Atomic origin of absent superconductivity in bulk infinite-layer nickelate

doi:10.21203/rs.3.rs-3607723/v1

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Atomic origin of absent superconductivity in bulk infinite-layer nickelate

https://doi.org/10.21203/rs.3.rs-3607723/v1

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The recent discovery of superconductivity in infinite-layer nickelate films has sparked significant interest and expanded the realm of superconductors, in which the infinite-layer structure and proper chemical doping are both of the essence. Nonetheless, the absence of superconductivity in bulk infinite-layer nickelates remains puzzling. Herein, we investigate atomic defects and electronic structures in bulk infinite-layer Nd_0.8Sr_0.2NiO₂ using scanning transmission electron microscopy. Our observations reveal the presence of three-dimensional (3D) block-like structural domains resulting from intersecting defect structures, disrupting the continuity within crystal grains, which could be a crucial factor in giving rise to the insulating character and inhibiting the emergence of superconductivity. Moreover, the infinite-layer structure, even without complete topotactic reduction, retains interstitial oxygen atoms on the Nd atomic plane in bulk nickelates, possibly further aggravating the local distortions of NiO₂ planes and hindering the superconductivity. These findings shed light on the existence of structural and atomic defects in bulk nickelates and provide valuable insights into the influence of proper topotactic reduction and structural orders on superconductivity.

Physical sciences/Materials science/Condensed-matter physics/Superconducting properties and materials

Physical sciences/Physics/Condensed-matter physics/Surfaces, interfaces and thin films

The emergent superconductivity in nickelates marks pivotal progress in the exploration of superconducting materials and mechanisms^1–3. In infinite-layer thin films, i.e. Nd_0.8Sr_0.2NiO₂ prepared by topotactic reduction using CaH₂, the NiO₂ layers hold similar atomic configuration and electronic characteristics to that in cuprates^4–11. Particularly, the superconductivity in nickelates is highly sensitive to subtle changes in chemical composition and crystal structures. For example, the infinite-layer nickelates R_1 − xA_xNiO₂(R = La, Pr, Nd, and A = Sr, Ca, Eu) could exhibit superconducting dome regions only under proper doping concentrations^12–19. Recent studies have confirmed the crucial role of hydrogen “impurity” in superconductivity, rendering the electron structure to be more quasi-2D-like²⁰. The superconducting characteristics precisely depend on 3d electron filling that is from d^8.75 (Ni^1.25+) to d^8.88 (Ni^1.12+), so that the emergent superconductivity is just obtained in the T′-type phase (R_n+1Ni_nO_2n+2, R is rare-earth) with the outer-shell electron filling of d^8.8 for Ni (n = 5) through the reduced dimensionality^21–24.

Moreover, the different ionic radii of doping will lead to the reduction of structural stability, making it challenging to synthesize pure-phase precursor Nd_0.8Sr_0.2NiO₃^{19, 25, 26}. Consequently, the Ruddlesden-Popper (RP, R_n+1Ni_nO_3n+1, R is rare-earth) phases are always easily introduced as observed in thin films^27–29. The layered square-planar T′-type phase (R_n+1Ni_nO_2n+2) is therefore obtained via oxygen deintercalation from the parent RP phase³⁰. More recently, the RP phase La₃Ni₂O₇ has even been found to be superconducting with a T_c of 80 K under pressure³¹. These extremely complicated phenomena manifest the strong correlation between local atomic structure and superconductivity in nickelates. Although a few studies of Nd_0.8Sr_0.2NiO₂ thin films have unveiled the interfacial reconstruction to stabilize the external crystal structure and alleviate the polar discontinuity^32–35, the absence of superconductivity in bulk nickelates remains elusive^36–38.

Herein, using integrated differential phase contrast (iDPC) and electron energy-loss spectra (EELS) combined with state-of-the-art aberration-corrected scanning transmission electron microscopy (STEM), the microstructure and atomic structure of polycrystalline Nd_0.8Sr_0.2NiO₂ are resolved to unveil the origin of absent superconductivity in bulk nickelate. A large number of structural defects and atomic distortions were found therein to contribute to a three-dimensional (3D) block-like structure, causing the breakdown of connections within the grains. The impurity fluorite-layers are unexpectedly discovered across the T′-type phase boundaries, attributing to the impurity of reducing agent CaH₂. Furthermore, iDPC-STEM imaging reveals the residual O atom at the apical of NiO₆ octahedra after topotactic reduction, introducing additional distortions in infinite layers. The effects of structural defects on the transport property and superconductivity of polycrystalline Nd_0.8Sr_0.2NiO₂ are discussed.

