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)
posted on 2016-01-04, 00:00authored byFederico Wrubl, Pietro Manfrinetti, Marcella Pani, Pavlo Solokha, Adriana Saccone
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 Fm3̅m, 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 Bärnighausen formalism within group
theory.