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CH10134 > Atomic Defects in Solids > Flashcards

Atomic Defects in Solids Flashcards

1
Q

What are point defects

A
  1. All crystalline solids contain defects (imperfections) of structure or composition (for T>0K)
  2. Point defects occur at single lattice sites
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2
Q

What do defects influence

A
  1. Optical/photographic (Ag defects in AgCl)
  2. Ionic conduction (mobile defects in batteries)
  3. Solid catalysts (surface defects)
  4. Gemstones (colour due to impurities)
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3
Q

What are the defect concentrations normally

A
  1. Normally very small < 1% in most solids
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4
Q

Why do solids have defects

A
  1. They introduce disorder so increase entropy
  2. Formation of defects is normally endothermic because the lattice is disrupted so the enthalpy of the solid increases
  3. But -TS becomes more negative as defects introduce disorder into the lattice and entropy rises
  4. If T>0 the Gibbs energy will have a minimum at a nonzero concentration of defects and their formation will be spontaneous
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5
Q

What happens to the number of defects as temperature increases

A
  1. As temperature of solid is raised, the minimum in G shifts to higher defect concentrations
  2. So solids have a greater number of defects as their melting points are approached
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6
Q

Where is defect formation

A
  1. Intrinsic

2. Occurs in pure compound

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7
Q

What are the different types of defects

A
  1. Schottky defect

2. Frenkel defect

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8
Q

What is a Schottky defect

A
  1. A pair of vacancies (anion + cation)
  2. Found in alkali halides (NaCl, KCl etc) and binary oxides (MgO, CaO)
  3. Intrinsic defect
  4. stoichiometric- overall composition is unchanged- equal numbers of the two vacancies to keep charge-balanced
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9
Q

What is the name for defect notation

A
  1. Kroger-Vink notation
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10
Q

What is the notation for a vacancy

A
  1. V= vacancy
  2. Superscript= effective charge (dot= +ve, dash= -ve)
  3. Subscript= atom symbol
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11
Q

Give a Schottky equation for NaCl

A
  1. Na(Na)x + Cl(Cl)x –> V(Na)I + V(Cl). + NaCl (surface)
  2. X= effective normal lattice charge
  3. Left hand side= normal lattice sites
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12
Q

Give the schottky equation for CaCl2

A
  1. Ca(Cl)x + 2Cl(Cl) x –> V(Ca)II + 2V(Cl). + CaCl2 (surface)
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13
Q

Give the schottky equation for MgO

A
  1. Mg(Mg)x + O(O) X –> V(Mg)II + V(O).. + MgO (surface)
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14
Q

How can you calculate the number of Schottky defects

A
  1. Nschottky = BNexp (-deltaH/2RT)
  2. N= total number of sites
  3. Delta H= enthalpy of defect formation
  4. T= temperature
  5. B= pre-exponential factor
  6. R= Gas constant
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15
Q

Where do Schottky defects most commonly occur

A
  1. Purely ionic solids (NaCl)
  2. Most commonly in structures with high coordination number- close packed ions and metals as enthalpy of reducing the average coordination number is relatively low
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16
Q

What is a Frenkel defect

A
  1. Atoms displaced off its lattice site into a site not normally occupied (interstitial site)
  2. Leaves a vacancy behind
  3. Occurs only on one type of site- cation or anion
  4. Also intrinsic and stoichiometric
17
Q

Give an example of a Frenkel defect

A
  1. AgCl or AgBr (rock-salt structure)

2. Thin film photography depends on Ag+ interstitials moving and clustering to form the latent image

18
Q

Give the defect notation for Frenkel defect

A
  1. Element notation
  2. Superscript- same charge notation as in schottky (dot or dash)
  3. Subscript- i
19
Q

Give the Frenkel defect equation for AgCl

A
  1. Ag(Ag)x –> Agi. + VAg I

2. Normal lattice site –> defects

20
Q

Give the Frenkel defect equation for ZnS with Zn defect

A
  1. Zn(Zn) x –> Zni.. + V(Zn) II
21
Q

Give the Frenkel defect equation for CaF2 with F defect

A
  1. F(F) x –> Fi I + V(F).
22
Q

What is an extrinsic defect

A
  1. Those resulting from the presence of impurities- dopant ions
  2. Dopants often improve of change properties (performance enhancing)
23
Q

What is an aliovalent dopant

A
  1. Dopant ions with a different valency to host lattice ion
24
Q

Give 3 examples of dopants

A
  1. MgO doped with Li+ = solid catalyst
  2. ZrO2 doped with Y3+ = ion conductors
  3. La2CaO4 doped with Ba 2+ = high Tc superconductor
25
Q

Name the colour, dopant and parent compound of ruby

A
  1. Red
  2. Cr3+ replacing Al3+
  3. Al2O3
26
Q

Name the colour, dopant and parent compound of saphire

A
  1. Blue
  2. Electron transfer between Fe2+ and Ti4+ replacing Al3+ in adjacent octahedral sites
  3. Al2O3
27
Q

Name the colour, dopant and parent compound of amethyst

A
  1. Purple
  2. Fe3+/4+
  3. SiO2
28
Q

What is the notation for a dopant e.g. NaCl doped with Ca2+ (as CaCl2)

A
  1. Ca(Na).

2. Charge-compensating defect- V(Na)I

29
Q

Give the equation for the dopant defect of NaCl doped with Ca2+ (as CaCl2)

A
  1. CaCl2(dopant) + 2Na(Na)x (host) –> Ca(Na). + V(Na)I + 2NaCl (surface)
30
Q

Give the equation for the dopant defect of MgO doped with LI+ (as LI2O)

A
  1. Li2O + 2Mg(Mg)x + O(O)x –> 2Li(Mg)I + V(O).. + 2MgO

CH10134 (15 decks)

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