Structural defects in bulk Nd_0.8Sr_0.2NiO₂

The precursor Nd_0.8Sr_0.2NiO₃ was synthesized via a two-step high-temperature and high-pressure solid-state synthesis method, followed by topotactic reduction using CaH₂ (Aladdin, 98.5%, Mg༜1%) at low temperature to obtain the polycrystalline bulk Nd_0.8Sr_0.2NiO₂ with the average grain size of 500 nm³⁶. The Energy Dispersive X-ray Spectra (EDS) mappings at low magnification show the uniform elemental distributions of Nd, Ni, O, and doped Sr in the polycrystalline in Fig. 1a. Figure 1b illustrates the temperature dependence of resistivity for bulk Nd_0.8Sr_0.2NiO₂ under ambient pressure. Notably, it exhibits strong insulating behavior over the temperature range of 2 to 300 K, indicating the absence of superconductivity in bulk Nd_0.8Sr_0.2NiO₂.

The magnifying high-angle annular dark-field (HAADF-STEM) image in Fig. 1c reveals the presence of numerous stripes within the Nd_0.8Sr_0.2NiO₂ grains. Further insight into the interior grains gives clear stripe contrast in Figs. 1d and 1e. Some of these stripes are arranged in parallel to [010] direction (marked by a yellow box in Fig. 1c), while the others exhibit a vertically staggered distribution (marked by a red box in Fig. 1c). The selected area electron diffraction (SAED) displays the elongated diffraction spots extending along the [001] direction in the inset of Fig. 1d, while the diffraction spots are elongated both along [010] and [001] directions in Fig. 1e, suggesting the formation of 90° interleaved stripe configurations.

Subsequently, a detailed analysis of the stripe-like structure is conducted. Atomic resolution HAADF-STEM and iDPC-STEM with the capability of imaging heavy and light elements simultaneously are used to identify these phases as the T′-type (R_n+1Ni_nO_2n+2, R is rare-earth) structure, which is classified by the number of NiO₂ layers n. As illustrated in Fig. 2a, taking n = 3 (R₄Ni₃O₈) as an example, the apical oxygen of NiO₆ octahedra in the parent RP phase (R₄Ni₃O₁₀) is deintercalated after the topotactic reduction. This conversion changes the RO planes by sole element R, accompanied by the transformation of NiO₆ octahedra into NiO₂ square-planar. Moreover, the pristine rock salt layers are changed into the fluorite structure after topotactic reduction, with additional oxygen atoms inserted between two R layers as indicated by the red spheres in Fig. 2a. Figure 2b displays a typical iDPC-STEM image of the T′-type phase, highlighting the formation of fluorite slabs and infinite NiO₂ layers. The inserted oxygen layer is indicated by orange arrow. The HAADF-STEM image in Fig. 2c reveals that the T′-type phases with different layers number n are alternatively arranged along the c axis separated by the fluorite layers. The intensity profile across the interface is plotted in Fig. 2d. The lattice spacing is reduced at the fluorite layers and the interface is well resolved from the geometrical phase analysis (GPA) in Figs. 2e and 2f. The compressive strain ε_yy reaches a negative maximum value at the fluorite layers. In addition, the intensity of R atom columns near the fluorite layers is relatively weaker than that in infinite layers, as indicated by the purple stars in Fig. 2d, implying the changed chemical composition according to the Z contrast character of HAADF-STEM imaging. In consideration of the co-occupying Sr and Nd atoms at R sites, the decreased intensity reveals a higher ratio of Sr/Nd localized at the fluorite layers.

