Among the minor actinides, only carbides of ameri­cium and curium have been identified and investi­gated so far.

2.04.7.1 Americium Carbides

The two most stable isotopes of americium, 241Am (432.2 years) 243Am (7370 years), are formed by p-decay of 241Pu and 243Pu, respectively. Therefore, a certain percentage (order of 10—1 at.%) of ameri­cium is commonly present in plutonium. As a conse­quence, traces of americium carbides can be formed in plutonium or mixed carbide matrices.226

Although the Am-C phase diagram has not been investigated in any detail, the monocarbide and the sesquicarbide have been identified, prepared by carbothermic reduction and arc-melting.227 Samples of nominal composition Am104 and Am125 were thus obtained and annealed at 1273 K for 24h, and then characterized by XRD at room temperature. The main phase displayed a fcc NaCl Fm 3m lattice, with lattice parameter a = 502 pm. Although the precise
composition and the oxygen and nitrogen contents were not determined, this phase corresponded most probably to the monocarbide. Traces of a bcc phase (most likely the sesquicarbide) were also detected. Mitchell and Lam199 prepared americium sesquicar­bide Am2C3 and analyzed it by XRD at room temper­ature. The material was found to be isostructural with Pu2C3, bcc, 143d, with eight formula units per unit cell. After annealing, the lattice parameter was found to be a = 827.57 ± 0.2 pm. Holley et al4 estimated the thermodynamic functions for the formation of ameri­cium sesquicarbide: AfH° (298) = —75 ± 20kJmol— AfS° (298) = 8 ± 8kJK—1mol—1, and AfG° (298) = — 77 ± 20kJmol—1. These values were estimated by assuming that Am2C3 is similar to Pu2C3 and that the trend toward lower values of AfH and AfG with increasing atomic number continues beyond Pu2C3.