Chemistry:Erbium trihydride
| Names | |
|---|---|
| Other names
Erbium hydride
| |
| Identifiers | |
| EC Number |
|
| Properties | |
| ErH3 | |
| Molar mass | 170.28 g/mol |
| Density | 7.66 g/cm3 |
| Structure | |
| Trigonal, HoH3 type | |
| P3c1, No. 165 | |
a = 0.6264 nm, c = 0.6520 nm
| |
Formula units (Z)
|
6 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). | |
| Infobox references | |
Erbium trihydride is an inorganic compound of erbium and hydrogen with the approximate composition ErH3. It is the hydrogen-rich γ phase of the erbium–hydrogen system and, under ordinary pressure, adopts a trigonal HoH3-type structure.[1][2] The compound is less thermodynamically stable than erbium dihydride, but metastable ErH3 powders and thin films can persist at ambient conditions.[3]
Preparation
Erbium trihydride can be prepared by direct hydrogenation of metallic erbium:
- 2 Er + 3 H2 → 2 ErH3
In one high-pressure preparation, pure erbium was exposed to hydrogen at 15 MPa and 300 °C.[1]
The γ phase forms only at sufficiently high hydrogen chemical potential. In the erbium–hydrogen phase diagram, increasing hydrogen concentration gives the sequence of an α solid solution, the cubic β-ErH2 phase, a β + γ two-phase region, and finally γ-ErH3 at hydrogen-to-erbium ratios above approximately 2.7.[1]
Metastable ErH3 has also been prepared as thin films and powders. Electron-beam-deposited erbium films heated in hydrogen can transform to the trihydride, which may remain intact after the hydrogen overpressure is removed.[3]
Properties
Crystal structure
At ambient pressure, γ-ErH3 adopts the HoH3 structure type, space group P3c1 (No. 165). Representative lattice parameters are a = 6.264 Å and c = 6.520 Å, with six formula units per unit cell.[2] The erbium atoms form an approximately hexagonal close-packed sublattice. Hydrogen occupies two different types of interstitial sites, displaced from ideal tetrahedral and octahedral positions. The structure differs markedly from cubic β-ErH2, in which the erbium atoms form a face-centred cubic lattice and hydrogen occupies the tetrahedral sites.
Stability and thermal decomposition
Bulk ErH3 is thermodynamically stable only under sufficient hydrogen pressure. Pressure–composition measurements indicate that, at temperatures between approximately 200 and 370 °C, removal of the hydrogen overpressure favors decomposition toward the dihydride.[3]
Thermal-desorption measurements on material of approximate composition ErH2.7 show a sequence of γ + β → β → α + β → α as hydrogen is released.[4]
Two principal hydrogen-desorption processes were observed, with activation energies of 160.7 and 212.3 kJ/mol. The lower-temperature feature was associated with loss of the γ phase, while the higher-temperature feature corresponded to decomposition of the β hydride.[4]
High-pressure behaviour
High-pressure X-ray diffraction shows that the ambient-pressure hexagonal phase transforms to a cubic phase at approximately 15 GPa.[1] The cubic phase has a face-centred cubic erbium sublattice; extrapolation of its equation of state gives a lattice parameter of approximately 5.23 Å at zero pressure. The transition is reversible on decompression.[1]
Later first-principles calculations predict a somewhat more complex sequence, with a monoclinic C2/m phase becoming stable near 15 GPa before transformation to cubic Fm3m ErH3 at about 40 GPa.[5]
The same calculations find the ambient-pressure P3c1 phase to be non-metallic, whereas the predicted high-pressure phases are metallic.[5]
Magnetic properties
The magnetic properties of ErH3 are dominated by the localized 4f electrons of erbium. Magnetic-susceptibility measurements between 1.3 and 4.2 K follow approximately Curie–Weiss behavior, with a Weiss temperature of −1.05 ± 0.05 K, indicating predominantly antiferromagnetic interactions.[6] The measured low-field effective moment is 6.77 ± 0.27 μB per erbium ion, while the saturation moment is approximately 3.84 ± 0.15 μB.[6] The reduced values relative to the free Er3+ ion arise from the crystal-field splitting of the erbium 4f states.
References
- ↑ 1.0 1.1 1.2 1.3 1.4 Palasyuk, T.; Tkacz, M. (2004). "Pressure induced hexagonal to cubic phase transformation in erbium trihydride". Solid State Communications 130 (3–4): 219–221. doi:10.1016/j.ssc.2004.01.040.
- ↑ 2.0 2.1 Gschneidner, Karl A.; Eyring, Leroy (1979). Handbook on the Physics and Chemistry of Rare Earths. 3. North-Holland.
- ↑ 3.0 3.1 3.2 Tewell, Craig R.; King, Saskia H. (2007). "Observation of metastable erbium trihydride". Applied Surface Science 253 (5): 2597–2602. doi:10.1016/j.apsusc.2006.05.025.
- ↑ 4.0 4.1 Ma, M.; Tan, X. (2023). "Phase-structural transformation of erbium trihydride studied by thermal desorption spectroscopy". International Journal of Hydrogen Energy 48 (37): 13913–13921. doi:10.1016/j.ijhydene.2022.12.328.
- ↑ 5.0 5.1 Hou, Pugeng; Tian, Fubo; Li, Da; Chu, Binhua; Zhao, Zhonglong; Liu, Bingbing; Cui, Tian (2014). "High-pressure phase transition of MH3 (M: Er, Ho)". The Journal of Chemical Physics 141 (5). doi:10.1063/1.4890657.
- ↑ 6.0 6.1 Flood, D. J. (1978). "Crystal fields and magnetic properties of ErH3". Journal of Applied Physics 49 (3): 1495–1497. doi:10.1063/1.324933.
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