Topotactic transformation of single crystals: From perovskite to infinite-layer nickelates

Abstract

Topotactic transformations between related crystal structures are a powerful emerging route for the synthesis of novel quantum materials. Whereas most such "soft chemistry" experiments have been carried out on polycrystalline powders or thin films, the topotactic modification of single crystals, the gold standard for physical property measurements on quantum materials, has been studied only sparsely. Here, we report the topotactic reduction of La 1−x Ca x NiO 3 single crystals to La 1−x Ca x NiO 2+ using CaH 2 as the reducing agent. The transformation from the three-dimensional perovskite to the quasi-two-dimensional infinite-layer phase was thoroughly characterized by x-ray diffraction, electron microscopy, Raman spectroscopy, magnetometry, and electrical transport measurements. Our work demonstrates that the infinite-layer structure can be realized as a bulk phase in crystals with micrometer-sized single domains. The electronic properties of these specimens resemble those of epitaxial thin films rather than powders with similar compositions.

Investigation of Hydrogen Incorporations in Bulk Infinite-Layer Nickelates

“…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 LaNiO 2 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 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.…”

“…Furthermore, topotactic hydrogen can be found in SrTiO 3 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)NiO 2 single crystals [29], where metallicity was observed.…”

Dynamical structural instability and its implication on the physical properties of infinite-layer nickelates

“…Another point that we want to note is that all the above results are about pristine infinite-layer nickelates RNiO 2 , with an emphasis on SmNiO 2 . In the experiment, superconductivity emerges when RNiO 2 is hole doped with the lanthanide element R being partially substituted with an alkaline earth element (such as Sr or Ca) [63][64][65][66][67][68].…”

“…Albeit there are many important differences, one thing that all the models have in common is the P 4/mmm crystal structure of NdNiO 2 (either experimental one or DFT optimized one), based on which oneparticle band structure calculations [44][45][46][47][48][49] or more sophisticated many-body electronic structure calculations [50][51][52][53][54][55][56][57][58][59][60][61][62] are performed. On the experimental side, in addition to Sr x Nd 1−x NiO 2 , recently superconductivity is also observed in Sr x Pr 1−x NiO 2 [63][64][65], Sr x La 1−x NiO 2 [66] and Ca x La 1−x NiO 2 [67,68]. Thus it is anticipated that superconductivity should be observed in the entire lanthanide series of infinite-layer nickelates RNiO 2 .…”

Emergent flat-band physics in $d^{9-δ}$ multilayer nickelates

“…Further layering and topotactic reduction should therefore be most effective in singling out the Ni-d x 2 −y 2 orbital in a low-energy regime for some (effective) Ni + setting 8,13 . However, single crystals for the "straightforward" infinite-layer (p→∞) materials were absent until very recently 17 , and therefore focus consolidated on the multilayer systems with small p. And indeed, single crystals of the reduced p = 3 system were successfully prepared by Zhang et al 15 . Yet superconductivity was still not detected, but then eventually the thin-film realization of hole-doped infinite-layer nickelates made the breaktrough 1,2 .…”