[en] Quantitative phase analysis by neutron powder diffractometry yields conclusive evidence for the irreversible decomposition of La₇Ni₃ into LaD₃ and LaNi₅ during deuteration at room temperature. Refinement of the LaD₃ structure confirms that the D atoms are distributed over the tetrahedral and octahedral interstices of the f.c.c. metal atom arrangement. The D atoms occupying the octahedral interstices are not exactly at the centre but appear to be shifted away from it along different directions. It is suggested that this delocalization results from large anharmonic thermal displacements of the D atoms mainly along (111) direction. The crystal structure of La₇Ni₃ was refined both by single-crystal X-ray analysis and by neutron powder diffractometry. The results are compared and discussed in relation to other Th₇Fe₃-type structures. (Auth.)