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Erbium trihydride
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| 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).
| |
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
[edit]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
[edit]Crystal structure
[edit]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
[edit]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
[edit]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
[edit]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
[edit]- ^ a b c d e 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.
- ^ a b Gschneidner, Karl A.; Eyring, Leroy (1979). Handbook on the Physics and Chemistry of Rare Earths. Vol. 3. North-Holland.
- ^ a b c 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.
- ^ a b 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.
- ^ a b 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) 054703. doi:10.1063/1.4890657.
- ^ a b Flood, D. J. (1978). "Crystal fields and magnetic properties of ErH3". Journal of Applied Physics. 49 (3): 1495–1497. doi:10.1063/1.324933.