Layered structure and re-entrant disproportionation observed in crystallineBeH2under pressure

The phase diagram of the Be-H system is measured up to 100 GPa. ${\mathrm{BeH}}₂$ is the only stoichiometry observed. Three crystalline phases are identified using synchrotron x-ray diffraction. The $Ibam$ structure, already known at ambient pressure, is stable up to 27 GPa. The $P{4}₁{2}₁2$ structure, isotypical to the crystoballite-low ${\mathrm{SiO}}₂$ structure, is stable above 600 K over the same pressure range. The well-known $P\overline{3}m1$ layered structure of transition metal dichalcogenides is stable above 80 GPa, making ${\mathrm{BeH}}₂$ a quasi-two-dimensional hydride. Surprisingly, between 27 and 72 GPa, a miscibility gap opens up in the Be-H system and an equilibrium between pure Be and pure H is observed. That is at odds with the current prevailing view of an increase in solubility of H in metals under pressure and contradicts calculations of stable compounds in the Be-H system.