Investigation of Hydrogen Incorporations in Bulk Infinite-Layer Nickelates

Infinite-layer (IL) nickelates are an emerging class of superconductors, where the Ni¹⁺ valence state in a square planar NiO₂ coordination can only be reached via topotactic reduction of the perovskite phase. However, this topotactic soft chemistry with hydrogenous reagents is still at a stage of rapid development, and there are a number of open issues, especially considering the possibility of hydrogen incorporation. Here, we study the time dependence of the topotactic transformation of LaNiO₃ to LaNiO₂ for powder samples with x-ray diffraction and gas extraction techniques. While the hydrogen content of the powder increases with time, neutron diffraction shows no negative scattering of hydrogen in the LaNiO₂ crystal lattice. The extra hydrogen appears to be confined to grain boundaries or secondary-phase precipitates. The average crystal structure, and possibly also the physical properties, of the primary LaNiO₂ phase are, therefore, not noticeably affected by hydrogen residues created by the topotactic transformation.

1 Introduction

Superconductivity exists in various hydrogen-containing compounds, highlighted by the recent discoveries of critical temperatures T_c as high as room temperature for hydride compounds formed under extreme pressures [1, 2]. Metal hydroxide–intercalated iron chalcogenides, such as (Li_0.8Fe_0.2)OHFeSe [3], show coexistence of antiferromagnetic order and superconductivity, a feature known from some high-T_c superconductors [4]. Electron-doped 1111 iron pnictides, such as RFeAsO_1−xH_x [5, 6], CeFeAsO_1−xH_x [7], and the pnictogen-free LaFeSiH [8] are a class, where the introduction of charge carriers via doping with hydrogen drives the system from an antiferromagnetically ordered state toward superconductivity. In layered sodium cobalt oxyhydrate, Na_xCoO₂·H₂O [9], superconductivity with a similar hole/electron-doping behavior by chemical substitution is observed as in the cuprate high-T_c superconductors [10].

For IL nickelates, a close relation and possible analogy to cuprate superconductors was suggested already in 1999 [11], and since the first discovery of superconductivity in the IL nickelate (Nd,Sr)NiO₂ [12], the observation of superconductivity has been confirmed [13–15] and extended to (Pr,Sr)NiO₂ [16], (La,Sr)NiO₂ [17], (La,Ca)NiO₂ [18], and Nd₆Ni₅O₁₂ [19]. Furthermore, a recent work reported superconductivity not only for films grown on SrTiO₃ substrates but also on LSAT [20], which provides enough evidence to consider thin-film nickelates as a novel class of superconductors. The possible presence of topotactic hydrogen in IL nickelates, which depends on the rare-earth ion and/or epitaxial strain [21, 22], was proposed in theoretical studies [23, 24] and might have substantial influence on the electronic and magnetic properties of the IL nickelates, as LaNiO₂H would realize a two-orbital Mott insulator [23]. A hint toward the possibility of hydrogen incorporation was provided by an early study of topotactically reduced NdNiO₃ films, which showed an oxyhydride NdNiO_3−xH_y phase with a defect-fluorite structure in the surface region [25]. Furthermore, topotactic hydrogen can be found in SrTiO₃ thin films [26], that is, the material that is commonly used as a substrate and capping layer for IL nickelate films, which again provides a possible route for inclusion of hydrogen in infinite-layer nickelate thin films. While superconductivity has remained elusive in IL nickelates in bulk form, with studies on powder samples reporting insulating behavior [27, 28], a first step toward superconductivity was taken by our recent investigation of (La,Ca)NiO₂ single crystals [29], where metallicity was observed.

In this study, we address the issue of possible hydrogen incorporations in IL nickelates by examining bulk LaNiO_3−xH_y powder samples with a combination of x-ray and neutron diffraction studies, gas extraction, and complementary high-pressure synthesis attempts of LaNiO₂H.

2 Methods

LaNiO₃ powder samples with grain sizes of 0.5 μm were synthesized via the citrate–nitrate method as described in Ref. [30]. The method is optimal as the relatively small grains enhance the surface to bulk ratio, reducing reduction times. Moreover, higher purity can be reached than by high-pressure powder synthesis. The IL phase LaNiO₂ can be obtained solely through topotactic reduction of the perovskite LaNiO₃ phase, which is here achieved by using CaH₂ as the reducing agent [27, 28]. We reduced 50 mg of LaNiO₃ powder wrapped in aluminum foil, spatially separated from 250 mg CaH₂ powder at 280°C, as described in detail in Ref. [30], for various times.

