TWO NEW YTTRIUM CARBIDE BORIDE IODIDES
Oliver Oeckler, Hansjürgen Mattausch, and Arndt Simon
Max-Planck-Institut für
Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart,
Germany,
E-mail: oliver@vaxff2.mpi-stuttgart.mpg.de
Keywords: yttrium carbide boride
iodides, preparation, single crystal structure, twinning, BC2
units, B2C4 units, misfit structure, HRTEM
Numerous halides of rare earth metals are stabilized by interstitial atoms [1], e.g. H, C, B, N, O, which center metal atom polyhedra. "Condensation" of these polyhedra may enable bond formation between interstitials, so that quasi-molecular units are surrounded by cage-like metal polyhedra. In many rare earth carbide borides [2] and carbide boride halides [3] the boron atoms are found in the center of trigonal prisms. In carbide borides with B-C-bonding the square faces of these prisms are capped by additional metal atoms and the carbon atoms are located in the resulting tetragonal pyramids.
The first yttrium carbide boride halides, Y16I19C8B4 and Y21I18C14B7, could be prepared in almost quantitative yield from stoichiometric mixtures of Y,YI3, C and B in closed Ta capsules at 1050 °C and 1180 °C, respectively. The compounds are very sensitive to moisture. Their crystal structures have been determined from single-crystal X-ray data (Stoe IPDS). Both structures are relatively complex.
Y16I19C8B4 (space group P1, a = 12.311(2) A, b = 13.966(3) A, c = 19.695(3) A, a = 74.96(2)°, a = 89.51(2)°, g = 67.03(2)°, V = 3000.9(9) A3, Z = 2) contains planar B2C4 units in polyhedra formed from 12 yttrium atoms. Two bicapped boron-centered trigonal prisms are fused via the remaining square face. The four carbon atoms are coordinated by the pyramidal caps. These Y12(B2C4) building units are connected via planar rhomboid faces to form rods (fig. 1a), which are surrounded by iodine atoms. Bridging iodine atoms connect the rods so that layers are formed (fig. 1b). Crystals show non-merohedral twinning (twin plane [100]) which can be explained by the approximately mmm pseudosymmetry of the individual layers. A formal charge distribution (Y3+)1648+(I-)1919-[(B2C4)12-]224-(e-)55- assumes (B2C4)12- units to be isoelectronic with B2F4 which is in agreement with the distances and angles observed. The compound is semiconducting, so the remaining yttrium valence electrons are probably localized in Y-Y bonding states (compare Gd2Cl3 [4]).
In Y21I18C14B7
bicapped trigonal prisms containing non-linear CBC units, Y8(BC2),
are fused via both triangular faces of the prisms and four edges
of the caps, building up a layer with pseudo-orthorhombic
symmetry (fig. 2a). Pseudo-hexagonal iodine layers sandwich the
metal atom layers. Due to the symmetry misfit the layers are
distorted and the structure must be described with a large
triclinic unit cell (space group P1, a = 10.660(2) A, b =
15.546(3) A, c = 18.416(3) A, a = 82.49(2)°, b =
85.01(2)°, g = 82.92(2)°, V = 2994.9(9) A3, Z = 2), compare fig.
2b. Probably this is only a good space-averaged approximation as
a certain lack of fit (gof = 1.57) is observed in the refinement
and displacement parameters show a relatively high degree of
anisotropy even at -180 °C. High-resolution transmission
electron microscopical (HRTEM) investigations are in progress in
order to obtain information on the local (not space-averaged)
structure.
Fig. 1 a) rods of "condensed" Y12(B2C4)
units in Y16I19C8B4,
b) projection of the structure along [001] showing the
arrangement of connected rods and the pseudosymmetry of the
layers ( 90°, so [100] is approximately parallel to the paper, B
and C atoms are not drawn).
Fig. 2 a) detail of a layer formed by condensed bicapped trigonal Y prisms containing CBC units in Y21I18C14B7, b) projection of the structure along [100] (B and C atoms are not drawn).