Enthalpy and Gibbs energy of formation of neodymium dicarbide

CO partial pressures over the univariant three-phase region containing NdO1.5(s), C(s) and NdC2(s) were measured by using a quadrupole mass spectrometer (QMS) between 1403 and 1588 K. The NdC2(s) phase was generated in situ starting from a mixture of NdO1.5(s) and C(s). The second-law and third-law enthalpies of the reaction NdO1.5(s)+3.5 C(s)=NdC2(s)+1.5 CO(g) at 298 K were calculated. The enthalpy of formation of NdC2(s) was derived from the enthalpy of reaction and the enthalpies of formation of NdO1.5(s) and CO(g) taken from the literature. The Gibbs energy of formation of NdC2(s) was derived from the Gibbs energy of formation of NdO1.5(s) and CO(g) from the literature. The third-law enthalpy of the reaction is (648.1±1.0) and (647.6±1.0) kJ mol−1 based on thermal functions of NdC2(s) derived from those of UC1.94 and ThC1.94, respectively. The recommended enthalpy and Gibbs energy of formation of NdC2(s) at 298 K are −(90.8±6.0) and −(122.2±6.0) kJ mol−1, respectively.