Chemistry:Erbium dihydride
| 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). | |
| Infobox references | |
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]
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]
References
- ↑ 1.0 1.1 1.2 1.3 1.4 Effect of Stoichiometry on Helium Release from Erbium Tritide (Report). 2008.
- ↑ 2.0 2.1 "Phase-structural transformation of erbium trihydride studied by thermal desorption spectroscopy". International Journal of Hydrogen Energy 48 (37): 13913–13921. 2023. doi:10.1016/j.ijhydene.2022.12.328.
- ↑ Bonnet, J. E.; Daou, J. N. (1977). "Rare-earth dihydride compounds: lattice thermal expansion and investigation of the thermal dissociation". Journal of Applied Physics 48: 964–968. doi:10.1063/1.323717.
- ↑ "X-ray powder diffraction data for ErH2−xDx". Powder Diffraction 23 (3): 259–264. 2008. doi:10.1154/1.2959575.
- ↑ 5.0 5.1 Gupta, M. (1978). "Electronic structure of ErH2". Solid State Communications 27 (12): 1355–1359. doi:10.1016/0038-1098(78)91571-5.
- ↑ Liu, Yiding; Tang, Yongjian (2012). "Elastic and thermodynamic properties of erbium dihydride under high temperature and pressure". High Power Laser and Particle Beams 24 (8): 1874–1878. doi:10.3788/HPLPB20122408.1874.
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