Chemical impurities in bulk Nd_0.8Sr_0.2NiO₂

Furthermore, high resolution STEM-EDS measurements were carried out to analyze the element distributions in Figs. 3a and 3b. The deficiency of Ni at the fluorite layers can be easily understood according to the atom model in Fig. 2a. Although the Nd seems to have a relatively uniform distribution, Sr is enriched at the fluorite layers, consistent with the increased ratio of Sr/Nd and thus the decreased contrast of HAADF-STEM image in Figs. 2c and 3a. The doped Sr plays a vital role in the multi-band properties and the minimum energy of infinite-layer nickelate, and the hole carriers provided by Sr are necessary for the superconductivity¹². Moreover, the element F is unexpectedly observed at the fluorite layers, accompanied by a deficient oxygen distribution in Fig. 3b. It is noteworthy that the element F is not involved during the whole topotactic reduction process. We suppose that it might come from the impure CaH₂ as the precursor of CaF₂ could be introduced even with a small amount. It is further confirmed by the X-ray photoelectron spectra (XPS) measurements of CaH₂ in Fig. S1. Therefore, the chemical compositions of ingredients and target samples should be carefully examined before and after the topotactic reduction, as the unexpected impurities might play a significant role in superconductivity, such as the impurity H as discovered before²⁰. The replacement of O by F atoms could change the valence state of Ni due to the strong electronegativity of F. Meanwhile, the fluorite layer might decouple the NiO₂ layers and suppress the hybridization between the c-axis as discussed in the case of La₂NiO₃F ^39–41.

Moreover, atomic resolution EELS elemental mappings show similar distributions to the EDS results in Fig. S2. The EELS near-edge fine structures of O K, F K, Ni L, and Nd M edges were measured both for the infinite-layer phase and T′-type phase, as depicted in Fig. 3c. The O K edge shows almost no changes between the two phases. The distinct K edge of F appears at around 685 eV for fluorite layers, further evidencing the existence of impurity F. Particularly, F only replaces oxygen at the fluorite layers and it is not discovered in the infinite NiO₂ layers. Although the fine structures of the Nd M edge show no difference, the prominent increased intensity of the L₃ edge for Ni is observed for T′-type phase. The larger ratio of L₃/L₂ corresponds to the higher valence of Ni. The resultant valence state of Ni leads to the modified 3d-orbital electron filling and thus non-superconducting states according to the superconducting phase diagram^{23, 42}. As the infinite-layer structure and the proper concentration of doped Sr are essential to superconductivity, the presence of various T′-type phases with such imperfects would probably lead to the insulating and non-superconducting characters in nickelates^{25, 36, 43}.

3D block-like configurations

In addition to the T′-type phases arranged vertically to the c-axis (parallel stripes), the stripes parallel to the c-axis (vertical stripes) are also formed as shown in Figs. 1e and 4a, highly intersected with the parallel ones. Figure 4b shows an enlarged iDPC image of the vertical stripes and the atomic structure model is displayed on the upper side. In contrast to the T′-type phases with additional oxygen/fluorine layers, the vertical stripes are formed just by a \(\frac{1}{2}\)c shift of the infinite-layer phase (left) with respect to the other one (right) along the [001] axis, similar to the stacking fault structure. The vertical and parallel stripes can be clearly distinguished by GPA in Figs. 4c and 4d. These two types of defects are coherent with the infinite-layer phase, finally forming a 3D block-like configuration in the bulk nickelates.

To clarify the 3D block-like configurations clearly, the joint region is shown in Fig. 4e with the sample oriented along [001] direction. The boundaries are indicated by the green dotted lines. The relatively uniform contrast at each atomic column for Area 2 indicates two overlapping regions along [001] direction with a translation vector of \(\frac{1}{2}\left[110\right]\), in contrast to that in Areas 1, 3, and 4. Based on the structural analysis along [001] direction above, the 3D atom model is built in Fig. 4f. When viewed along [001] direction, the intersecting domain structures are consistent with the observations of the central region in Fig. 4e. We also compared the EELS fine structure with that of the infinite-layer phase in Fig. S3, no obvious difference is observed for O K, Ni L and Nd M edges, further confirming that the regions are still the infinite-layer phases. The polycrystalline Nd_0.8Sr_0.2NiO₂ are not only segmented by the grain boundaries but also disrupted by the stripe domains as schematically shown in Fig. 4g. These stripe structures are presented with the modified chemical composition and electronic structure within the crystal grains as discussed in Fig. 3, which could be the possible origin of insulating behavior and absent superconductivity in bulk nickelates.

