Experimental clathrate superhydrides EuH_{6} and EuH_{9} at extreme pressure conditions

Abstract

The recent discovery of a class of sodalitelike clathrate superhydrides (e.g., YH_{6}, YH_{9}, ThH_{9}, ThH_{10}, and LaH_{10}) at extreme pressures, which commonly exhibit high-temperature superconductivity with the highest T_{c} approaching 260 K for LaH_{10}, 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 (NH_{3}BH_{3}) in a diamond-anvil cell and successfully synthesized the clathrate structured EuH_{6} and EuH_{9} at conditions of 152 GPa and 1700 K, and 170 GPa and 2800 K, respectively. Two nonclathrate structured phases of EuH_{5} and EuH_{6} 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 EuH_{5} and clathrate EuH_{6}. 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.