Glossary - Michaelmas 2013 Flashcards
Activation energy, Q
in diffusion, this is related to the energy required to move an atom from one lattice site to another (Jmol^-1 or eVatom^-1)
Anode
the electrode at which oxidation occurs
Anisotropic
having properties that vary according to the direction of measurement
Alloy
A metallic substance that is composed of 2 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
Atomic form factor/Atomic scattering factor
Amplitude of radiation scattered by a single atom. It varies with atomic number and with the angle of scattering
Antiferromagnetism
opposition of adjacent magnetic dipoles causing zero net magnetisation
Arrhenius plot
Plot of ln(rate) for a process or reaction vs. 1/T. The slope is proportional to the activation energy
Atactic polymer
polymer with configurational base units in a random sequence
Backbone
main structure of a polymer onto which substituents are attached
Birefringence, delta-n
difference in refractive index between 2 permitted vibration directions
Bloch walls
boundaries between magnetic domains
Body-centred unit cell
non-primitive unit cell that is described with a lattice point at the centre of each unit cell
Bragg equation
basic diffraction equation which relates interplanar spacings to the angle through which beams of radiation are diffracted, for a given wavelength of radiation
Bravais lattice/Lattice type
Classification of lattices based on symmetry, allowing for all possible lattices consistent with a given symmetry. There are 14 Bravais lattices
Centre of symmetry/Inversion centre
point through which an object can be inverted (i.e. all x,y,z transformed to -x,-y,-z) 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
Compounds of metallic anions with non-metallic cations, which commonly have very high melting temperatures
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 structures are ccp and hcp
Constructive interference
combination of rays which are in phase and give an intense beam
Coordination number
number of atoms forming a polyhedron around a central atom in a structure
Coordination polyhedron
polyhedron (commonly tetrahedron or octahedron) that can be constructed around a cation with the centres of the surrounding anions forming the vertices
Conventional unit cell
unit cell that is orientated 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
ratio of cation radius to anion radius for the condition where the surrounding anions are touching each other as well as the central cation
Crossed polars
2 sheets of polaroid (polariser and analyser) orientated at 90 degrees to each other between which a sample is placed for optical examination
Crystal
solid form of matter showing translational periodicity in 3d in its atomic arrangement
Crystal structure
arrangement of atoms in a single crystal
Crystal system
Classification based on the symmetry of the lattice. There are 7 crystal systems
Capacitance,C
charge per unit voltage (Farads or CV^-1)
Capacitor
electrical device consisting of alternating layers of dielectric and conductor, which is capable of storing charge
Cathode
electrode at which reduction occurs
Chiral nematic (cholesteric)
molecules in adjacent layers are orientated at a slight angle relative to each other (rather than parallel as in nematic). Therefore the Director rotates helically, around an axis perpendicular to the layers.
Compensator
sample of an optically anisotropic crystal with a known birefringence (often quartz)
Coercive field Ec/Hc
electric/magnetic field that is required to depolarise/demagnetise a ferroelectric/magnetic material (Vm^-1/Am^-1)
Concentration cell
electrochemical cell whose electromotive force is driven by differences in concentrations across the electrolyte
Concentration gradient
rate of change of composition with distance (m^-4)
Conformation
structure of a polymer chain that arises from rotation about the single bonds
Cross-linking
process in which bonds are formed joining adjacent molecules. At low density these bonds add to the elasticity of a polymer, at higher densities they produce rigidity
Curie temperature
temperature above which ferroelectric/ferromagnetic behaviour is lost (Celsius or K)
Current density, j
current flowing through unit cross-sectional area (Am^-2)
Diamagnetism
effect caused by the magnetic moment due to orbiting electrons which produces a slight opposition to the imposed magnetic field
Dielectric
an electrical insulator which is polarisable
Dielectric breakdown
the passage of current through a dielectric material upon application of a large electric field. The maximum field that a dielectric can sustain without breakdown is the dielectric strength
Dielectric constant, κ
a measure of the rate of change of polarisation with electric field; the ratio of the permittivity of a material to the permittivity of free space
Diffusion