We measured the stoichiometry including the hydrogen content with a combination of inductively coupled plasma mass spectroscopy (ICP-OES) and gas extraction; the former with a SPECTRO CIROS CCD and the latter with an Eltra ONH-2000 analyzer. For the determination of oxygen and hydrogen content, the powder samples were placed in a Ni crucible and clipped and heated, where the carrier gas takes the oxygen and hydrogen out of the sample. The oxygen reacts with carbon, and CO₂ is detected in an infrared cell, while hydrogen is detected by a thermal conductivity cell. Each measurement is repeated three times and compared to a standard. The quoted error bars provide the statistical error.

Powder x-ray diffraction (PXRD) data were collected using a Rigaku MiniFlex with a Cu K_α tube at room temperature. Neutron diffraction was performed at the high-resolution powder neutron diffractometer HRPT [31] at the Spallation Neutron Source SINQ at the Paul Scherrer Institute in Villigen. For the HRPT experiments, an amount of 340 mg of LaNiO₂ was enclosed into a vanadium can with an inner diameter of 6 mm, where the remaining space of the vanadium can was shielded with Cd foil, and the measurement was carried out at 1.5 K in a ⁴He bath cryostat with a neutron wavelength of 1.15 Å.

3 Results

As a first step, we checked the purity and stoichiometry of our starting material in the perovskite phase. In PXRD, we found no detectable impurities, and ICP combined with measurements of gas extraction indicated a starting stoichiometry of La_0.99(1)Ni_0.99(1)O_2.99(3)H_0.005(2). We studied the reduction progress with time in detail by preparation of several ampules and extracting samples after varying reduction times and analyzed all the products with PXRD. We noticed that the progress of the reduction depends on the purity and sample amount and grain size for a given time as the reduction process is surface-dependent. Motivated by these observations, we carried out a standardized reduction on the same batch of perovskite precursors. After 1 day of reduction in the described process (see Section 2) we found a full structural transition to LaNiO_2.5 (see Figure 1). Our Rietveld refinement of the LaNiO_2.5 crystal structure is in good agreement with previous reports on reduced powder [32–34]. Notably, reduction times of 1 day and longer are in stark contrast to thin-film samples, which can be fully reduced to the IL phase within hours [12–18, 35, 36], presumably due to the enhanced thickness of our grains, with sizes of 0.5 μm as extracted from scanning electron microscopy (SEM) images. The reduction appears to be happening in domains, as we found cluster spin glass behavior in a detailed magnetic characterization [30], and thus after longer reduction times than 1 day, we observe phase mixtures of LaNiO_2.5 in P2₁/n (#14) [33] and LaNiO₂ in P4/mmm (#123) [37], with a slowly increasing fraction of the LaNiO₂ phase as a function of time (see Figure 2). Extracting the weight percentages from our refinement, we present the reduction progress for the CaH₂ reduction in analogy to a thermogravimetry (TG) curve, which is shown in Figure 2. The reduction process follows an exponential decay, which we also observed in thermogravimetry studies in hydrogen gas flow (not shown here). After a reduction time of approximately 316 h, the crystal structure can be refined assuming a single phase of IL LaNiO₂ (Figure 2, and Ref. [30]). Notably, while the structural transition does not progress further on an exponential time scale, a small amount of apical oxygen remains in the crystal lattice, as will be revealed by neutron diffraction below. However, for even longer reduction times, the sample begins to decompose, forming Ni and La₂O₃, which becomes clearly visible in PXRD after 400 h (Figures 1, 2).

FIGURE 1

FIGURE 1. Powder x-ray diffraction (PXRD) of reduced LaNiO₃ after different reduction times. Data were measured at T = 300 K with Cu K_α radiation. Curves are offset in vertical direction for clarity. The 0- and 316-h data are reproduced from Ref. [30].

FIGURE 2

FIGURE 2. Oxygen and hydrogen content of reduced LaNiO₃ after different reduction times. Red symbols correspond to the oxygen content x in LaNiO_3−x, extracted from Rietveld refinements of the PXRD data shown in Figure 1. The blue datapoint is extracted from Rietveld refinement of our neutron diffraction data shown in Figure 3. The solid red line is a guide to the eye. The black symbols in the inset correspond to the hydrogen content y in LaNiO_3−xH_y, obtained from gas extraction. The solid black line is a guide to the eye.

