Biomedical Engineering Reference
In-Depth Information
According to the available literature, for each particular atom of
any solid there are a SRO of 2-5 Å, a medium-range order (MRO)
of 5-20 Å, and a LRO at distances exceeding ~20 Å [87, 88]. In the
case of covalent materials, in which a directed chemical bonding is
dominant, a SRO can be characterized in terms of the well-defined
coordination polyhedra, which, in many cases, appear to concur
with the unit cells. The definition of a MRO is more contentious and
is helpful to subdivide MRO into 3 subcategories. At the shortest
length scale (~5 Å), a near-MRO describes the connections among
the coordination polyhedra. At the next length scale (~5-8 Å), an
intermediate MRO can be associated with correlations between pairs
of preferred dihedral angles for neighboring bonds. Finally, on a yet
larger length scale (~8-20 Å), a far-MRO can be associated with the
total dimensionality of the covalently bonded amorphous network.
Thus, characterizing the nature of MRO in disordered solids is very
important for understanding their structure. Figure 2.1 represents
an excellent visual demonstration of the differences between a
SRO and a MRO [89]. Farther details on this topic are available in
literature [87, 88].
Figure 2.1
A formation of skydivers illustrates disorders on an intermediate
length scale. Each skydiver has a simple set of rules for bonding
to the next skydiver (SRO) but there is a sufficient flexibility for
different patterns of ordering to be created on the scale of a
few body lengths (MRO).
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