transport of atoms in which the diffusing atom moves relative to its neighbours
Diffusion coefficient (diffusivity), D
a temperature dependent coefficient related to the rate at which atoms diffuse, which also depends on the activation energy (m^2s^-1)
Dipole
2 equal charges of opposite sign, separated by a small distance
Dipole moment, μ
electric field generated by a spatial separation of positive and negative charges (Cm) or the magnetic moment generated by electron orbitals and spin
Director, D
the direction of preferred orientation of molecules in liquid crystals
Disclination
point at which the Director in a liquid crystal undergoes an abrupt change in orientation
Displacement field, D
resultant total polarisation field created in a material by application of an electric field (Cm^-2)
Domains
regions within a material in which all the electric/magnetic dipoles are aligned
Domain wall
boundary between ferroelectric/magnetic domains
Drift velocity
average rate at which charge carriers move through a material under the application of an electric field (ms^-1)
dhkl
spacing between lattice planes (hkl)
Destructive interference
combination of rays which are out of phase, giving zero intensity
Diffraction pattern
distribution of intensity of radiation scattered by an object
Diffractometer
instrument used for recording intensities of diffracted X-rays as a function of diffraction angle and sample orientation. There are many different types of diffractometers
Disordered materials
In general this term is used to refer to materials that lack the 3d long-range periodicity of a crystalline substance (e.g. liquids, glasses, polymers, liquid crystals)
Exchange interaction/exchange energy
quantum mechanical electron - electron interaction which favours parallel alignment of magnetic moments. A negative exchange interaction favours anti-parallel alignment of moments.
Extinction positions
orientations in which an anisotropic material appears black between crossed polars. (They occur every 90 degrees, when the PVDs of the sample are parallel to the polariser/analyser)
Fatigue
accumulation of defects in a ferroelectric material which gradually degrades the amount of switched charge
Ferrimagnetism
magnetic behaviour obtained when 2 types of dipole, with different strengths, oppose one another, leaving a net magnetisation
Ferroelectric
a material possessing a spontaneous net electrical polarisation which can be switched by an external electric field. A ferroelectric material must be polar and switchable
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
Ferroelectric phase transition
transition from a high temperature paraelectric phase to a low temperature ferroelectric phase. Characterised by onset of spontaneous polarisation
Ferroelectric/magnetic hysteresis
difference in polarisation/magnetisation behaviour upon reversing electric/magnetic field
Ferromagnetism
existence of a spontaneous magnetic dipole; alignment of magnetic domains can result in a net magnetisation after removal of an imposed magnetic field
Flux density, J
number of atoms/ions crossing a unit cross sectional area in unit time (m^-2s^-1)
Freedericksz Transition
reorientation of liquid crystal molecules on the application of an electric field
Frenkel defect
crystalline defect consisting of an interstitial and a vacancy
Fuel Cell
energy conversion device that produces electricity by the electrochemical oxidation of a fuel
Face -centred unit cell
non-primitive unit cell that is described with a lattice point at the centre of each face of the unit cell
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 length-scale the bonding may resemble that found in a crystal
Hard magnet
a ferromagnetic material that has a wide hyteresis loop and high remanence
Hysteresis loop
loop traced out by polarisation/magnetisation as an applied electric/magnetic field is cycled
Glide plane
glide plane is symmetry operation describing how a reflection in a plane, followed by a transition parallel with that plane, may leave the crystal unchanged
Ionic bond
primary bond arising from the electrostatic attraction between 2 oppositely charged ions
Isotropic
Having properties that are the same regardless of the direction of measurement. In the isotropic state, all directions are indistinguishable from each other
Isotactic polymer
polymer with configurational base units in a single sequential arrangement
Kuhn length/segment
theoretical treatment of a polymer chain, divided into n Kuhn segments, with Kuhn length, l, so that each segment can be thought of as freely jointed. Complex polymers may be simply modeled as a random walk
Lambda (lambda sensor)
the air/fuel weight ratio for a petrol engine normalised by the corresponding ratio for complete stoichiometric combustion of petrol
Liquid crystal
A phase characterised by anisotropy of properties without the existence of a 3d crystal lattice i.e. orientational order, without positional order
