Praseodymium sesquicarbide is an inorganic compound of praseodymium and carbon with the formula Pr2C3. It is a rare-earth sesquicarbide with the cubic Pu2C3 structure type and becomes antiferromagnetic below about 8 K.
Preparation Praseodymium sesquicarbide can be prepared by heating praseodymium and carbon at high temperature:
2 Pr + 3 C → Pr2C3
The phase forms peritectically in the Pr–C system rather than melting congruently. Phase-diagram assessments place its decomposition/peritectic temperature at approximately 1545–1560 °C.
Properties Crystal structure Pr2C3 crystallizes in the cubic crystal system, space group I3d (No. 220), with the Pu2C3 structure type and eight formula units per unit cell. The lattice parameter varies with carbon content across the homogeneity range. Spedding et al. reported a = 8.5731 Å on the praseodymium-rich side and a = 8.6072 Å on the carbon-rich side. The corresponding calculated density at the carbon-rich composition is about 6.62 g/cm3. The structure contains discrete C2 units. Neutron diffraction gave a C–C distance of about 1.239 Å in Pr2C3, intermediate between that of CaC2 and the longer C–C distances found in many rare-earth dicarbides. Paramagnetic neutron scattering indicates that praseodymium is essentially trivalent in Pr2C3.
Phase stability Pr2C3 has a substantial homogeneity range rather than being restricted to exact 40 at.% praseodymium / 60 at.% carbon stoichiometry. Phase-diagram assessments place the stable composition range at approximately 55–60 at.% carbon.
It decomposes peritectically at about 1550 °C to liquid and another carbide phase.
Magnetic properties Pr2C3 orders antiferromagnetically below approximately 8 K. Neutron diffraction gives a Néel temperature of about 8 K and an ordered moment of approximately 1.3 μB per Pr atom, only about 41% of the free-ion value. The reduced moment was attributed to a strong crystal-field effect. The magnetic structure is the same general type found in Nd2C3, Tb2C3 and Dy2C3. In the uniaxial model, two sets of body-diagonal praseodymium chains order antiferromagnetically while the other two remain magnetically disordered. The antiferromagnetic arrangement was reported to remain unchanged in applied fields up to 21 kOe.
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