Topic 4 - Characterising Defects Flashcards
(43 cards)
Give 2 examples of intrinsic (stoichiometric) defects?
Schottky defects
- pair of vacancies where anion and cation are both missing to maintain charge neutrality, common in AB solids.
Frenkel defects
- ion (usually cation) moves into an interstitial site that is normally unoccupied, common in AgX halides
Both are intrinsic point defects and occur in pure materials, causing no overall change in composition
What equation gives the number of Schottky defects in a crystal?
Ns = number of defects
N = number of lattice sites
∆Hs = the enthalpy of formation of 1 mole of Schottky defects
What equation gives the number of Frenkel defects in a crystal?
NF = number of defects
N = number of lattice sites
Ni = number of interstitial sites
∆HF = enthalpy of formation of 1 mole of Frenkel defects
Why in both cases of intrinsic defects is it usually cations which move?
Because cations are usually smaller and so cause less disruption in the solid
In what structure type would anions move to give intrinsic defects?
Fluorite
- CaF2
- Oct holes are free
- Anion can diffuse & fill interstitial hole
What are extrinsic (non-stoichiometric) defects?
These are associated with a change of composition or incorporation of an impurity species.
They are also common when several valences are possible for an ion in the structure, such as Fe or Cu.
i.e. doping
Why are materials doped?
Because properties vary dependent on the composition, therefore non-stoichiometry can be exploited to tune the properties of a material
What are aliovalent impurities?
Aliovalent impurities are where the valency of the impurity atom is different from that of the host crystal
Why is doping with aliovalent impurities done?
Frequently done to introduce vacancy defects, retaining the structure type but forming vacancies to maintain charge balance
What are colour centres?
They are observed in alkali halides, such as NaCl
- F-centre is best known
Consists of a free electron trapped on a vacant anion site
Colour is then emitted and is related to the transitions between available energy levels
- is characteristic of host lattice
How are colour centres formed?
They are formed by high energy radiation (x-rays) or exposure to alkali metal vapour
What are solids that have high dopant concentrations called?
Solid solutions
What is Vegard’s law?
Vegard’s law states that the unit cell parameters of a material should change linearly with the solid solution composition
How is charge carried by ionic conductors (electronic insulators)?
Charge is carried by interstitial ions or vacancies which can move in the structure
This is called ionic compensation
What does the type of interstitial or vacancies generated depend on?
They depend on whether the dopant has a higher or lower valence than the host
What’re the 2 possibilities for ionic compensation of non-stoichiometric solids when doping with cations?
Cation interstitial
- some of host cation is retained in interstitial sites
Anion vacancies
- anions are removed to attain charge neutrality
How does ionic conductivity occur?
Though the presence of point defects, which allow the diffusion of atoms through the lattice
What equation gives the ionic conductivity of a solid?
What is Zirconia?
Zirconia (ZrO2) is a very high melting point ceramic - 2700˚C - that undergoes several crystal phase transitions on cooling
Why is Zirconia doped with Yttria?
Replacing some Zr4+ with aliovalent cations - such as Y3+ - suppresses the phase transitions.
Whats the importance of Yttria-stabilised Zirconia?
YSZ is important because it is stable over a large temperature range and is a fast oxide ion conductor
Whats the importance of solid oxide fuel cells (SOFCs)?
SOFCs are clean energy devices that can produce electricity from fuels such as hydrogen, air & methanol
What are extended defects?
These are defects that extend beyond a small number of sites throughout a larger volume of the crystal
These may be seen as clusters or along lines or planes of the crystal structure
What different forms of extended defects are there?