A thermodynamic stable phase characterised by anisotropy of properties without the existence of a 3d crystal lattice, generally lying in the temperature range between the solid and isotropic liquid phase, hence the term mesophase
Lattice
An infinite array of points repeated periodically throughout space. The view from each lattice point is the same as from any other
Lattice parameters/Unit cell parameters
set of lengths of the edges of the unit cell, a,b,c, and the angles between the unit cell axes, α, β, γ
Lattice plane
one of a set of parallel planes containing 2 non-parallel intersecting lattice vectors, defined by the Miller indices (hkl)
Lattice vector
vector between 2 lattice points, defined by the symbol [UVW] = Ua + Vb + Wc
Long-range order
order between particles correlated over large distances in a solid
Miller indices
set of parallel, equally spaced lattice planes is specified by the Miller indices (hkl) with reference to the unit cell, where h,k,l are integers. The first plane out from the origin makes intercepts of length a/h,b/k,c/l along the 3 unit cell axes. If one of the indices is 0, the planes are parallel to the associated axis
Microstructure
the arrangement of phases and other structural features that make up a solid material
Mirror plane
plane through which reflection of an object brings it back into coincidence with itself
Motif
element of a structure associated with any lattice point
Multiplicity
number of equivalent lattice planes which are related by symmetry; important for understanding the intensities of X-ray powder diffraction patterns
Magnetic Flux density (Inductance, B)
total magnetic flux per unit area (Tesla)
Magnetic moment, m
strength of the magnetic field associated with electron orbitals/spin (Am^2)
Magnetic permeability, μ
ratio between magnetic flux density, B, & applied magnetic field, H (Hm^-1)
Magnetic susceptibility, χ (Susceptibility)
measure of the effect of a magnetic field on the magnetisation of a material; ratio between magnetisation, M, and the applied field, H (dimensionless)
Magnetisation, M
sum of all magnetic moments per unit volume (Am^-1)
Magnetocrystalline anisotropy energy, K
extra energy, per unit volume, required to magnetise a material along the ‘hard’ crystallographic axis (Jm^-3)
Magnetostriction coefficient, Λ
fractional change in length on changing magnetisation from zero to saturation (dimensionless)
Monomer
simple chemical unit which, when many are joined together, form a polymer
Non-crystalline
solid state wherein there is no long-range atomic order. Sometimes the terms amorphous, glassy and vitreous are used synonymously
Non-primitive unit cell
unit cell that encompasses more than one lattice point. 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
Nematic
spontaneously anisotropic liquid, composed of rod-shaped molecules with parallel alignment
Optic axis
for a birefringent sample, a direction along which no birefringence is observed i.e. a direction perpendicular to an isotropic section. The optic axis of a nematic LC is along the director
Optical Path difference
extra distance traveled by the fast ray outside an anisotropic material in the time it takes for the slow ray to reach the edge of the sample (m)
Order parameter
describes the orientational order of a liquid crystalline material, taking account of the deviation of orientation of individual molecules from the Director ( 0 for complete disorder, 1 for perfect order)
Paramagnetism
net magnetic moment caused by alignment of the electron spins when a magnetic field is applied
Permitted Vibration Directions
2 perpendicular directions in an anisotropic material along which the transverse electric field of light can vibrate
Permittivity, ε
describes how an electric field is affected by a dielectric (Fm^-1)
Perovskite
family of ABX3 compounds which commonly show displacive phase transitions by octahedral tilts or atomic displacements
Phase
a part of a system, homogeneous in composition and structure
a physically distinct form of a given material, when the material can exist as different phases under different conditions of temperature or pressure. Also used to refer to a homogeneous portion of a system that has uniform physical and chemical characteristics
Phase diagram
a diagram showing the equilibrium phases and phase compositions at each combination of temperature (and/or pressure) and overall composition
Photoelasticity
stress-induced birefringence. In a polymer, this is caused by stress-induced alignment of polymer chains
Piezoelectricity
application of stress produces a change in the dielectric polarisation of a material or, conversely, application of an electric field results in strain. Piezoelectric materials must be non-centrosymmetric
Piezoelectric coefficient, d
measure of the rate of change of dielectric polarisation with applied stress (CN^-1)
Pitch