In the inset of Figure 2, we plot the hydrogen content in the resulting powder samples versus reduction time, obtained by a gas extraction method. While the initial perovskite nickelate shows a hydrogen content of 0.005(2) wt%, we find increase of hydrogen with time (opposite to the oxygen content), with 0.065(3) wt% for 24 h, 0.094(3) wt% for 38 h, 0.015(5) wt% for 240 h, and finally 0.169(7) wt% after 320–376 h. Via gas extraction, we find an oxygen content of 14.1(2) wt% after 316 h and 14.0(2) wt% after 320–376 h of reduction, where 14 wt% corresponds to a full reduction to the LaNiO₂ IL phase. If we assume that the amount of hydrogen would be incorporated into the average crystal structure, the corresponding effective stoichiometry of the sample investigated with neutrons would be La_0.99(1)Ni_0.99(1)O_2.00(2)H_0.39(2), where La and Ni are determined via ICP. However, a first hint that this might not be the case comes from the presence of small amounts of elemental Ni in the PXRD of the 400-h sample (Figure 1), which is known to trap hydrogen [38]. The Rietveld refinement of the neutron diffraction data below also indicates a small amount of Ni. Furthermore, signatures of Ni precipitates (below the detection threshold of our PXRD) were also detected in powders after shorter reduction times since they give rise to ferromagnetic contributions in the magnetic signal [30]. The amount of Ni likely increases with reduction time, which could explain the increasing capability of incorporating hydrogen.

As a next step, we performed high-resolution neutron diffraction experiments on 340 mg of a sample prepared by mixing batches that had been subject to reduction times of 320, 350, and 376 h, respectively which are all in the range just before the start of the clear decay shown in Figure 2. The obtained neutron diffraction pattern can be well refined (see Figure 3) assuming LaNiO₂ in the IL P4/mmm structure (see Table 1), with a c-axis parameter of 3.3588(3) Å, which is lower than previously reported values [29, 30, 37], and 1.1(2) wt% of elemental Ni was included as a secondary phase.

FIGURE 3

FIGURE 3. Neutron powder diffraction. The data are taken from mixed samples reduced in the range of 320–376 h. The Rietveld refinement includes the LaNiO₂ phase and a Ni minority phase. Data were collected at 1.5 K with a neutron wavelength of 1.15 Å.

TABLE 1

TABLE 1. Refined atomic coordinates of LaNiO₂ in tetragonal space group P4/mmm with preferred orientation extracted from powder neutron diffraction data at 1.5 K (Figure 3).

Figure 4 shows a Fourier map of the scattering density resulting from the Rietveld refinement of the neutron data, where the range of the color scheme is chosen to focus on the negative scattering length of hydrogen. While the used reduction temperatures (see Section 2) are too low to enable significant mobility of the La and Ni ions, the purpose of our topotactic treatment is to reduce oxygen, which becomes mobile and is extracted by reaction with hydrogen. We, therefore, focus on the scattering lengths of the two elements O and H. According to the theoretical prediction of Ref. [23], intercalation of H in IL nickelates is most favorable on the vacant apical oxygen sites with Wyckoff position (1b), or on interstitial positions between the rare-earth ions (Wyckoff positions (2e) and (1c)). Another theoretical work [24] predicts that NdNiO₂ is unstable, while NdNiO₂H is not and also proposes the apical oxygen site with Wyckoff position (1b) for hydrogen. However, as shown in the Fourier maps of Figure 4, we find no significant negative scattering, which would appear as blue in our plots. The slightly negative scattering realized as a ring around a reflex, here in close proximity to La, is an artifact from the Fourier transformation, typically seen around larger ions [39] due to a signal cutoff from a finite q range. This is best seen in the Fourier maps Figures 4E,F. We note that the data can be refined with similar reliability factors as in Table 1 by putting a mixture of O and H on Wyckoff position (1b) and constraining their occupation. As the positive scattering observed at this site could be a mixture of negative scattering of H and positive scattering of O, which effectively clouds the negative scattering. Realized in a substitution series with LaNiO_3−xH_x, which would yield occupations of O: 0.430(8) and H: 0.570(8) in refinements of our data, but this oxygen content is way higher than what can be found in multiple gas extraction experiments (La_0.99(1)Ni_0.99(1)O_2.00(2)H_0.39(2)). Most importantly, the scenario of LaNiO_3−xH_x would hint toward the possibility to synthesize the phase directly, similar as observed in the iron pnictides. However, such synthesis attempts have been carried out as described below and were not successful. Furthermore, in the case of iron pnictides, hydrogen substitution leads to no structural transitions [5–7], contrasting the evolution of the underlying structural transitions in LaNiO_3−x according to the PXRD data (see Figure 1) and literature [37]. Another possibility to refine mixed occupancy is by constraining an equal occupation of O and H as LaNiO₂(OH)_x, which converges to 0.18(4). However, this would rather suggest an OH molecule with typical distances around 0.84 Å, which would realize negative scattering [39] and, thus, reduces the quality of the fit and shifts the occupation again down to 0.06(1). Thus, we conclude that intercalated hydrogen is unlikely in our LaNiO_2+δ powder sample. Nevertheless, we find the presence of a small amount of residual apical oxygen in Wyckoff position (1b), which corresponds to a deviation of δ = 0.06(1) from the ideal stoichiometry.

