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. At high temperature it transforms to a cubic phase and has a narrow range of non-stoichiometry.
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. Reported lattice parameters are a = 3.8517 Å and c = 6.4337 Å, with two formula units per unit cell. 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. On heating, the tetragonal structure transforms to a cubic phase of space group Fmm. Phase-diagram assessments place this transformation near 1130–1150 °C. Above about 1000 °C, PrC2 has a narrow homogeneity range of roughly 1–2 at.% carbon rather than existing only at exact stoichiometry. 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. 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. 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.
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