Amorphous solid

In

The term comes from the Greek a ("without"), and morphé ("shape, form").

Structure

Amorphous materials have an internal structure consisting of interconnected structural blocks that can be similar to the basic structural units found in the corresponding crystalline phase of the same compound.^[4] Unlike in crystalline materials, however, no long-range order exists. Amorphous materials therefore cannot be defined by a finite unit cell. Statistical methods, such as the atomic density function and radial distribution function, are more useful in describing the structure of amorphous solids.^[1][3]

Although amorphous materials lack long range order, they exhibit localized order on small length scales. Localized order in amorphous materials can be categorized as short or medium range order.^[1] By convention, short range order extends only to the nearest neighbor shell, typically only 1-2 atomic spacings.^[5] Medium range order is then defined as the structural organization extending beyond the short range order, usually by 1-2 nm.^[5]

Fundamental properties of amorphous solids

Glass transition at high temperatures

The freezing from liquid state to amorphous solid -

Universal low-temperature properties of amorphous solids

At very low temperatures (below 1-10 K), large family of amorphous solids have various similar low-temperature properties. Although there are various theoretical models, neither

Amorphous solids is an important area of

On the phenomenological level, many of these properties were described by a collection of tunneling two-level systems.^[8][9] Nevertheless, the microscopic theory of these properties is still missing after more than 50 years of the research.^[10]

Remarkably, a dimensionless quantity of internal friction is nearly universal in these materials.

Due to the lack of long-range order, standard crystallographic techniques are often inadequate in determining the structure of amorphous solids.[14] A variety of electron, X-ray, and computation-based techniques have been used to characterize amorphous materials. Multi-modal analysis is very common for amorphous materials.

X-ray and neutron diffraction

Unlike crystalline materials which exhibit strong Bragg diffraction, the diffraction patterns of amorphous materials are characterized by broad and diffuse peaks.^[15] As a result, detailed analysis and complementary techniques are required to extract real space structural information from the diffraction patterns of amorphous materials. It is useful to obtain diffraction data from both X-ray and neutron sources as they have different scattering properties and provide complementary data.^[16] Pair distribution function analysis can be performed on diffraction data to determine the probability of finding a pair of atoms separated by a certain distance.^[15] Another type of analysis that is done with diffraction data of amorphous materials is radial distribution function analysis, which measures the number of atoms found at varying radial distances away from an arbitrary reference atom.^[17] From these techniques, the local order of an amorphous material can be elucidated.

X-ray absorption fine-structure spectroscopy

X-ray absorption fine-structure spectroscopy is an atomic scale probe making it useful for studying materials lacking in long range order. Spectra obtained using this method provide information on the oxidation state, coordination number, and species surrounding the atom in question as well as the distances at which they are found.^[18]

Atomic electron tomography

The atomic electron tomography technique is performed in transmission electron microscopes capable of reaching sub-Angstrom resolution. A collection of 2D images taken at numerous different tilt angles is acquired from the sample in question, and then used to reconstruct a 3D image.^[19] After image acquisition, a significant amount of processing must be done to correct for issues such as drift, noise, and scan distortion.^[19] High quality analysis and processing using atomic electron tomography results in a 3D reconstruction of an amorphous material detailing the atomic positions of the different species that are present.

Fluctuation electron microscopy

Fluctuation electron microscopy is another transmission electron microscopy based technique that is sensitive to the medium range order of amorphous materials. Structural fluctuations arising from different forms of medium range order can be detected with this method.^[20] Fluctuation electron microscopy experiments can be done in conventional or scanning transmission electron microscope mode.^[20]

Computational techniques

Simulation and modeling techniques are often combined with experimental methods to characterize structures of amorphous materials. Commonly used computational techniques include density functional theory, molecular dynamics, and reverse Monte Carlo.^[14]

Uses and observations

Amorphous thin films

Amorphous phases are important constituents of thin films. Thin films are solid layers of a few nanometres to tens of micrometres thickness that are deposited onto a substrate. So-called structure zone models were developed to describe the microstructure of thin films as a function of the homologous temperature (T_h), which is the ratio of deposition temperature to melting temperature.^[21][22] According to these models, a necessary condition for the occurrence of amorphous phases is that (T_h) has to be smaller than 0.3. The deposition temperature must be below 30% of the melting temperature.^{[b][citation needed]}

Superconductivity

Regarding their applications, amorphous metallic layers played an important role in the discovery of

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.^[29][30]

In soils

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.^[31]

Phase

The occurrence of amorphous phases turned out to be a phenomenon of particular interest for the studying of thin-film growth.

References