distance it takes for the Director in a cholesteric/chiral nematic LC to go through one complete 360 rotation (m)
Polar material
material with crystal symmetry such that it contains one (or more) unique direction
Polarisation, P
the dipole moment per unit volume in a medium, or charge per unit area (Cm^-2)
Polarisation (of light)
property of electromagnetisation waves that describes the direction of the transverse electric field
Polarised light
light that has passed through a sheet of polariser, and has a transverse electric field that vibrates in one direction only
Poled ferroelectric
ferroelectric material that has been brought to saturation by a strong electric field and then allowed to relax back to the remanent condition
Polymer
long chain of covalently bonded atoms
Pyroelectricity
change in electrical polarisation due to a change in temperature. Pyroelectric materials must be polar
Pyroelectric coefficient, p
measure of the rate of change of dielectric polarisation with temperature (Cm^-2K^-1)
Packing efficiency
ratio of the volume contained within the atoms in a crystal structure to the volume of the crystal. Put another way, it is the fraction of the volume of the crystal that is contained within the constituent atoms
Phase angle
the phase difference between radiation scattered from an atom at the origin of a unit cell and from an atom at a general position within the unit cell
Polymorphism
existence of a single compound as 2 or more phase with different crystal structures stable under different conditions
Primitive unit cell
a unit cell that encompasses only one lattice point i.e. a unit cell where the lattice points are only found at the corners of the unit cell
Radius ratio
ratio of the radii of the cations and anions in a simple ionic structure, which is sometimes useful as a measure of the relative stabilities of different structures possible for a given material
Rotation axis
the axis, of order n, about which rotation by a given fraction 1/n of a complete rotation brings an object back into coincidence with itself
Refractive index, n
property that describes the speed of light in a material relative to that in a vacuum. The higher the refractive index, the slower the light travels
Remanence
the polarisation/magnetisation that remains in a material after it has been removed from an applied electric/magnetic field, due to permanent alignment of dipoles (Cm^-2/Am^-1)
Saturation polarisation/magnetisation
the state in which all electric/magnetic dipoles have been aligned by an electric/magnetic field, producing maximum polarisation/magnetisation (Cm^-2/Am^-1)
Sawyer-Tower circuit
test circuit for measuring the hysteresis loop of ferroelectric materials
Schottky defect
an anion and cation vacancy pair
Smectic
liquid crystal phase in which the molecules organise themselves into layers
SOFC
Solid Oxide Fuel Cell
Soft magnet
ferromagnetic material that has a narrow hysteresis loop and little energy loss on cycling the field
Spontaneous polarisation/magnetisation
cooperative phenomenon of electric/magnetic dipole ordering below the Curie temperature (Cm^-2/Am^-1)
Stoichiometric
obeying a strict chemical formula e.g. anion/cation ratio
Syndiotactic polymer
a polymer with configurational base units in a regular, alternating sequence
Screw Axis
a screw axis is a symmetry operation describing how a combination of rotation about an axis and a translation parallel to that axis leaves a crystal unchanged
Structure factor
amplitude of radiation scattered by all atoms in a unit cell. It depends on (a) the fractional coordinates, x, y, z, of atoms in a unit cell and (b) diffraction geometry, as specified by the planes (hkl) from which the diffracted beams are said to be reflected
Systematic absences
Diffracted beams from a crystal which have strictly zero intensity, due to repeating units of structure which scatter exactly out of phase with each other. They are most easily summarised in terms of lattice type
Short range order
Order which implies strong correlations between the positions of atoms, in a structure such as glass, over distances of a few Angstroms
Translational order
A condition when molecules have some arrangement in space. Crystals have 3 degrees of translational order (each molecule is fixed in space with an x,y,z coordinate) and liquids have no translational order
Tacticity
arrangement of side-groups on a polymer backbone
Transducer
device that receives one type of input (such as strain) and provides a different type of output (such as an electrical signal)
Transformation/transition
denote a change in phase (terms used interchangeably)
Twisted nematic
liquid crystals physically forced to adopt a chiral structure
Unique direction
a lattice vector in a crystal which is not repeated by any of the symmetry elements present
Unit cell
any parallepiped in a lattice having lattice points at all corners
