The Phase Transition Nobody Noticed express
The nickelate family consists of layered crystals, like a stack of pancakes: nickel and oxygen layers alternate with lanthanum spacers. Two years ago, superconductivity was discovered in them — the ability to conduct electricity without resistance when cooled. But data from different labs varied so much that it seemed each was measuring a different material. A group of researchers decided to get to the bottom of it.
They grew flawless single crystals of the entire Laₙ₊₁NiₙO₃ₙ₊₁ series and put them through dozens of tests: from X-ray to magnetic measurements, from two degrees above absolute zero to a thousand degrees Celsius. And they found something that had eluded others: at high temperatures, another atomic rearrangement occurs in the crystal — not the one leading to the tetragonal phase, but a completely different one. It had gone unnoticed because it hides behind a more obvious transition.
This "quiet" phase transition explains the discrepancies in results: depending on synthesis conditions, a crystal can end up on either side of an invisible boundary. Now that we know about it, we can grow nickelates with tailored properties, and get closer to understanding high-temperature superconductivity.
Original abstract
as written by the authors arXiv:2606.27040
The discovery of superconductivity at $$15\,\mathrm{K}$$ in the infinite-layer nickelate $$(\mathrm{Nd},\mathrm{Sr})\mathrm{NiO}_2$$, followed by superconductivity at $$80\,\mathrm{K}$$ in the Ruddlesden--Popper phase $$\mathrm{La}_3\mathrm{Ni}_2\mathrm{O}_7$$, has ushered in a new era of nickelate research. Despite this progress, large discrepancies between reports exist. Here, we investigate the complete series of bulk-stable $$\mathrm{La}_{n+1}\mathrm{Ni}_n\mathrm{O}_{3n+1}$$ compounds using a comprehensive set of experimental techniques, including PXRD, single-crystal XRD, electron microscopy, heat capacity, differential scanning calorimetry, magnetic susceptibility, and transport measurements, over a broad temperature range from $$2$$ to $$1000\,\mathrm{K}$$. By studying high-quality single crystals, we identify a previously underappreciated high-temperature phase transition in Ruddlesden--Popper nickelates $$\mathrm{La}_{n+1}\mathrm{Ni}_n\mathrm{O}_{3n+1}$$ distinct from the one going to a tetragonal phase.
Superconductivity in nickelates was only discovered in 2019, even though the first nickelate was synthesized back in the 1970s — for half a century, no one thought to cool it down and check its resistance.