Title: ETH_Sem4 / ACII / AC 2 ZSF VCS

Q: X-Ray diffraction
A: • X-rays are scattered by electron cloud
• the scattering factor increases linearly with its atomic number, making heavy atoms more effective in scattering
• intensity drops at hight theta values

Q: Neutron diffraction
A: • neutrons are scattered by nucleus
• large sample size required

Q: Linear density:
A: the number of atoms per length whose centers lie on the the direction vector of a crystallographic direction.

Q: Planar density
A: the number of atoms per unit area that are centered on a particular crystallographic plane.

ZnS (zinc blende)

NaCl

CsCl

CaF_2, Na_2O

diamond

ZnS (wurtzite)

NiAs

CdI_2

MoS_2 (molybdenite)

TiO_2 (rutile)

ReO_3 (bronze)

MgAl_2O_4 (Spinell)

alpha-Al_2O_3
(Corundum)

Q: General information on crystal structures
A: • Ionic conductivity often found in structure types NaCl, CaF2, MgAl2O4
• Semiconductive properties found in structure types diamond, zinc blende, wurtzite
• Metallic conduction found in NiAs
• Layered structures (anisotropic properties) found in CdI2, CdCl2, MoS2
• In oxides (TiO2, ReO3), changes in the oxidation state of the metal alters the electronic properties
• Structures with Td groups may show piezoelctricity (ZnS-blende, ZnS-wurtzite)
• Wurtzite is more ionic than zinc blende

ZnS radius

NaCl radius

Q: 0.732 - 1
A: CsCl radius

Q: 1.633
A: ZnS (wurtzite) radius

Q: 1.391
A: NiAs radius

electrical properties? magneti properties?

Q: Czochralski growth of single crystals from melts
A: - Refined polycrystalline silicon is loaded and melted.
- Then it is pulled out of the melt on a seed crystal, thereby forming a single crystal of silicon.
- Silicon melting point is 1414°C.
- Growth rate: several cm/hour upon vertical pulling.
- Typical crystal sizes: up to 30 cm in diameter, 1-2 meter long.
- This type of Si is standard for the electronic industry. Very high quality of ingots. Possible contamination with SiO_2.

Q: Flat-zone growth of single crystals and zone-refining
A: - Polycrystalline silicon rod is passed through an RF heating coil, which creates a localized molten zone from which the crystal grows.
- The molten zone carries the impurities away with it and hence reduces impurity concentration (most impurities are more soluble in the melt than the solid).
- After cooling, the end containing the impurities is cut off.
- The FZ process is carried out in an evacuated chamber or in an inert gas purge.
- Concentrations of carbon and oxygen impurities are extremely low.
- Nitrogen helps to control micro-defects and also improves mechanical strength.
- Other dopant gases can be introduced.
- The diameters of flat-zone crystals are generally not greater than 150 mm.
- As a variation, solid state refining can be done as well.
- Related methods include the Bridgman and Stockbarger methods, where a temperature gradient is maintained across a melt so that crystallisation starts at the cooler end on a seed crystal; this can be achieved either by using a furnace with a temperature gradient or by pulling the sample through a furnace. The crystallisation is in the same orientation as the seed.

Q: Verneuil growth of single crystals
A: - The powdered sample is melted in a high-temperature oxyhydrogen (H_2 + O_2) flame
- droplets are allowed to fall onto a seed crystal where they crystallise.
- Using the same method but using a plasma torch to melt the powder can achieve even higher temperatures.

Q: Directional Solidification of polycrystalline Silicon
A: - Silicon is melted by induction inside a melting crucible
- droplets fall into a growth crucible made of graphite or quartz coated with Si_3N_4 or other compounds (to discourage sticking or enable reuse) where they crystallise.
- it is a cheaper method used by the photovoltaics industry yielding polycrystalline silicon.

Q: Growing nanowires
A: - A surface with gold impurities is used.
- Gaseous precursors of Si (zB silane) are brought to the surface where silicon will dissolve inside the Au droplets
- and then solidify onto the surface.
- New silicon atoms will diffuse into the gold droplet from the gas and a nanowire will form.

Q: Molecular Beam Epitaxy (MBE)
A: - Si-atoms emitted by an effusion cell (that contains heated elemental Si) arrive at the substrate as an atomic beam.
- In an ultra-high vacuum environment (UHV, < 10^-8 torr)
- UHV is needed to avoid any collision of Si beam with other molecules (this will destroy the beam).
- MBE allows very precise growth of thin films of Si and many other materials.
- the method is very complex and hard to up-scale.

Q: Liquid precursors to silicon
A: - To produce films of silicon, cyclopentasilane (CPS) is used:
- Cyclopentasilane converts into amorphous silicon film between two parallel substrates under atmospheric pressure by thermal decomposition at 350-400 °C
- Combines the advantages of high throughput with cost reduction and high quality film formation.

technique?

use/products?

