Amorphous and Crystalline Solids

Solids are further divided into two categories based on the degree of order in the arrangement of their

constituent particles: crystalline solids and amorphous solids.

A crystalline solid has its constituent particles (atoms, ions or molecules) arranged in a highly ordered,

regularly repeating three-dimensional pattern that extends over the whole sample — this is called long-range order. Because this repeating pattern is geometrically precise, a crystalline solid has a sharp, well-defined

melting point: at that exact temperature, the ordered lattice collapses into the disordered liquid state all at

once. Crystalline solids are also anisotropic — properties such as refractive index, electrical conductivity or

thermal expansion have different values when measured along different crystallographic directions, because the

arrangement (and hence spacing) of particles differs from one direction to another. When cut with a sharp blow,

a crystalline solid cleaves cleanly along specific, flat crystal planes, giving smooth faces. Quartz, sodium

chloride and diamond are crystalline.

An amorphous solid, by contrast, has particles arranged with only short-range order — a roughly regular

arrangement persists only over a few atomic or molecular diameters, with no long-range periodicity. Structurally,

an amorphous solid is best thought of as a liquid that has been cooled and "frozen" too quickly for its particles

to settle into an ordered crystal lattice; for this reason amorphous solids are sometimes called supercooled liquids or "pseudo-solids." Because there is no single, well-defined lattice spacing to break down, an amorphous

solid does not melt sharply — it softens gradually over a range of temperatures and can be moulded, which is why

glass and plastics can be shaped by heating. Amorphous solids are isotropic — since the arrangement of

particles is, on average, the same in every direction, properties like refractive index are the same no matter

which direction they are measured along. When broken, an amorphous solid does not cleave along flat planes;

instead it fractures with an irregular, often curved surface — glass shattering into jagged, curved pieces is the

everyday example. Common glass and amorphous silica, rubber, and most plastics are amorphous.

A useful comparative example is glass versus quartz, both built from $\text{SiO}_2$ networks: quartz has its

$\text{SiO}_4$ tetrahedra arranged in a perfectly repeating crystalline lattice, giving it a sharp melting point,

anisotropic optical properties, and clean cleavage; ordinary glass has the same $\text{SiO}_4$ tetrahedral …