Amorphous solid

Amorphous solids, also known as non-crystalline solids, are materials that lack the long-range periodic atomic arrangement characteristic of crystalline structures. They are widespread in nature and technology, appearing in forms such as glass, metallic glasses, polymers, and specialised thin films. Although amorphous materials do not exhibit the repeating unit cells seen in crystals, they possess unique structural and physical properties that make them central to condensed matter physics and materials science. Their study provides essential insights into structural disorder, glass transition behaviour, and low-temperature physics, as well as a foundation for significant technological applications.

Background and Etymology

The term amorphous originates from the Greek words a (without) and morphē (shape or form), reflecting the absence of a long-range ordered atomic arrangement. In everyday contexts the word is often associated with glass, but in scientific usage it encompasses a much wider class of materials. A key distinction is that the term glassy solid specifically refers to amorphous solids that undergo a glass transition, whereas not all amorphous substances display this behaviour.
Amorphous solids commonly arise when a liquid is cooled rapidly enough to prevent the atoms or molecules from arranging themselves into an ordered crystalline structure. They may also form during vapour deposition, sputtering, ion implantation, or other non-equilibrium processes. Their lack of crystallinity does not imply complete disorder; instead, they exhibit identifiable short-range and medium-range structural correlations that give rise to distinct physical properties.

Structural Characteristics

At the atomic scale, amorphous materials contain molecular or atomic building blocks that may resemble the local motifs of their crystalline counterparts. However, these motifs do not repeat periodically. Because long-range order is absent, crystallographic methods based on unit cell repetition are insufficient to describe their structure.
Short-range order typically extends only across a single coordination shell, generally one to two atomic spacings. At this scale, atoms still maintain typical bonding environments—for example, tetrahedral coordination in amorphous silicon. Medium-range order may extend over distances of 1–2 nm, capturing correlations among clusters of atoms, although these arrangements do not lead to fully periodic networks.
Statistical descriptors are vital for structural characterisation. Key quantities include:

• Atomic density function, describing the distribution of atoms within a given volume.
• Radial distribution function (RDF), which provides the probability of finding an atom at a particular radial distance from a reference atom.

These tools help distinguish amorphous materials from nanocrystalline systems where extremely small crystalline domains may complicate structural identification.

Fundamental Properties of Amorphous Solids

Amorphous solids exhibit physical behaviours that differ markedly from those of crystalline materials. Their properties are especially unusual and scientifically significant at both high temperatures during glass formation and at very low temperatures near absolute zero.
Glass transition behaviour is a central unresolved issue in condensed matter physics. When a liquid becomes an amorphous solid upon cooling, it does not freeze in the traditional sense but instead undergoes a gradual slowing of molecular motion. The resulting glass transition temperature depends strongly on cooling rate and composition. The exact microscopic mechanism of this transition remains a major theoretical challenge.
At temperatures below approximately 1–10 K, amorphous solids display a remarkable set of universal low-temperature properties:

• Specific heat varies nearly linearly with temperature.
• Thermal conductivity follows an approximate quadratic temperature dependence.
• Internal friction exhibits a nearly universal dimensionless value across diverse materials.

These anomalous behaviours contrast sharply with corresponding crystalline properties and are often explained phenomenologically using a model based on tunnelling two-level systems (TLS). However, the origin, distribution, and density of TLS remain unresolved, leaving a gap between phenomenology and microscopic theory. The universality of internal friction, in particular, highlights the need for a deeper theoretical framework.

Amorphous Solids in Nanostructured Materials

In nanostructured systems, distinguishing between amorphous and crystalline regions becomes increasingly difficult. Very small crystals may display reduced long-range order due to surface relaxation effects. Structural distortions at interfaces and high surface-to-volume ratios may render traditional diffraction-based classification ambiguous. Consequently, advanced analytical approaches are essential for accurate interpretation of nanoscale materials.

Characterisation Techniques

Because amorphous solids lack long-range periodicity, their study relies on a combination of complementary structural probes and computational methods. No single technique is sufficient; multimodal analysis is frequently employed.
X-ray and neutron diffraction remain fundamental tools. Amorphous materials display diffuse and broad diffraction peaks rather than sharp Bragg reflections. Analysis of diffraction patterns enables determination of RDFs and pair distribution functions, helping to elucidate near-neighbour coordination. X-rays and neutrons possess different scattering sensitivities, so using both allows improved structural refinement.
X-ray absorption fine-structure (XAFS) spectroscopy provides atomic-scale information on oxidation states, coordination environments, and interatomic distances. It is well suited to amorphous materials because it does not rely on long-range periodic order.
Atomic electron tomography (AET), employing transmission electron microscopy with sub-ångström resolution, produces three-dimensional reconstructions of atomic arrangements. This technique involves acquiring numerous tilted projections and applying extensive computational correction for drift, noise, and distortion. AET can reveal the spatial distribution of different atomic species in amorphous networks.
Fluctuation electron microscopy (FEM) is sensitive to medium-range order and can distinguish structural fluctuations that conventional diffraction fails to resolve. It can be performed in both conventional and scanning TEM modes.
In addition to experimental tools, computational techniques such as molecular dynamics, density functional theory, and Reverse Monte Carlo simulations are invaluable for modelling amorphous structures and interpreting experimental observations.

Uses and Technological Significance

Amorphous materials are essential in a wide range of scientific and industrial applications.
Amorphous thin films are widely used in electronics, optics, and protective coatings. Thin films, typically with a thickness from nanometres to tens of micrometres, may naturally form amorphous structures depending on deposition temperature relative to melting temperature. Structure zone models indicate that an amorphous phase arises when the homologous temperature is below approximately 0.3.
Amorphous metallic layers played a foundational role in the discovery of superconductivity in amorphous metals. Their superconducting behaviour arises from phonon-mediated Cooper pairing, consistent with strong-coupling Eliashberg theory. Despite their generally low toughness, metallic glasses possess high yield strength, making them valuable for specific high-performance applications.
Thermal protection systems benefit from the low thermal conductivity of amorphous solids, attributed to strong localisation of heat carriers. Ultrathin thermal barrier coatings and lightweight insulating materials commonly employ amorphous phases to achieve enhanced thermal resistance.
Optical coatings frequently use amorphous films of titanium dioxide, silicon dioxide, and tantalum pentoxide due to their desirable refractive properties and ease of deposition. These coatings are critical components of mirrors, filters, and laser systems.
Microelectronic devices rely heavily on thin amorphous layers. The most important example is the nanometre-scale layer of silicon dioxide that serves as the insulating gate oxide in metal-oxide-semiconductor field-effect transistors (MOSFETs). Amorphous silicon, often hydrogenated (a-Si:H), is used in photovoltaic devices and various semiconductor applications due to its tunable electronic structure and compatibility with large-area deposition.
Amorphous films are also investigated for use as gas-separation membranes or biomimetic layers due to their structural versatility and ability to incorporate functional additives.