Advantages? Disadvantages?

important considerations?

Q: Ionic conductivity often found in structure types ....
A: NaCl, CaF_2, MgAl_2O_4

Q: Semiconductive properties found in structure types ...
A: diamond, zinc blende, wurtzite

Q: Metallic conduction found in
A: NiAs

Q: Layered structures (anisotropic properties) found in
A: CdI_2, CdCl_2, MoS_2

Q: piezoelectricity found in structure types...
A: ZnS-blende, ZnS-wurtzite

Q: Why are large samples required in Neutron diffraction?
A: - low intensity neutron beams
- not really monochromatic

Overview of all crystallographic systems and their bravais lattices

Q: the essential symmetry for a cubic system is [...]
A: four 3-fold axes

Q: the essential symmetry for a tetragonal system is [...]
A: one 4-fold axis

Q: the essential symmetry for a orthorhombic system is [...]
A: three mirrors or three 2-fold axes

Q: the essential symmetry for a hexagonal system is [...]
A: one 6-fold axis

Q: the essential symmetry for a trigonal system is [...]
A: one 3-fold axis

Q: the essential symmetry for a monoclinic system is [...]
A: one 2-fold axis or a mirror plane

Q: the essential symmetry for a triclinic system is [...]
A: no symmetry

Q: The Herrmann-Mauguin symbol 'm' means [...]
A: mirror plane

Q: The Herrmann-Mauguin symbol 'n' means [...]
A: rotation axis n=2,3,4,...

Q: The Herrmann-Mauguin symbol 'n^_' means [...]
A: inversion axis

Q: In space symmetry, the Herrmann-Mauguin symbol 'a,b,c,d,n' means [...]
A: glide plane

Q: In space symmetry, the Herrmann-Mauguin symbol '2_1,3_1,...' means [...]
A: screw axis

Q: rule for reflection to be observed in primitive P lattice type
A: none

Q: rule for reflection to be observed in body centered I lattice type
A: h+k+l = 2n

Q: rule for reflection to be observed in face centered F lattice type
A: either h k and l all even or all odd.
0 is considered even

Q: rule for reflection to be observed in side centered C lattice type
A: h+k = 2n

Q: rule for reflection to be observed in rhomboheadral R lattice type
A: -h+k+l=3n or h-k+l = 3n

Q: Perovskites
A: General Formula is ABO_3
A^2+, B^4+,O^2-
zB CaTiO_3, BaTiO_3

Q: The ions size effect in perovskites can be determined by calculating [...]
A: the retention factor

retention factor of perovskites

Q: Octahedrally coordinated high valent d^0 cations (e.g Ti pch[4]) and cations containing filled valence shells (e.g. Sn2+) lead to a [...]
A: second-order Jahn-Teller distortion

Q: Random dipole orientations are called [...]
A: paraelectric

Q: aligned dipole orientations are [...]
A: ferroelectric.

The equilibrium constant K for the formation of Schottky defects is

Q: The molar concentration of Schottky defects is given by
A: where N_A is the Avogadro constant, R is the universal gas constant, T is the temperature and DeltaH_S is the molar entropy
needed to form Schottky defects.

Q: the concentration of Frenkel defects n_F can be dermined by
A: where N_i is the number of interstitial sites available

Q: Slip  direction is given by [...]
A: Burger's vector

Q: Polymorphs are substances [...]
A: who have different phases for the same composition.
Rückseite Extra: Beispiele:
Carbon: Graphite, Diamond, Fullerene
CaCO_3: Aragonite, Vaterite, Calcite

Q: Wurtzite is [...] ionic than zinc blende
A: more

Q: Ceramic/Shake’n’bake
A: - Mix the pure elements in a 1:2 ratio.
- Heat inside an autoclave as sulfur might be volatile under inert gas conditions as oxides are more stable than chalcogenides to very high temperatures.
- Cool slowly down for clean crystalisation.

Q: Explain why X is transparent to visible light.
A: Some reasons for transparency:
- covalent bonds
- tightly bound electrons
- high band gap energy, ~5 eV
- no dopants that would introduce donor or acceptor levels, e.g. the Hope diamond.

Q: In the context of amorphous inorganic compounds, name two network formers, two network modifiers, and one intermediate.
A: formers: SiO2, P2O5, B2O3, GeO2
modifiers: CaO, MgO, Na2O
intermediates: Al2O3, TiO2, ZnO

Q: Cite two desirable characteristics of glasses.
A: optical transparency and ease of fabrication

Q: The glass transition temperature is, for a non crystalline ceramic, that temperature at which [...]
A: there^ is a change of slope for the specific-volume-versus-temperature curve.

Q: The melting temperature is, for a crystalline material, that temperature at which [...]
A: there is a sudden and discontinuous decrease in the specific-volume-versus-temperature curve.