Abstract
The discovery of superconducting H(3)S 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(c)s among binary compounds and discuss the contradictions between different theoretical calculations and experimental data. We synthesized yttrium hydrides with the compositions of YH(3), YH(4), YH(6) and YH(9) 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 YH(6) and P6(3)/mmc YH(9) phases with maximal T(c)s of ∼220 K at 183 GPa and ∼243 K at 201 GPa, respectively. Fm-3m YH(10) with the highest predicted T(c )> 300 K was not observed in our experiments, and instead, YH(9) was found to be the hydrogen-richest yttrium hydride in the studied pressure and temperature range up to record 410 GPa and 2250 K.