Atomic defects Flashcards
What solid state properties can defects affect?
Solid catalysis, ionic conduction, photography and gem stones.
Why do solids have defects?
There is increased entropy (disorder) which is favoured due to gibbs energy. The enthalpy term is more than offset by the gain in entropy, so gibbs energy decreases.
What is the Schottky defect?
A pair of vacancies (anion and cation) found in alkali halides. The overally composition is unchanged and they are stoichiometric.
What is the equation for the number of schottky defects?
=BN e(-deltaH/2RT) where N=the number of sites, delta H is the enthalpy of defect formation, B is a pre exponential factor and R is the gas constant.
What is the Frenkel defect?
When the atom or ion is disordered off its lattice site into a site not normally occupied, leaving a vacancy.
What are intrinsic defects?
They occur only in the pure compounds.
What are extrinsic defects?
Defects due to dopant ions/impurities.
What is the dopant ion in ruby and what does it cause?
Chromium - causes red colour.
What is the dopant ion in sapphire and what does it cause?
Iron and titanium - causes blue colour.
What is the dopant ion in amethyst and what does it cause?
Iron - causes a purple colour.
What are the three main diffusion mechanisms?
Vacancy mechanisms, direct interstitial mechanism and the interstitialcy/knock-on mechanism.
What is the vacancy mechanism?
Migration into an adjacent vacant site.
What is the direct interstitial mechanism?
Interstitial migration - an ion in an interstitial site moves into an adjacent unoccupied site.
What is the knock-on mechanism?
An interstitial ion moves into an adjacent lattice site, displacing the ion into a near interstitial site.
What is the equation for ionic conductivity?
=AE (/Ea/RT)
