Crystal Chemistry of the New Families of Interstitial Compounds R6Mg23C (R = La, Ce, Pr, Nd, Sm, or Gd) and Ce6Mg23Z (Z = C, Si, Ge, Sn, Pb, P, As, or Sb)

The crystal chemical features of the new series of
compounds R6Mg23C with R = La-Sm or Gd and Ce6Mg23Z with Z =
C, Si, Ge, Sn, Pb, P, As, or Sb have been studied by means of single-crystal
and powder X-ray diffraction techniques. All phases crystallize with the
cubic Zr6Zn23Si prototype (cF120, space group Fm3?m, Z = 4), a filled
variant of the Th6Mn23 structure. While no Th6Mn23-type binary rare
earth-magnesium compound is known to exist, the addition of a third
element Z (only 3 atom %), located into the octahedral cavity of the
Th6Mn23 cell (Wyckoff site 4a), stabilizes this structural arrangement and
makes possible the formation of the ternary R6Mg23Z compounds. The
results of both structural and topological analyses as well as of LMTO
electronic structure calculations show that the interstitial element plays a crucial role in the stability of these phases, forming a
strongly bonded [R6Z] octahedral moiety spaced by zeolite cage-like [Mg45] clusters. Considering these two building units, the
crystal structure of these apparently complex intermetallics can be simplified to the NaCl-type topology. Moreover, a structural
relationship between RMg3 and R6Mg23C compounds has been unveiled; the latter can be described as substitutional derivatives
of the former. The geometrical distortions and the consequent symmetry reduction that accompany this transformation are
explicitly described by means of the Ba?rnighausen formalism within group theory.