The lattice parameter of cubic U2C3 was studied up to 2073 K by XRD, and no anomalies were detected either at low or high temperature. Its values vary from 807.3 pm at 10 K123 to 825.6 pm at 2073 K.114

Oetting et a/.124 determined the energy of forma­tion for vacancies in the U2C3 lattice to be ^0.8 eV, from the heat capacity increase above 1000 K.

The temperature coefficient of the electronic heat capacity was estimated to be g«84mJK-2mol-1 from low T heat capacity measurements, in agree­ment with the metallic character of uranium sesqui — carbide. U2C3 is antiferromagnetic below the Neel temperature TN « 55 ± 4 K.8

2.04.4.2.1.2 Uranium dicarbide UC2

Tagawa et a/.125 showed that the lattice parameter of a-UC2 increases linearly between UC180 and UC196 according to the following relation:

a = 352.45 + 0.75 x (C/U — 1.80) [7]

An uncertainty of ±0.01 pm stems from the different sample preparation methods. Tagawa et a/.126 also showed that the c/a ratio is 1.702 at room tempera­ture, and does not detectably vary as a function of the C/U ratio between UC180 and UC196, where c stays approximately constant and close to 600 pm. Atoji127 measured the lattice parameters of UC186 at 5 K by neutron diffraction, finding a = 351.7 ± 0.1pm and c = 598.9 ± 0.1pm. No phase transitions were de­tected between 5 and 300 K. The c/a ratio decreases with increasing temperature above 1473 K. Whereas a increases from 353.6 pm at 1073 K to 362.5 pm at 1973 K, there is no complete agreement about the behavior of c. Laugier and Blum108 suggested that c decreases from 605.6pm at 1073 K to 594.9pm at 1700 K on the U-rich side of the tetragonal UO2-x phase field, whereas it varies from 605.6 to 603.9 pm on the C-rich side.

The transformation a! p is diffusionless of the martensitic type. It occurs without movement of
the U atoms, and with a slight deformation of the C sublattice. The transformation shear angle is between 4° and 6°. p-UC2-x crystallizes in a fcc structure of the KCN-type with a0 = 548.8 pm.109

UC2-x is a metal. The UC2 electronic state den­sity at the Fermi level was recently calculated by Shi et al., 9 in reasonable agreement with the tempera­ture coefficient g of the electronic heat capacity. This was estimated to be 16.3 mJ K-2 mol-1, to yield N (Ef) ~ 3.45 eV-1 atom-1 for UC190 and 16.7 mJK-2 mol-1, to yield N(Ef) « 3.53 eV-1 atom-1 for UC194.

Atoji127 showed that a-UC2-x is paramagnetic down to 5 K, without superconductivity.

2.04.4.2.2 Thermodynamic properties