Amorphous
solids have no long-range order. The atoms or molecules in these solids are not
periodically located over large distances. An amorphous structure is shown
below.
Many
amorphous materials have internal structures similar to liquids. In fact, the
only obvious distinction between amorphous materials, such as glass, and
liquids is the high viscosity (resistance to flow) of the amorphous solids.
Amorphous Solids
All
solids tend to exist in the crystalline state rather than the amorphous state
because the crystalline structure always has a larger binding energy. However,
in numerous instances amorphous solids are formed when liquids are cooled below
the melting temperature. This occurs for two reasons:
1)
the structure of
the molecules is so complex that they cannot easily rearrange themselves to
form a crystalline structure, and/or
2)
the solid forms
so rapidly that the atoms or molecules do not have time enough to rearrange
themselves in a crystalline structure.
Generally,
amorphous solids have one of two distinct atomic arrangements:
either
a tangled mass of long-chained molecules or a 3-dimentional network of atoms
with no long-range order.
Amorphous
materials with long-chained molecules (e.g. polymers) have a structure similar
to that shown below.
Each
segment in above figure represents one of the repeating units of the polymer
chain.
The
arrangement of the molecules is fairly random, resulting in a loosely packed
structure. Network amorphous solids are usually Oxides, the most common being
Silica (SiO2). The amorphous SiO2 structure is shown
below.
Only
Oxygen atoms are shown (corners of tetrahedral) in this amorphous SiO2
structure. There is a Silicon atom at the center of each tetrahedral which is
not shown here.
This
structure has short-range order but none of the long-range order found in
crystalline Silica. Thus, in both amorphous and crystalline Silica, each
Silicon atom and each Oxygen atom have essentially the same local surroundings,
even though there is no long-range periodicity in the amorphous structure.