Page 305 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN010H-470 July 16, 2001 16:53

314 Nanosized Inorganic Clusters

FIGURE 18 Ball and stick representation of the “packing” of the linked rings (in the direction of the b axis) in crystals
V
of Na 16 [Mo VI 124 Mo 28 O 429 (µ 3 -O) 28 H 14 (H 2 O) 66.5 ]. XH 2 O(X ≈300) ({Mo 152 }) viewed along the crystallographic b axis.
(a) Each ring is connected to surrounding rings via Mo-O-Mo bridges of the O=Mo-O-Mo-OH 2 units, thus forming
layer networks parallel to the ac plane. One ring is shown as basic unit in polyhedral representation. (b) Detailed view
of the bridging region between two cluster rings. One {Mo 8 } unit of each ring along with one {Mo 1 } unit is shown in
2+ VI 2+
polyhedral representation and one {Mo 2 } (={Mo 2 O 5 (H 2 O) 2 } ) unit per ring in ball and stick representation. (c)
Perspective view along the crystallographic c axis showing the framework with nanotubes that are filled with H 2 O
molecules and sodium cations. For clarity, only one ring is shown in a polyhedral representation. For the other rings
only the equatorial {Mo 1 } units are given and connected.

{Mo 144 } defect cluster can be linked to chains (see Fig. 17) units are “added” to each side of the cluster forming a
and an {Mo 152 } cluster can be linked in the construction spherical disk-shaped cluster comprising 248 Mo atoms.
of a layered compound (see Fig. 18). The hubcaps were found in the mixed-crystal compound
to have an occupation of 50%, see Fig. 19. This appears
to be a remarkable result when it is considered that the
D. Nucleation Processes within a
larger wheel cluster “cap,” the [Mo 36 O 96 (H 2 O) 24 ]-type
Cluster Cavity—from an {Mo 176 }
fragment, is nearly identical to a segment of the solid-state
to an {Mo 248 } Cluster
structure of the compound Mo 5 O 14 (see Fig. 20). This ex-
Under special types of reducing conditions (using ascor- traordinary structure (the largest discrete inorganic struc-
bic acid as a reducing agent) the {Mo 176 } cluster can be ture to be characterized by single crystal X-ray diffrac-
observed to grow; during this process a further two {Mo 36 } tion to date) offers the possibility of model crystal growth