Course A: Atomic Structure of Material

Question 1

Alloy

Answer 1

A metallic substance that is composed of two or more elements.

Question 2

Amorphous

Answer 2

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.

Question 3

Anisotropic

Answer 3

Having properties that vary according to the direction of measurement.

Question 4

Body-centred unit cell

Answer 4

A non-primitive unit cell that is described with a lattice point at the centre of each unit cell.

Question 5

Centre of symmetry

Answer 5

A point through which an object can be inverted to bring the object into coincidence with itself.

Question 6

Centrosymmetric

Answer 6

Possessing a centre of symmetry.

Question 7

Ceramic

Answer 7

A compound of metallic and nonmetallic elements, in which the interatomic bonding is predominantly ionic.

Question 8

Close-packed structure

Answer 8

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.

Question 9

Coordination number

Answer 9

The number of atoms forming a polyhedron around a central atom in a structure.

Question 10

Coordination polyhedron

Answer 10

The polyhedron (commonly a tetrahedron or octahedron) that can be constructed around a cation with the centres os the surrounding anions forming the vertices.

Question 11

Conventional unit cell

Answer 11

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.

Question 12

Critical radius ratio

Answer 12

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.

Question 13

Crystal

Answer 13

A solid form of matter showing transitional periodicity in three dimensions in its atomic arrangement.

Question 14

Crystal structure

Answer 14

The arrangement of atoms in a single crystal.

Question 15

Crystal system

Answer 15

Classification based on the symmetry of the lattice.

There are 7 crystal systems.

Question 16

d_hkl

Answer 16

The spacing between lattice planes (hkl)

Question 17

Disordered materials

Answer 17

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)

Question 18

Face-centred unit cell

Answer 18

A non-primitive unit cell that is described with a lattice point a the centre of each face of the unit cell.

Question 19

Ferroelectric material

Answer 19

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.

Question 20

Fractional coordinates

Answer 20

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.

Question 21

Glass

Answer 21

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.

Question 22

Glide plane

Answer 22

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.

Question 23

Ionic bond

Answer 23

A primary bond arising from the electrostatic attraction between two oppositely charged ions.

Question 24

Inversion centre

Answer 24

Same as centre of symmetry.

Question 25

Isomorphous (isomorphic)

Answer 25

of a crystal, having essentially the same crystal structure (which often manifests as having the same shape)

Question 26

Isotropic

Answer 26

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.

Question 27

Lattice

Answer 27

An infinitely array of points repeated periodically throughout space. The view from each lattice point is the same as from any other.

Question 28

Lattice parameters

Answer 28

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.

Question 29

Lattice plane

Answer 29

One of a set of parallel planes containing two non-parallel intersecting lattice vectors, defined by the Miller indices (hkl).

Question 30

Lattice type

Answer 30

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.

Question 31

Lattice vector

Answer 31

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)

Question 32

Long-range order

Answer 32

Order between particles correlated over large distances in a solid.

Question 33

Miller indices

Answer 33

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.

Question 34

Microstructure

Answer 34

The arrangement of phases and other structural features that make up a solid material.

Question 35

Mirror plane

Answer 35

A plane through which reflection of an object brings it back into coincidence with itself.

Question 36

Motif

Answer 36

The element of a structure associated with any lattice point.

Question 37

Multiplicity

Answer 37

The number of equivalent lattice planes in a crystal which are related by its symmetry.

Question 38

Non-crystalline

Answer 38

The solid state wherein there is no long-range atomic order. Sometimes the terms amorphous, glassy and vitreous are used synonymously.

Question 39

Non-primitive unit cell

Answer 39

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.

Question 40

Packing efficiency

Answer 40

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.

Question 41

Perovskite

Answer 41

Family of ABX_3 compounds which commonly show displacive phase transitions by octahedral tilts or atomic displacements.