Course A: Atomic Structure of Material Flashcards
Alloy
A metallic substance that is composed of two or more elements.
Amorphous
Irregular; having no discernible order or shape.
In the context of solids, the molecules are randomly arranged, as in glass, rather than periodically arranged, as in a crystalline material.
A non-crystalline structure.
Anisotropic
Having properties that vary according to the direction of measurement.
Body-centred unit cell
A non-primitive unit cell that is described with a lattice point at the centre of each unit cell.
Centre of symmetry
A point through which an object can be inverted to bring the object into coincidence with itself.
Centrosymmetric
Possessing a centre of symmetry.
Ceramic
A compound of metallic and nonmetallic elements, in which the interatomic bonding is predominantly ionic.
Close-packed structure
A structure in which the atoms are packed closely together.
For structures made of only one atom type, the common close-packed structure are cubic close-packed and hexagonal close-packed.
Coordination number
The number of atoms forming a polyhedron around a central atom in a structure.
Coordination polyhedron
The polyhedron (commonly a tetrahedron or octahedron) that can be constructed around a cation with the centres os the surrounding anions forming the vertices.
Conventional unit cell
A unit cell that is oriented in a specific way with respect to the symmetry elements of the crystal.
The conventional cell may or may not be primitive.
Critical radius ratio
The ratio of cation radius to anion radius for the condition where the surrounding anions are touching each other as well as the central atom.
Crystal
A solid form of matter showing transitional periodicity in three dimensions in its atomic arrangement.
Crystal structure
The arrangement of atoms in a single crystal.
Crystal system
Classification based on the symmetry of the lattice.
There are 7 crystal systems.
d_hkl
The spacing between lattice planes (hkl)
Disordered materials
In general, this term is used to refer to materials that lack the three-dimensional long-range periodicity of a crystalline substance (e.g. liquids, glasses, polymers, liquid crystals)
Face-centred unit cell
A non-primitive unit cell that is described with a lattice point a the centre of each face of the unit cell.
Ferroelectric material
One that produces domains of spontaneous polarisation whose polar axis can be reversed in an electric field directed opposite to the total dipole moment of the lattice.
Fractional coordinates
Set of coordinates x, y, z that define the position of an atom in a unit cell in terms of fractions of the unit cell lengths.
Glass
A solid form of matter formed by cooling a liquid sufficiently fast to avoid crystallisation.
The arrangement of atoms in a glass does not exhibit periodicity, but on a short-scale the bonding may resemble that found in a crystal.
Glide plane
A glide plane is symmetry operation describing how a reflection in a plane, followed by a translation parallel with that plane, may leave the crystal unchanged.
Ionic bond
A primary bond arising from the electrostatic attraction between two oppositely charged ions.
Inversion centre
Same as centre of symmetry.
Isomorphous (isomorphic)
of a crystal, having essentially the same crystal structure (which often manifests as having the same shape)
Isotropic
Having properties that are the same regardless of the direction of measurement.
In the isotropic state, all directions are indistinguishable from each other.
See also anisotropic.
Lattice
An infinitely array of points repeated periodically throughout space.
The view from each lattice point is the same as from any other.
Lattice parameters
The set of lengths of the edges of the unit cell, a, b and c, and the angle between the unit cell axis, alpha, beta and gamma.
Lattice plane
One of a set of parallel planes containing two non-parallel intersecting lattice vectors, defined by the Miller indices (hkl).
Lattice type
Classification of lattice based on the number of lattice points per unit cell.
Primitive lattices have on lattice point per cell, and non-primitive lattices have more than one lattice point per cell.
Lattice vector
The vector between two lattice points, defined by the symbol [UVW] = Ua+Vb+Wc, where a, b and c are the basis vectors of crystal lattice and U, V and W are integers (components of basis vectors)
Long-range order
Order between particles correlated over large distances in a solid.
Miller indices
A set of parallel, equally spaced lattice planes is specified by Miller indices (hkl) with reference to the unit cell, where h, k and l are integers. The first plane out from the origin makes intercepts of lengths a/h, b/k and c/l along the three unit cell axes. If one, or more, of the indices is zero, the plane are parallel to the associated axis, or axes.
Microstructure
The arrangement of phases and other structural features that make up a solid material.
Mirror plane
A plane through which reflection of an object brings it back into coincidence with itself.
Motif
The element of a structure associated with any lattice point.
Multiplicity
The number of equivalent lattice planes in a crystal which are related by its symmetry.
Non-crystalline
The solid state wherein there is no long-range atomic order. Sometimes the terms amorphous, glassy and vitreous are used synonymously.
Non-primitive unit cell
A unit cell that encompasses more than one lattice points.
Lattice points will be found at the corners of the unit cell, and also in positions such as the centre of the unit cell or in the centres of some or all of the faces of the unit cell.
Packing efficiency
The ratio of the volume contained within the atoms in a crystal structure to the volume of the crystal.
Put another way, the fraction of the volume of the crystal that is contained within the constituent atoms.
Perovskite
Family of ABX_3 compounds which commonly show displacive phase transitions by octahedral tilts or atomic displacements.
