Amorphous solid

In condensed matter physics and materials science, an amorphous (from the Greek a, without, morphé, shape, form) or non-crystalline solid is a solid that lacks the long-range order, which is a characteristic of a crystal. In some older articles and books, the term was used synonymously with glass. Today, however, "glassy solid" or "amorphous solid" is considered to be the overarching concept, and glass is considered to be a special case: glass is an amorphous solid maintained below its glass transition temperature.[1] Polymers are often amorphous.[2]

Amorphous materials have an internal structure comprising interconnected structural blocks that can be similar to the basic structural units found in the corresponding crystalline phase of the same compound.[3] Whether a material is liquid or solid depends primarily on the connectivity between its elementary building blocks; solids are characterized by a high degree of connectivity whereas structural blocks in fluids have lower connectivity.[4]

In the pharmaceutical industry, some amorphous drugs have been shown to offer higher bioavailability than their crystalline counterparts as a result of the higher solubility of the amorphous phase. However, certain compounds can undergo precipitation in their amorphous form in vivo, and can then decrease mutual bioavailability if administered together.[5][6]

Even amorphous materials have some degree of short-range order at the atomic length scale as a result of the nature of intermolecular chemical bonding (see structure of liquids and glasses for more information on non-crystalline material structure). Furthermore, in very small crystals, short-range order encompasses a large fraction of the atoms; nevertheless, relaxation at the surface, along with interfacial effects, distort the atomic positions and decrease structural order. Even the most advanced structural characterization techniques, such as x-ray diffraction and transmission electron microscopy, have difficulty in distinguishing amorphous and crystalline structures at short length scales.[7]

Amorphous phases are important constituents of thin films, which are solid layers of a few nanometres to some tens of micrometres thickness deposited upon a substrate. So-called structure zone models were developed to describe the microstructure of thin films and ceramics as a function of the homologous temperature Th that is the ratio of deposition temperature over melting temperature.[8][9] According to these models, a necessary (but not sufficient) condition for the occurrence of amorphous phases is that Th has to be smaller than 0.3, that is the deposition temperature must be below 30% of the melting temperature. For higher values, the surface diffusion of deposited atomic species would allow for the formation of crystallites with long-range atomic order.

Regarding their applications, amorphous metallic layers played an important role in the discovery of superconductivity in amorphous metals by Buckel and Hilsch.[10][11] The superconductivity of amorphous metals, including amorphous metallic thin films, is now understood to be due to phonon-mediated Cooper pairing, and the role of structural disorder can be rationalized based on the strong-coupling Eliashberg theory of superconductivity.[12] Today, optical coatings made from TiO2, SiO2, Ta2O5 etc. and combinations of them in most cases consist of amorphous phases of these compounds. Much research is carried out into thin amorphous films as a gas separating membrane layer.[13] The technologically most important thin amorphous film is probably represented by few nm thin SiO2 layers serving as isolator above the conducting channel of a metal-oxide semiconductor field-effect transistor (MOSFET). Also, hydrogenated amorphous silicon, a-Si:H in short, is of technical significance for thin-film solar cells. In case of a-Si:H the missing long-range order between silicon atoms is partly induced by the presence of hydrogen in the percent range.

The occurrence of amorphous phases turned out as a phenomenon of particular interest for studying thin-film growth.[14] Remarkably, the growth of polycrystalline films is often used and preceded[needs copy edit] by an initial amorphous layer, the thickness of which may amount to only a few nm. The most investigated example is represented by thin polycrystalline silicon films, where such as the unoriented molecule[needs copy edit]. An initial amorphous layer was observed in many studies.[15] Wedge-shaped polycrystals were identified by transmission electron microscopy to grow out of the amorphous phase only after the latter has exceeded a certain thickness, the precise value of which depends on deposition temperature, background pressure and various other process parameters. The phenomenon has been interpreted in the framework of Ostwald's rule of stages[16] that predicts the formation of phases to proceed with increasing condensation time towards increasing stability.[11][15] Experimental studies of the phenomenon require a clearly defined state of the substrate surface and its contaminant density etc., upon which the thin film is deposited.

Amorphous materials in soil strongly influence bulk density, aggregate stability, plasticity and water holding capacity of soils. The low bulk density and high void ratios are mostly due to glass shards and other porous minerals not becoming compacted. Andisol soils contain the highest amounts of amorphous materials.[17]