oriented. Additionally, SIMS measurement revealed that this sample contained 6 × 10²¹ cm⁻³ of hydrogen (0.5/fu), comparable to that of the sample prepared at the lower temperature of 240 °C (0.7/fu). The EDS measurement confirmed that the neodymium and nickel contents in this film were invariant after the heat-treatment, while the oxygen content decreased from 2.3 (at 240 °C) to 1.7 (at 400 °C). If hydrogen is present as H⁻, H, or H⁺, the nominal valence state of nickel is calculated to be +0.9, +0.4, or −0.1, respectively. Though +3 (as in NdNiO₃), +2 (as in NiO), and +1 (as in NdNiO₂) states of nickel are known in extended oxides, to the best of our knowledge, there are no reports on zero or negative charge states of nickel oxides. Therefore, it seems natural to assume that hydrogen exists as hydride anions rather than protons or charge-neutral atoms.

Fig. 4 (a) Out-of-plane (χ = 90°) and (b) asymmetric (χ = 35°) XRD patterns of the films reacted at 400 °C for 24 h. HAADF-STEM images of the film reacted at 400 °C for 24 h (c) at the middle of the film and (d) at the interface between the film and the substrate.

In the [001]-oriented fluorite film obtained at 240 °C, the [110] and [1−10] axes of NdNiO_xH_y are parallel to the [100] and [010] directions of the STO substrate, respectively. The cubic lattice constant a of the substrate (3.905 Å) and a/√2 of the fluorite phase (≈3.9 Å) are nearly equal; thus, it is natural that [001]-orientated fluorite is grown on STO (100). Meanwhile, the orientation change mentioned above is contrary to our prediction based on the epitaxial relationship between the fluorite film and the substrate. In the [110]-oriented NdNiO_xH_y film obtained at 400 °C, the [1−10] and [001] axes are parallel to the [100] and [010] directions of the substrate, respectively. In this orientation, the in-plane lattice of NdNiO_xH_y has a rectangular shape with axes of 3.9 Å (along [100]_substrate) and 5.5 Å (along [010]_substrate). The latter value is incommensurable with that of the substrate and thus the [110]-oriented NdNiO_xH_y film is expected to have a lattice match poorer than that of the [001]-oriented crystal. A similar orientation change was reported in LaNiO₂ thin films obtained by the topotactic reduction of LaNiO₃, in which c-axis-orientated LaNiO₂ with better lattice matching transformed to a-axis-orientated LaNiO₂ with poorer lattice matching as the reaction time or temperature increased.²⁶

These results suggest that the crystal growth direction is determined by kinetics rather than by thermodynamics including epitaxial stabilization. One possible explanation is that the anisotropic diffusion of atoms promotes a specific arrangement of atoms with respect to the substrate lattice. Because the topotactic reaction on thin films mainly occurs from their surfaces, the atomic diffusion perpendicular to the film surface dominates the reaction.²⁷ The growth of (110) oriented NdNiO_xH_y, contrary to our expectation, may be the consequence of such perpendicular diffusion of the constituting atoms. Further studies on chemical transport properties such as anisotropic ionic conductivity may help to elucidate the detailed mechanisms.

Conclusions

We have investigated the soft chemical reaction of NdNiO₃ thin films with CaH₂. When the precursor film was reacted at 240 °C, [001]-oriented NdNiO_xH_y (x ∼ 2.3, y ∼ 0.7) with a defect-fluorite structure was grown on a thin interfacial layer of infinite-layer NdNiO₂. At the elevated reaction temperature of 400 °C, we observed the growth of a [110]-oriented defect-fluorite phase, although it seemed more poorly lattice-matched to SrTiO₃ [100] than the [001]-oriented NdNiO_xH_y did.

The incorporation of a large amount of hydrogen into the films suggests the potential of NdNiO_x for hydrogen storage. Hydrogen storage using single crystals is not practical because of the small surface/volume ratio. However, they are useful for the observation of fundamental kinetics of diffusion-controlled reactions.

Acknowledgements

This work was supported by the Kurata Memorial Hitachi Science and Technology Foundation, Core Research for Evolutionary Science and Technology of Japan Science and Technology Agency, and a Grant-in-Aid for Scientific Research (No. 15H05424) from the Japan Society for the Promotion of Science (JSPS). One of the authors (TO) was supported by the Japan Society for the Promotion of Science through Program for Leading Graduate Schools (MERIT).

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