Atomic Defects in Solids Flashcards
What are point defects
- All crystalline solids contain defects (imperfections) of structure or composition (for T>0K)
- Point defects occur at single lattice sites
What do defects influence
- Optical/photographic (Ag defects in AgCl)
- Ionic conduction (mobile defects in batteries)
- Solid catalysts (surface defects)
- Gemstones (colour due to impurities)
What are the defect concentrations normally
- Normally very small < 1% in most solids
Why do solids have defects
- They introduce disorder so increase entropy
- Formation of defects is normally endothermic because the lattice is disrupted so the enthalpy of the solid increases
- But -TS becomes more negative as defects introduce disorder into the lattice and entropy rises
- If T>0 the Gibbs energy will have a minimum at a nonzero concentration of defects and their formation will be spontaneous
What happens to the number of defects as temperature increases
- As temperature of solid is raised, the minimum in G shifts to higher defect concentrations
- So solids have a greater number of defects as their melting points are approached
Where is defect formation
- Intrinsic
2. Occurs in pure compound
What are the different types of defects
- Schottky defect
2. Frenkel defect
What is a Schottky defect
- A pair of vacancies (anion + cation)
- Found in alkali halides (NaCl, KCl etc) and binary oxides (MgO, CaO)
- Intrinsic defect
- stoichiometric- overall composition is unchanged- equal numbers of the two vacancies to keep charge-balanced
What is the name for defect notation
- Kroger-Vink notation
What is the notation for a vacancy
- V= vacancy
- Superscript= effective charge (dot= +ve, dash= -ve)
- Subscript= atom symbol
Give a Schottky equation for NaCl
- Na(Na)x + Cl(Cl)x –> V(Na)I + V(Cl). + NaCl (surface)
- X= effective normal lattice charge
- Left hand side= normal lattice sites
Give the schottky equation for CaCl2
- Ca(Cl)x + 2Cl(Cl) x –> V(Ca)II + 2V(Cl). + CaCl2 (surface)
Give the schottky equation for MgO
- Mg(Mg)x + O(O) X –> V(Mg)II + V(O).. + MgO (surface)
How can you calculate the number of Schottky defects
- Nschottky = BNexp (-deltaH/2RT)
- N= total number of sites
- Delta H= enthalpy of defect formation
- T= temperature
- B= pre-exponential factor
- R= Gas constant
Where do Schottky defects most commonly occur
- Purely ionic solids (NaCl)
- Most commonly in structures with high coordination number- close packed ions and metals as enthalpy of reducing the average coordination number is relatively low
What is a Frenkel defect
- Atoms displaced off its lattice site into a site not normally occupied (interstitial site)
- Leaves a vacancy behind
- Occurs only on one type of site- cation or anion
- Also intrinsic and stoichiometric
Give an example of a Frenkel defect
- AgCl or AgBr (rock-salt structure)
2. Thin film photography depends on Ag+ interstitials moving and clustering to form the latent image
Give the defect notation for Frenkel defect
- Element notation
- Superscript- same charge notation as in schottky (dot or dash)
- Subscript- i
Give the Frenkel defect equation for AgCl
- Ag(Ag)x –> Agi. + VAg I
2. Normal lattice site –> defects
Give the Frenkel defect equation for ZnS with Zn defect
- Zn(Zn) x –> Zni.. + V(Zn) II
Give the Frenkel defect equation for CaF2 with F defect
- F(F) x –> Fi I + V(F).
What is an extrinsic defect
- Those resulting from the presence of impurities- dopant ions
- Dopants often improve of change properties (performance enhancing)
What is an aliovalent dopant
- Dopant ions with a different valency to host lattice ion
Give 3 examples of dopants
- MgO doped with Li+ = solid catalyst
- ZrO2 doped with Y3+ = ion conductors
- La2CaO4 doped with Ba 2+ = high Tc superconductor
Name the colour, dopant and parent compound of ruby
- Red
- Cr3+ replacing Al3+
- Al2O3
Name the colour, dopant and parent compound of saphire
- Blue
- Electron transfer between Fe2+ and Ti4+ replacing Al3+ in adjacent octahedral sites
- Al2O3
Name the colour, dopant and parent compound of amethyst
- Purple
- Fe3+/4+
- SiO2
What is the notation for a dopant e.g. NaCl doped with Ca2+ (as CaCl2)
- Ca(Na).
2. Charge-compensating defect- V(Na)I
Give the equation for the dopant defect of NaCl doped with Ca2+ (as CaCl2)
- CaCl2(dopant) + 2Na(Na)x (host) –> Ca(Na). + V(Na)I + 2NaCl (surface)
Give the equation for the dopant defect of MgO doped with LI+ (as LI2O)
- Li2O + 2Mg(Mg)x + O(O)x –> 2Li(Mg)I + V(O).. + 2MgO