Distortions in infinite layers

At last, the local lattice distortions of the infinite-layer phase are analyzed as there is always a strong correlation between different degrees of freedom in perovskite oxides⁴⁴. The soft chemical topotactic reduction reduces the perovskite RNiO₃ to the infinite layer RNiO₂ by removing the apical oxygen atoms in the distorted NiO₆ octahedron and thus generates a two-dimensional NiO₂ plane with square coordination, as schematically shown in Fig. 5a. iDPC-STEM technique was used to visualize the oxygen⁴⁵ in the infinite layers in Fig. 5b. Remarkably, there is still residual apical oxygen in Nd/Sr plane after topotactic reduction, as indicated by the orange arrows in Fig. 5c − 5e with a random distribution. To exclude the possible imaging artifacts, iDPC-STEM simulations are conducted based on the perfect atomic model of the infinite-layer in Fig. S4. No extra contrast is observed at the positions of apical oxygen atoms. Therefore, it is suggested that a few residual apical oxygen atoms are still present in the bulk, which could be caused by the insufficient topotactic reduction due to the large volume of the bulk sample, unlike the thin films only with a thickness of tens of nanometers. Besides, the strain effect in thin films could degenerate the equivalent occupancy of oxygen atoms in NiO₆ octahedron and facilitate the removal of apical oxygen more easily, in contrast to the similar oxygen coordination environment in the bulk.

The presence of residual apical oxygen would generate a large interlayer coupling between intralayer Ni cations by altering the electron energy levels³¹ and impact the flatness of the infinite NiO₂ planes by introducing local structural distortions, which could serve a crucial role in superconductivity. Hence, the detailed atom position analysis was performed to evaluate the local distortions in Fig. 5f, including the lattice parameter of c/a, the O-Ni-O bond angle, and the deviations of Ni atoms from the center of four-nearest Nd/Sr. The c/a ratio exhibits a wide fluctuation ranging from 0.81 to 0.99 in Fig. 5g, indicating the nonuniform structural distortions. The values c/a measured on a large number of iDPC-STEM images are statistically plotted in Fig. 5h, leading to the average value of approximately 0.886 ± 0.021, comparable to the value of 0.86 in thin films at the same concentration of Sr doping¹.

The displacement of Ni atoms is plotted here in Fig. 5i with yellow arrows, and the contour map of the bond angle is overlaid together. Obviously, the bond angles O-Ni-O are largely deviated from the 180°, thereby suppressing the flatness of superconducting NiO₂ layers. The regions with large distortions of the NiO₂ plane correspond to a large displacement of Ni as indicated by arrows in Fig. 5i. The disorders within the NiO₂ layers are consistent with previous X-ray analysis of bulk polycrystalline samples with broaden diffraction peaks³⁶. These observations suggest the complicated distortions in Ni-O planes for polycrystalline Nd_0.8Sr_0.2NiO₂ after chemical reduction. Referring to the widely observed superconducting characters in nickelate thin films, it could therefore be inferred that the strain could suppress the local distortions and potentially flatten the NiO₂ planes, which might be necessary for accommodating the superconductivity but absent in its bulk form^46–49.

In summary, the microstructure and atomic structure of polycrystalline nickelates were thoroughly analyzed by scanning transmission electron microscopy to reveal the origin of absent superconductivity in its bulk form. A large number of T′-type phases and stacking-fault-like defects are found to form the 3D block-like configurations, disrupting the continuity within crystal grains. The impurity F is unexpectedly discovered to replace the oxygen atoms in the fluorite layers. The fluorite layers in the T′-type phases are found to be Sr-rich and F-rich, leading to the change of valence state of Ni accordingly. Moreover, the residual oxygen atoms were observed in the Nd/Sr plane due to the incomplete topotactic reduction, which could distort the local structure and hinder the flatness of the NiO₂ plane. These structural defects and local distortions in infinite layers could be responsible for the insulating and non-superconducting characters in bulk nickelates. Our findings highlight the importance of proper topotactic reduction and structural order to superconducting properties and suggest the possible origin of absent superconductivity in nickelates.

