Novel Strongly Correlated Europium Superhydrides

Abstract

We conducted a joint experimental-theoretical investigation of the high-pressure chemistry of europium polyhydrides at pressures of 86-130 GPa. We discovered several novel magnetic Eu superhydrides stabilized by anharmonic effects: cubic EuH₉, hexagonal EuH₉, and an unexpected cubic (<i>Pm</i>3<i>n</i>) clathrate phase, Eu₈H₄₆. Monte Carlo simulations indicate that cubic EuH₉ has antiferromagnetic ordering with <i>T</i>_<i>N</i> of up to 24 K, whereas hexagonal EuH₉ and <i>Pm</i>3<i>n</i>-Eu₈H₄₆ possess ferromagnetic ordering with <i>T</i>_C = 137 and 336 K, respectively. The electron-phonon interaction is weak in all studied europium hydrides, and their magnetic ordering excludes <i>s</i>-wave superconductivity, except, perhaps, for distorted pseudohexagonal EuH₉. The equations of state predicted within the DFT+U approach (<i>U - J</i> were found within linear response theory) are in close agreement with the experimental data. This work shows the great influence of the atomic radius on symmetry-breaking distortions of the crystal structures of superhydrides and on their thermodynamic stability.