Etude par diffraction de neutrons des deplacements atomiques moyens dans ThC et ThC_0.77 à 4 et 250 K

A neutron diffraction study of Debye-Waller factors has been performed on ThC and ThC_0.77 powder samples at 4 and 250 K on the high-resolution diffractometers D1A and D1B of HFR (Grenoble); 25 diffraction lines were resolved and analysed with wavelength λ = 1.21 Å on D 1A, and 22 lines with λ = 1.28 Å on D1B. In the case of ThC, it is shown that the results (B_Th = 0.07 ± 0.05 Ų and B_C = 0.40 ± 0.08 Ų at 4 K; B_Th = 0.32 ± 0.08 Ų and B_C = 0.68 ± 0.08 Ų at 250 K) are mainly due to the thermal vibrations. For ThC_0.77, we find B_Th = 0.23 ± 0.03 Ų and B_C = 0.47 ± 0.05 Ų at 4 K, and B_Th = 0.48 ± 0.04 Ų and B_C = 0.69 ± 0.05 Ų at 250 K. The enhancements of the Debye-Waller factors compared to the stoichiometric case are attributed to the static distortions induced by the presence of 23% vacancies in the carbon sublattice. The mean-square static displacements of thorium and carbon atoms are found to be about 1% of the lattice parameter = 0.08 ± 0.03 Å and 0.05 ± 0.03 Å.