High-pressure synthesis and physical properties of the single-layer nickelate oxychalcogenides Ba2NiO2Cu2X2 (X=S and Se)

Abstract

New single-layer nickelate oxychalcogenides Ba2NiO2Cu2X2 (X=S and Se) were successfully synthesized by using a high-pressure high-temperature method. They crystallize into the tetragonal space group I4/mmm with alternating NiO2 square planar and Cu2Se2 antifluorite layers along the c direction. High-pressure synchrotron x-ray diffraction manifests that the Ba2NiO2Cu2Se2 can be fairly compressed along the c direction, whereas the ab plane is quite rigid, corresponding to its layered feature. The valence states of Ni2+ and Cu1+ were validated from both bond valence sums and x-ray absorption spectroscopy. The magnetization of Ba2NiO2Cu2S2 follows the Curie-Weiss law, whereas a spin-glass-like behavior emerges below Tsg=5 K. On the other hand, Ba2NiO2Cu2Se2 experiences a low-dimensional antiferromagnetic transition at TN=200 K. The resistivity of both compounds follows the thermal activation model at elevated temperatures and follows the three-dimensional variable-range hopping model at low temperatures, indicating the semiconducting or insulating nature. DFT+U calculations indicate that weak antiferromagnetic coupling in the NiO2 layer suppresses the Fermi surface, thereby opening a gap in the electronic structure. The electrical resistance gradually decreases under high pressure, no superconductivity was observed up to 21.4 GPa. This work provides an insight to explore new layered nickelates with emergent physical properties and potential superconductivity.

Source-reported materials — not catalogue approval

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