Experimental clathrate superhydrides EuH6 and EuH9 at extreme pressure conditions

Abstract

The recent discovery of a class of sodalitelike clathrate superhydrides (e.g., ${\mathrm{YH}}_{6}$, ${\mathrm{YH}}_{9}$, ${\mathrm{ThH}}_{9}$, ${\mathrm{ThH}}_{10}$, and ${\mathrm{LaH}}_{10}$) at extreme pressures, which commonly exhibit high-temperature superconductivity with the highest ${T}_{c}$ approaching 260 K for ${\mathrm{LaH}}_{10}$, 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 $({\mathrm{NH}}_{3}{\mathrm{BH}}_{3})$ in a diamond-anvil cell and successfully synthesized the clathrate structured ${\mathrm{EuH}}_{6}$ and ${\mathrm{EuH}}_{9}$ at conditions of 152 GPa and 1700 K, and 170 GPa and 2800 K, respectively. Two nonclathrate structured phases of ${\mathrm{EuH}}_{5}$ and ${\mathrm{EuH}}_{6}$ 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 ${\mathrm{EuH}}_{5}$ and clathrate ${\mathrm{EuH}}_{6}$. 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.