Topic 4 - Characterising Defects Flashcards
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?
How is WO3 an example of crystallographic shear structures?
WO3 forms a ReO3 - type structure
On reduction to W5+, oxide anion vacancies occur on specific crystal planes
This layer of vacancies is unstable and the structure collapses to remove these vacancies
- this creates a crystallographic shear plane.
What techniques can be used to analyse defect structures?
Pair distribution function (PDF) analysis
X-ray absorption spectroscopy
Solid state NMR
Transmission electron microscopy (TEM)
Give detail on Transmission Electron Microscopy.
High resolution (≤1Å)
Electron beam (10^6eV) passes through very thin sample
- lenses focus beam
Image is produced is 2D projection of structure, including defects
What is the cause of the dark spots observed in the image of La(4)Sr(n-4)Ti(n)O(3n+2) where n=12?
Randomly distributed oxygen-rich defects appear as dark spots in the image
- from relaxation of crystal structure around the place where oxides are
What happens in La(4)Sr(n-4)Ti(n)O(3n+2) when n=8?
Defects form layers randomly distributed in the crystal
What happens in La(4)Sr(n-4)Ti(n)O(3n+2) when n=5?
Defects form fully ordered layers
What is the relationship between microstructure, composition and conductivity of the La(4)Sr(n-4)Ti(n)O(3n+2) series?
Conductivity is proportional to the number of charge carriers…
What information do total scattering (Bragg + diffuse) experiments give?
Diffuse scattering provides info on short-range structure of materials
Total scattering can even be used for amorphous solids & liquids
Whats the use of total scattering (Bragg + diffuse) experiments for disordered crystalline materials?
They help characterise the periodic structure AND the deviations from long-range order
In total scattering (Bragg + diffuse) experiments what does the intensity of diffraction tell you?
How is total scattering and the pair distribution function (PDF) related?
Total scattering is mathematically related to the PDF via Fourier transform
(Same relation as reciprocal lattice and real crystal)
What information does the PDF give on crystal structure?
Peak positions give interatomic separations
Area - under curve - provides info about coordination number
Whats the importance of Bi perovskite oxides?
Bi3+ on A-site are potential lead-free ferroelectrics (PZT)
What must the cations must be on the B-site of Bi perovskite oxides?
B-cations must be mixed to stabilise Bi3+ A-cation
What does diffraction analysis provide about the structure of Bi perovskite oxides?
Rhombohedral distortion - 2 different oxide distances
Disordered distribution of B-cations
What is the Big-box model?
Build a model of many 1000s of unit cells (supercell) and make changes to fit the PDF
What does fitting the experimental PDF data show about B-O distances in (see image) ?
It shows a significant difference from experimental PDF at short-range
- distances related to the B-O distances
What does fitting the experimental PDF data show about Ti4+ in (see image) ?
Fitting the experimental PDF data shows that Ti4+ has a markedly different coordination geometry, not apparent in XRD.
- due to small size of Ti4+ cation
What additional information does PDF analysis give over XRD alone?
PDF analysis resolves differences in B-cation coordination, not possible from diffraction alone
B-cation coordination differences suggest role in stabilising Bi3+ on A-site
- need flexible coordination at B-site
