Chemistry:Praseodymium dicarbide
| Names | |
|---|---|
| Other names
Praseodymium carbide
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| Identifiers | |
3D model (JSmol)
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| Properties | |
| PrC2 | |
| Molar mass | 164.93 g/mol |
| Appearance | Yellow crystalline solid |
| Density | 5.10 g/cm3 |
| Melting point | 2320 °C |
| Structure | |
| Tetragonal, calcium carbide type | |
| I4/mmm, No. 139 | |
a = 0.38517 nm, c = 0.64337 nm
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Formula units (Z)
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2 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). | |
| Infobox references | |
Praseodymium dicarbide is an inorganic compound of praseodymium and carbon with the formula PrC2. It is a yellow crystalline rare-earth dicarbide and adopts the tetragonal calcium carbide structure type at room temperature.[1] At high temperature it transforms to a cubic phase and has a narrow range of non-stoichiometry.[2][3]
Preparation
Praseodymium dicarbide can be prepared by direct reaction of praseodymium with carbon at very high temperature:
- Pr + 2 C → PrC2
Older preparations used electric-furnace heating at temperatures around 2300 °C, sometimes under hydrogen or inert atmosphere.
Properties
Crystal structure
At room temperature, PrC2 crystallizes in the tetragonal crystal system, space group I4/mmm (No. 139), with the CaC2 structure type.[1] Reported lattice parameters are a = 3.8517 Å and c = 6.4337 Å, with two formula units per unit cell.[1] As in other rare-earth dicarbides, the carbon atoms occur as discrete C2 units. The structure is commonly described in an acetylide-like picture containing Pr3+, C22− units and one conduction electron per formula unit.
High-temperature behaviour
PrC2 melts congruently at approximately 2320 °C.[2] On heating, the tetragonal structure transforms to a cubic phase of space group Fm3m. Phase-diagram assessments place this transformation near 1130–1150 °C.[2][3] Above about 1000 °C, PrC2 has a narrow homogeneity range of roughly 1–2 at.% carbon rather than existing only at exact stoichiometry.[2][3] The high-temperature cubic phase is related to the orientationally disordered cubic forms of other rare-earth dicarbides, in which the C2 groups are no longer ordered as in the low-temperature tetragonal structure.
Magnetic and electrical properties
PrC2 is metallic and displays unusual low-temperature magnetic behaviour. Neutron-diffraction work found antiferromagnetic ordering near 15 ± 2 K. Electrical-resistivity measurements show a clear change in slope around 18 K, consistent with this transition.[4] The same study found a ferromagnetic state below about 7 K in low applied fields. The discrepancy with the earlier antiferromagnetic result was interpreted as evidence for a low-field metamagnetic transition, occurring below approximately 700 Oe.[4] This behaviour distinguishes PrC2 from many neighboring rare-earth dicarbides, which remain simply antiferromagnetic at low temperature.
Thermochemistry
A standard enthalpy of formation of approximately −94 kJ mol−1 has been tabulated for PrC2.[5]
Reactions
Praseodymium dicarbide reacts with water, producing praseodymium hydroxide and hydrocarbons including acetylene and ethane:
- 4 PrC2 + 12 H2O → 4 Pr(OH)3 + 3 C2H2 + C2H6
References
- ↑ 1.0 1.1 1.2 Spedding, F. H.; Gschneidner, K.; Daane, A. H. (1958). "The Crystal Structures of Some of the Rare Earth Carbides". Journal of the American Chemical Society 80 (17): 4499–4503. doi:10.1021/ja01550a017.
- ↑ 2.0 2.1 2.2 2.3 Predel, B. (1992). C-Pr (Carbon-Praseodymium). Landolt-Börnstein - Group IV Physical Chemistry. 5b. pp. 1–2. doi:10.1007/10040476_654.
- ↑ 3.0 3.1 3.2 Gschneidner, K. A.; Calderwood, F. W. (1986). "The C-Pr (Carbon-Praseodymium) system". Bulletin of Alloy Phase Diagrams 7 (6): 558–559. doi:10.1007/BF02869867.
- ↑ 4.0 4.1 Sakai, Tetsuo; Adachi, Gin-ya; Yoshida, Takeshi; Shiokawa, Jiro (1981). "Magnetic and electrical properties of LaC2, CeC2, PrC2, NdC2, and SmC2". The Journal of Chemical Physics 75 (6): 3027–3032. doi:10.1063/1.442396.
- ↑ Handbook of High Temperature Compounds: Properties, Production, Applications. CRC Press. 1990. ISBN 0-89116-849-4.
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