Experimental clathrate superhydrides EuH6 and EuH9 at extreme pressure conditions

Abstract

The recent discovery of a class of sodalitelike clathrate superhydrides (e.g., YH6, YH9, ThH9, ThH10, and LaH10) at extreme pressures, which commonly exhibit high-temperature superconductivity with the highest Tc approaching 260 K for LaH10, opened up a new era in the search for high-temperature superconductors in metal superhydrides. There is high interest in finding alternative clathrate superhydrides that might witness the long-dreamed room-temperature superconductivity. Here, we target the experimental synthesis of europium (Eu) superhydrides where theory can fail for the prediction of superconductivity. We pressurized and laser heated a mixture of metal Eu and ammonia borane (NH3BH3) in a diamond-anvil cell and successfully synthesized the clathrate structured EuH6 and EuH9 at conditions of 152 GPa and 1700 K, and 170 GPa and 2800 K, respectively. Two nonclathrate structured phases of EuH5 and EuH6 were also synthesized that are not reported in lanthanide superhydrides. Theoretical simulations predicted that all the synthesized europium hydrides are magnetic, where the electrical resistance measurements suggest a possible magnetic order transition temperature at around 225 and 258 K, respectively, for EuH5 and clathrate EuH6. Our work has created a model superhydride platform for subsequent investigations on how a strongly correlated effect and magnetism can affect the superconductivity of superhydrides.