In amorphous solids (literally “solids without form” or non-crystalline solids) the particles do not have a repeating lattice pattern. They are also called “pseudo solids.” Amorphous materials have an internal structure made of interconnected structural blocks. These blocks can be similar to the basic structural units found in the corresponding crystalline phase of the same compound.
The amorphous solids are formed due to particular conditions during the solidification process (for example by increasing the cooling speed), which do not allow the atoms to stabilize thermodynamically in an ordered condition, thus preventing the formation of a periodic crystal structure. Amorphous solids are isotropic.
Examples of amorphous solids include glass, rubber, gels, and most plastics. An amorphous solid does not have a definite melting point; instead, it melts gradually over a range of temperatures, because the bonds do not break all at once. This means an amorphous solid will melt into a soft, malleable state (think candle wax or molten glass) before turning completely into a liquid.
Amorphous solids have no characteristic symmetry, so they do not have regular planes of cleavage when cut; the edges may be curved. They are called isotropic because properties such as refractive index, conductivity and tensile strength are equal regardless of the direction in which a force is applied.
Characteristics of amorphous solids
An amorphous solid depicts the following properties, which are as follows:
- The constituent particles of matter inside a solid are arranged randomly, that is, the position of atoms and molecules is not fixed and varies from one solid to another.
- Amorphous Solids don’t have definite shape or geometry due to the random arrangement of atoms and molecules inside the solid lattice.
- Short-range order (arrangement of atoms) is found in amorphous solids; due to the absence of long-range order, the amorphous solids present X-ray diffraction without peaks. Compared to liquids, the amorphous state has closer atoms and a lower free volume.
- Amorphous solids are also called pseudo-solids or supercooled Liquids because they don’t form crystalline structure and can flow.
- The nature of amorphous solids is isotropic in nature that is, the properties measured in all directions come out to be the same (in other words their physical properties do not depend on the direction in which the sample is analyzed), for example, the refractive index of amorphous solids is the same.
- Amorphous solids don’t show a sharp melting point, this is because of the irregular packing of amorphous solids (in other words the melting of an amorphous solid does not occur at a constant temperature since the bonds present in the solid do not all break at the same temperature).
- When we cut an amorphous solid, we find the broken constituent particles to be irregular in shape and geometry.
- Amorphous solids are unsymmetrical in nature, due to the irregular packing of atoms and molecules inside the solid lattice.
- Amorphous solids don’t have fixed heat of fusion because of the absence of a sharp melting point.
- The Amorphous solid-state has a metastable structure that can be transformed into crystalline structures only under certain thermodynamic and kinetic conditions.
- As the temperature increases, the viscosity of the amorphous solids decreases exponentially.
- They can exist in two distinct phases: the gummy one and the glassy one.