Under High Pressure

 Recently, La₃Ni₂O₇ was reported to show superconductivity at 80 K under pressure of 15 GPa [1]. La₃Ni₂O₇ is a bilayer Ruddlesden-Popper phase nickelate that have two layers of NiO₂ plane in its unit cell, while there is trilayer family compound: La₄Ni₃O₁₀. As there had been no paper yet that reports superconductivity in La₄Ni₃O₁₀, we examined the possibility of superconductivity in La₄Ni₃O₁₀ under high pressure [2].

 The polycrystalline sample was synthesized via solid-phase reaction and hot isostatic pressing process from La₂O₃ and NiO, and characterized by powder X-ray diffraction and thermogravimetry to find it to be single phase La₄Ni₃O_9.99. We used diamond anvil cell with boron-doped diamond electrodes designed for four-terminal resistance measurement under high pressure [3]. Cubic boron nitride powder was used as a pressure-transmitting medium.

 The synthesized La₄Ni₃O_9.99 displayed metallic behavior around room temperature in all measured pressures. As we applied the pressure, a drop in resistance suddenly appeared below 5 K at 32.8 GPa. The temperature where the drop appears tended to increase by increasing the pressure beyond 46.2 GPa. Measurements of electrical resistance under magnetic fields were also conducted at 69.4 GPa, where the drop in electrical resistance was suppressed by magnetic field. Therefore, the observed decrease in electrical resistance is most likely to be the result of a superconducting transition in La₄Ni₃O_9.99.