Superconductivity up to 243 K in the yttrium-hydrogen system under high pressure

The discovery of superconducting H₃S with a critical temperature T_c∼200 K opened a door to room temperature superconductivity and stimulated further extensive studies of hydrogen-rich compounds stabilized by high pressure. Here, we report a comprehensive study of the yttrium-hydrogen system with the highest predicted T_cs among binary compounds and discuss the contradictions between different theoretical calculations and experimental data. We synthesized yttrium hydrides with the compositions of YH₃, YH₄, YH₆ and YH₉ in a diamond anvil cell and studied their crystal structures, electrical and magnetic transport properties, and isotopic effects. We found superconductivity in the Im-3m YH₆ and P6₃/mmc YH₉ phases with maximal T_cs of ∼220 K at 183 GPa and ∼243 K at 201 GPa, respectively. Fm-3m YH₁₀ with the highest predicted T_c> 300 K was not observed in our experiments, and instead, YH₉ was found to be the hydrogen-richest yttrium hydride in the studied pressure and temperature range up to record 410 GPa and 2250 K.