FIGURE 4

FIGURE 4. Fourier maps of the scattering length density within the unit cell of LaNiO₂ extracted from neutron powder diffraction. (A,B) Refined unit cells with cuts corresponding to the Fourier maps in (C,D), with the intensity range focused on the negative scattering length (yellow to blue), with −3.74 fm corresponding to hydrogen [40]. The positive scattering (orange to red) is capped at 1 (with 5.8 fm corresponding to oxygen). The black lines mark the isosurface lines with zero scattering intensity. The theoretically proposed hydrogen positions [23, 24] are highlighted with blue dashed circles. (E,F) Fourier maps are shown as a 3D colormap.

In addition, we attempted direct synthesis approaches of LaNiO_3−xH_x as, for example, LaNiO₂H via a Walker-type high-pressure synthesis in NaCl crucibles. We mixed La₂O₃ and NiO with NaH or CaH₂ and heated it to 650–1000°C under a pressure of 5–7 GPa, but were unable to synthesize any oxyhydride nickelate. Instead, we obtained La(OH)₃ and Ni, which is in contrast to related cases, such as SmFeAsO_1−xH_x [5], BaCrO₂H [41], BaScO₂H [42], and SrVO₂H [43].

4 Conclusion

In summary, we have synthesized high-quality LaNiO_2+δ powders and found some density of residual oxygen (δ = 0.06(1)) even at the final stage of the topotactic reaction, just prior to decomposition of our samples. While our gas extraction method reveals partial hydrogen inclusions in the powder samples, high-resolution neutron diffraction refinements show that there is no clear topotactic hydrogen in LaNiO₂, and direct attempts to synthesize LaNiO_3−xH_x were unsuccessful. The hydrogen detected by gas extraction could be trapped in Ni impurities that increase with increasing reduction time and/or ascribed to phenomena at surfaces or grain boundaries, such as water adsorption/intercalation [44] on powder surfaces or partial formation of LaNiO_3−xH_x on the nanometer scale [25]. Notably, a relatively high density of grain boundaries/crystallographic defects was reported for IL nickelate thin films [45], and also surface effects can play a more decisive role in films. Thus, future studies clarifying the presence and impact of hydrogen in nickelate thin-film samples are highly desirable.

Data Availability Statement

The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.

Author Contributions

PP, MI, BK, and MH conceived the project. RO carried out the topotactic reductions. PP and VP conducted the neutron diffraction experiments. SH executed the ICP and gas extraction measurements. PP analyzed the data and prepared the manuscript with input from all authors.

Funding

We acknowledge financial support by the Center for Integrated Quantum Science and Technology (IQ^ST) and the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation): Projektnummer 107745057—TRR 80. The Max Planck Society is acknowledged for funding of the open-access fee.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

We thank R. Merkle and A. Fuchs for the synthesis of LaNiO₃ and H. Hoier for preliminary PXRD characterizations. We acknowledge PXRD measurements by C. Stefani from the X-ray Diffraction Scientific Facility at an early stage of this work. The use of facilities of the Quantum Materials Department of H. Takagi for the attempted high-pressure synthesis of LaNiO₂H is gratefully acknowledged.

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