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 spectromet er (QMS) between 1403 and 1588 K. The NdC2(s) phase was generated in situ s tarting 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 29 8 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 C O(g) taken from the literature. The Gibbs energy of formation of NdC2(s) wa s derived from the Gibbs energy of formation of NdO1.5(s) and CO(g) from th e 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 NdCz(s) derived f rom 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 -(12 2.2 +/- 6.0) kJ mol-l, respectively. (C) 2001 Elsevier Science B.V. All rig hts reserved.