[Paper Review] Synthesis and Characterization of Ca-Substituted Infinite-Layer Nickelate Crystals

Pascal Puphal, Yu‐Mi Wu+10 more|arXiv (Cornell University)|Jun 24, 2021

Chemical and Physical Properties of Materials1 references4 citations

TL;DR

This study synthesizes high-quality single crystals of Ca-substituted infinite-layer nickelates via high-pressure synthesis and topotactic oxygen reduction. The resulting crystals exhibit metal-like electrical conductivity and electronic structure similar to hole-doped thin films, demonstrating macroscopic single-crystalline samples suitable for probing superconductivity in heavily doped systems.

ABSTRACT

Rare-earth nickelates with the infinite-layer crystal structure have been synthesized in thin film and powder form via topotactic oxygen reduction of the perovskite phase. The infinite-layer phase exhibits remarkable properties, such as superconductivity and magnetic excitations with extraordinarily large bandwidth. Yet, superconductivity was exclusively reported for infinite-layer nickelate films, while polycrystalline powder samples of similar composition were insulating at all measured temperatures. Here, a high-pressure method was used to synthesize high-quality single crystals of the perovskite nickelate La$_{1-x}$Ca$_{x}$NiO$_3$ that were subsequently reduced to the infinite-layer phase La$_{1-x}$Ca$_{x}$NiO$_{2+\delta}$. The obtained samples were characterized by X-ray diffraction, electron microscopy, Raman spectroscopy, magnetometry, and electrical transport measurements. Notably, the metal-like electrical conductivity of the infinite-layer crystals is reminiscent of weakly hole-doped infinite-layer thin films. Moreover, local electron energy-loss spectroscopy reveals close similarities between the electronic structures of the crystals and thin films. This work demonstrates the realization of infinite-layer nickelate crystals with macroscopic size as well as superior crystalline quality, and paves the way for future studies exploring whether more heavily Ca-substituted crystals host superconductivity in analogy to sufficiently hole-doped films.

Motivation & Objective

• To overcome the lack of macroscopic single crystals of infinite-layer nickelates for systematic electronic property studies.
• To investigate whether Ca-substituted infinite-layer nickelates can exhibit superconductivity, as seen in hole-doped thin films.
• To establish a high-pressure synthesis route for high-quality perovskite precursor crystals followed by topotactic reduction to the infinite-layer phase.
• To compare the electronic structure and transport properties of single crystals with those of thin films using advanced spectroscopic and structural techniques.

Proposed method

• High-pressure solid-state synthesis was used to grow single crystals of the perovskite phase La₁₋ₓCaₓNiO₃.
• Topotactic oxygen reduction was applied to convert the perovskite phase into the infinite-layer phase La₁₋ₓCaₓNiO₂₊δ while preserving crystal structure.
• X-ray diffraction and electron microscopy were used to confirm phase purity and crystalline quality.
• Raman spectroscopy provided vibrational characterization of the crystal structure.
• Magnetometry and electrical transport measurements assessed magnetic and electronic behavior.
• Local electron energy-loss spectroscopy (EELS) was employed to compare the electronic structure between crystals and thin films.

Experimental results

Research questions

• RQ1 Can high-quality single crystals of Ca-substituted infinite-layer nickelates be synthesized using high-pressure methods?
• RQ2 Do these single crystals exhibit metal-like electrical conductivity similar to hole-doped infinite-layer thin films?
• RQ3 How do the electronic structures of the synthesized crystals compare to those of epitaxial thin films?
• RQ4 Is there evidence of superconductivity in heavily Ca-doped infinite-layer nickelate crystals, as observed in thin films?
• RQ5 Can the crystalline quality and macroscopic size of these materials enable future studies on unconventional superconductivity?

Key findings

• High-quality single crystals of the perovskite phase La₁₋ₓCaₓNiO₃ were successfully synthesized under high pressure.
• Topotactic oxygen reduction yielded phase-pure infinite-layer crystals of La₁₋ₓCaₓNiO₂₊δ with excellent crystallinity.
• Electrical transport measurements revealed metal-like conductivity in the infinite-layer crystals, analogous to weakly hole-doped thin films.
• Local electron energy-loss spectroscopy showed close electronic structure similarities between the single crystals and epitaxial thin films.
• The macroscopic size and superior crystalline quality of the synthesized samples enable future studies on electronic properties, including potential superconductivity.
• The results suggest that more heavily Ca-substituted crystals may host superconductivity, similar to sufficiently hole-doped thin films.