Sample synthesis

To synthesize polycrystalline samples of Nd_0.8Sr_0.2NiO₂, the following procedure was employed³⁶. Initially, polycrystalline precursor Nd_0.8Sr_0.2NiO₃were prepared through a two-step solid-state reaction under high pressure and high temperature conditions with KClO₄ as an excess oxygen source. The resulting Nd_0.8Sr_0.2NiO₃ samples were washed with distilled water. Subsequently, samples of Nd_0.8Sr_0.2NiO₂ were obtained via a topochemical reduction process from Nd_0.8Sr_0.2NiO₃ using CaH₂ as a reducing agent. Three molar additions of CaH₂ and one molar of Nd_0.8Sr_0.2NiO₃ were sealed in an evacuated quartz tube and subjected to heating at 280°C for 20 hours. The resulting pellets were then crushed and washed with saturated NH₄Cl in anhydrous ethanol to remove any residual CaH₂ and reacted byproduct CaO. Finally, the samples were dried in an evacuated oven to obtain the target sample of the infinite-layer phase in powder form. For resistivity measurements, the powders were pressed into a pellet and heat treated at 180°C for 10 h together with a separately pellet of CaH₂.

Preparation of transmission electron microscopy sample

The cross-sectional TEM lamellas of Nd_0.8Sr_0.2NiO₂ was thinned to electron beam transparency at 30kV by using Carl Zeiss Crossbeam 550L FIB-SEM, and then the surface amorphous layer was swept away at 2kV. The HAADF-STEM, iDPC-STEM and STEM-EELS experiments were conducted using a Thermo Fischer Scientific Titan Themis Z microscope operating at 300 kV, equipped with a probe corrector, a four segment DF4 detector, a super EDX detector and a Gatan GIF Continuum dual-EELS system. The convergence semi-angle for imaging is 25 mrad, and the collection semi-angle is 50–200 mrad for the HAADF imaging and 8–42 mrad for iDPC. The energy dispersive X-ray spectroscopy (EDS) elemental maps were recorded with probe current ~ 300 pA. The STEM-EELS experiments were performed with a probe current of 70 pA. The EELS data were acquired using a collection angle of 100 mrad with 0.03 s per pixel and a dispersion of 0.3 eV per channel.

iDPC-STEM simulations

The iDPC-STEM simulations were performed using Dr. Probe software developed by Dr. Juri Barthel. The imaging parameters are as follows: defocus value of -6 nm, accelerating voltage of 300 kV, convergence semi-angle of 25 mrad, and the collection angle ranging from 8 mrad to 42 mrad.

Acknowledgements

This work is financially supported by the National Key R&D Program of China (Grant No. 2022YFA1403200), the National Natural Science Foundation of China (No. 11874394, No. 52173215, No. 12061131001, No. A0402/11927809, No. 12204231), the National Natural Science Fund for Excellent Young Scientists Fund Program (Overseas), the Natural Science Foundation of Anhui Province for Excellent Young Scientist (2108085Y03), and the University Synergy Innovation Program of Anhui Province (No. GXXT-2020–003). The work made use of the resources of the Center for Electron Microscopy at Anhui University.

Author contributions

D.S., B.G. and H.W. supervised the project. K.H. performed the experiment. Q.L. synthesized Nd_0.8Sr_0.2NiO₂. K.H. and Y.J. performed the statistical data. Z.L. completed the image simulation. H.D. participated in guidance and discussions. K.H and D.S. wrote the manuscript with input from all authors. All authors discussed the results and contributed to the manuscript.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary information is available for this paper.

Correspondence and requests for materials should be addressed to Dongsheng Song, Binghui Ge or Hai-Hu Wen.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Atomic origin of absent superconductivity in bulk infinite-layer nickelate

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Abstract

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Introduction

Results

Structural defects in bulk Nd_0.8Sr_0.2NiO₂

Chemical impurities in bulk Nd_0.8Sr_0.2NiO₂

3D block-like configurations

Distortions in infinite layers

Methods

Sample synthesis

Preparation of transmission electron microscopy sample

iDPC-STEM simulations

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References

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Supplementary Files

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Atomic origin of absent superconductivity in bulk infinite-layer nickelate

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Results

Structural defects in bulk Nd0.8Sr0.2NiO2

Chemical impurities in bulk Nd0.8Sr0.2NiO2

3D block-like configurations

Distortions in infinite layers

Methods

Sample synthesis

Preparation of transmission electron microscopy sample

iDPC-STEM simulations

Declarations

References

Additional Declarations

Supplementary Files

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Structural defects in bulk Nd_0.8Sr_0.2NiO₂

Chemical impurities in bulk Nd_0.8Sr_0.2NiO₂