solid state physics Solid-state physics is the study of rigid matter, or solids, through methods such as quantum mechanics, crystallography, electromagnetism, and metallurgy. It is the largest branch of condensed matter physics. Solid-state physics studies how the l ...

, a superstructure is some additional structure that is superimposed on a higher symmetry

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

line structure. A typical and important example is

ferromagnet Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...

ic ordering. In a wider sense, the term "superstructure" is applied to

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

s and

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

s to describe ordering on a length scale larger than that of monomeric segments.

In crystals

In a crystal, a superstructure manifests itself through additional reflections in

diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...

patterns, e.g., in

low energy electron diffraction Low-energy electron diffraction (LEED) is a technique for the determination of the surface structure of single-crystalline materials by bombardment with a collimated beam of low-energy electrons (30–200 eV) and observation of diffracted el ...

( LEED) or X-ray diffraction

experiment An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into Causality, cause-and-effect by demonstrating what outcome oc ...

s. Often a set of weak diffraction spots appears between the stronger spots belonging to what is referred to as the substructure. In some cases a phase transition occurs, e.g., at higher temperatures, where the superstructure disappears and the material reverts to the simpler substructure. Not all compounds exhibit a superstructure. The superspots in diffraction patterns represent a ''modulation'' of the substructure that causes the inherent translation symmetry of the (substructure) lattice to be violated slightly or the size of the repeat motif of the structure to be increased. One could speak of symmetry breaking of the translation symmetry of the lattice, although rotational symmetry may be lost simultaneously.

Commensurate cases

If the superspots are located at ''simple fractions'' of the vectors of the

reciprocal lattice In physics, the reciprocal lattice represents the Fourier transform of another lattice (group) (usually a Bravais lattice). In normal usage, the initial lattice (whose transform is represented by the reciprocal lattice) is a periodic spatial fu ...

of the substructure, e.g., at q=(½,0,0), the resulting broken symmetry is a multiple of the unit cell along that axis. Such a modulation is called a commensurate superstructure.

Incommensurate cases

In some materials, superspots will occur at positions that do not represent a simple fraction, say q=(0.5234,0,0). In this case the structure strictly speaking has lost all translational symmetry in a particular direction. This is called an incommensurate structure.

Causes

There are basically three types of superstructures in crystals:

Magnetic superstructures

When a crystalline material that contains atoms with uncompensated electron spins is cooled down, ordering of these spins generally occurs once the thermal energy is small enough not to overrule the interactions between neighboring spins. If the ordering does not exhibit the same symmetry as the original unit cell of the crystallographic lattice, a superstructure will result. In this case, the superspots are typically only visible in

neutron diffraction Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material. A sample to be examined is placed in a beam of thermal or cold neutrons to o ...

patterns, because the neutron is scattered both by the nucleus and by the magnetic moments of the electron spins.

Defect ordering

Many alloys of elements that resemble each other chemically will form a structure at higher temperatures where the two elements occupy similar positions in the lattice at random. At lower temperatures ordering may occur where crystallographic positions are no longer equivalent because one element preferentially occupies one site and the other the other. This partial ordering process may lower the translation symmetry and result in a different, larger unit cell.

Displacive transitions

In some transitions a number of atoms occupying crystallographic positions that were originally equivalent will move away slightly from their ideal positions according to a certain pattern. This pattern or repeat motif may span multiple unit cells. The cause of this phenomenon is the small changes in chemical bonding that favor formations of semi-regular and larger clusters of atoms. Although having the undistorted substructure, these materials are typically 'unsaturated' in the sense that one of the bands in the band structure is only partially filled. The distortion changes the band structure, in part splitting the bands up into smaller bands that can be more completely filled or emptied to lower the energy of the system. This process may not go to completion, however, because the substructure only allows for a certain amount of distortion. Superstructures of this type are often incommensurate. A good example is found in the structural transitions of 1T- TaS₂, a compound with a partially filled, narrow d band (Ta(IV) has a d¹ configuration).

References

{{Reflist Condensed matter physics Crystallography Diffraction