Erbium dihydride is an inorganic compound of erbium and hydrogen with the approximate composition ErH2. It is the hydrogen-rich β phase of the erbium–hydrogen system and crystallizes in the cubic fluorite structure, with erbium atoms forming a face-centred cubic lattice and hydrogen occupying tetrahedral interstitial sites.
Preparation Erbium dihydride can be prepared by direct hydrogenation of metallic erbium:
Er + H2 → ErH2
Hydrogen absorption by erbium proceeds through the Er–H phase diagram. At low hydrogen contents, hydrogen forms a solid solution in hexagonal erbium (the α phase), while compositions near ErH2 form the cubic β phase.
Further hydrogen uptake produces hydrogen-richer material approaching ErH3, corresponding to the γ phase.
Properties Crystal structure ErH2 crystallizes in the cubic crystal system, space group Fmm (No. 225), with the fluorite or CaF2 structure type.
In this structure, the erbium atoms form a face-centred cubic lattice, while hydrogen occupies all eight tetrahedral sites of the conventional unit cell.
A room-temperature lattice parameter of approximately 5.128 Å has been reported.
X-ray diffraction measurements on mixed hydrogen/deuterium samples ErH2−xDx show that the cubic lattice parameter decreases approximately linearly with increasing deuterium content.
Electronic properties Erbium dihydride is metallic. Band-structure calculations show that its Fermi level lies near the bottom of the erbium 5d band, in a region of relatively low electronic density of states.
Below the erbium-derived d states lie two overlapping metal–hydrogen bonding bands. This electronic structure differs substantially from that of elemental erbium and affects the Fermi-surface geometry, magnetic behaviour and electrical resistivity.
High-pressure behaviour First-principles calculations indicate that the cubic fluorite structure remains mechanically stable to high pressure. A structural instability has been predicted near 20 GPa.
The same calculations give a zero-pressure lattice constant of about 5.232 Å, close to experimental values, and predict the expected decrease in volume and increase in bulk modulus with pressure.
Hydrogen stoichiometry The β phase has a finite homogeneity range around ErH2 rather than existing only at an exact 2:1 hydrogen-to-erbium ratio.
For stoichiometric ErH2, the tetrahedral interstitial sites are occupied by hydrogen. In hydrogen-rich compositions above ErH2, additional hydrogen begins to occupy octahedral interstitial sites.
On heating hydrogen-rich erbium hydrides, the sequence of phase changes can pass from β + γ to β, then α + β, and finally the α solid solution as hydrogen is released.
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