Erbium dihydride
Chemical compound
From Wikipedia, the free encyclopedia
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.[1]
| Names | |
|---|---|
| Other names
Erbium hydride | |
| Identifiers | |
| Properties | |
| ErH2 | |
| Molar mass | 169.28 g/mol |
| Appearance | Metallic solid |
| Structure | |
| Cubic, fluorite type | |
| Fm3m, No. 225 | |
a = 0.5128 nm | |
Formula units (Z) |
4 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
| |
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.[1]
Further hydrogen uptake produces hydrogen-richer material approaching ErH3, corresponding to the γ phase.[2]
Properties
Crystal structure
ErH2 crystallizes in the cubic crystal system, space group Fm3m (No. 225), with the fluorite or CaF2 structure type.[1]
In this structure, the erbium atoms form a face-centred cubic lattice, while hydrogen occupies all eight tetrahedral sites of the conventional unit cell.[1]
A room-temperature lattice parameter of approximately 5.128 Å has been reported.[3]
X-ray diffraction measurements on mixed hydrogen/deuterium samples ErH2−xDx show that the cubic lattice parameter decreases approximately linearly with increasing deuterium content.[4]
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.[5]
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.[5]
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.[6]
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.[1]
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.[2]