Cristobalite (SiO₂) and Network Solids

Network Solids: Cristobalite (SiO₂)

• Cristobalite is an example of an AB₂ network covalent solid with strong covalent bonds.
• Its high melting points are a result of its strong bonds.
• The lattice structure is based on [SiO₄]⁴⁻ tetrahedra.
• Its structure resembles 'zinc blende' structure.
• It features an FCC lattice of Si centers, with Si occupying 50% of the tetrahedral holes.
• Shared oxygen atoms bridge each pair of Si atoms.
• The number of formula units per unit cell is 8 for Si and 16 for O.
• The coordination number of Si is 4 (tetrahedra).
• The coordination number of O is 2.
• Quartz (alpha form: tetragonal, beta form: hexagonal) is a polymorph of SiO₂ and a more common mineral structure.
• BeF₂ and H₂O are other substances that adopt related structures.
• H₂O is a molecular solid with an extensive hydrogen bonding network and similar geometry.

Revisiting Allotropes: Carbon Network Solids

• Graphite is the most stable allotrope of carbon under standard conditions.
• It consists of stacked sheets of trigonal planar carbon forming a strong sigma network with a C-C bond of 142 pm.
• Delocalized pi electrons allow it to conduct electricity along the sheet.
• Layers are held together by weak van der Waals interactions, making it a longer axis with an Inter-layer distance of 335 pm.
• Graphite has a hexagonal solid state structure with ABAB layers.
• Its layers can slide easily, making it a lubricant.
• Graphite is conducting (in plane), shiny black, soft, and greasy.
• Diamond is a tetrahedrally arranged network solid formed under high pressure.
• It exists as an FCC lattice, with carbon in 50% of the tetrahedral holes.
• Every carbon is linked to 4 others, giving it a coordination number of 4, with a C-C bond of 154 pm.
• Each unit cell has 8 carbon atoms.
• Diamond is non-conducting, colorless transparent, and hard.
• Materials like silicon, germanium, and grey tin also adopt the 'Diamond' solid state structure.
• 'Diamond' solid state structure is related to the structures of SiO₂ (cristobalite) and 'zinc blende'.

Revisiting Allotropes: C₆₀ Molecular Solid

• C₆₀ (Buckminsterfullerene) is a spherical molecule, not a covalent network, which forms solid state structures.
• Buckminsterfullerene has a face-centered cubic crystal structure.
• Each unit cell contains 4 C₆₀ molecules.

Molecular Solids

• Carbon dioxide (dry ice) crystallizes as face-centered cubic (FCC).
• Other gases like Argon (Ar) and Methane (CH₄) also freeze into FCC structures.
• Iodine (I₂) adopts a cubic close packing arrangement.
• Sucrose crystallizes in a lower-symmetry monoclinic lattice.
• Molecular solids with complicated structures often exhibit lower overall symmetry.

Ice (H₂O)

• Water can adopt a cubic 'cristobalite'-type structure due to hydrogen bonds.
• Commonly found hexagonal crystals appear in snowflakes and winter windscreens.

Defects in Solid State Structures

• Crystalline solids may have occasional defects without distorting the overall structure.
• Controlled defects can lead to beneficial material properties.
• Vacancies are missing atoms/ions.
• Interstitial impurities are atoms/ions that fit into the gaps within the crystal lattice.
• Substitution impurities are atoms/ions that replace others, either of similar or dissimilar size.

Doping of Semiconductors

• Substitutional defects introduce extra valence electrons.
• Examples are the addition of approximately 0.00001% of n-type elements (e.g., P) or p-type elements (e.g., B dopant) into pure silicon wafers.
• Doping changes the conducting properties by bridging the band gap of the semiconductor in 'diamond'-type structures.

Alloys

• An alloy is a metal-like mixture that contains a mixture of solid phases of two or more pure elements.
• They are ductile, malleable, and good electrical conductors.
• Humans have been making alloys since copper and tin were combined in the Bronze Age (ca. 3000 BC).
• It produces an alloy harder than iron.
• Iron is often used in alloyed form due to its softness and corrosion vulnerability; steel and stainless steel mitigate these issues.
• Interstitial alloys is when a smaller element fills the gaps in the solid state structure.
• Substitutional alloys is when the less abundant element takes the place of the main metal.
• Occurs only if radii are within about 15% of each other.
• Alloying can combine both interstitial and substitutional properties.

Interstitial Alloys: Steels

• At room temperature, iron is most stable as bcc 'α-ferrite'.
  • Carbon doesn't fit well into bcc lattice interstitial holes
• At high temperature, a fcc structure becomes more stable as 'austentite'.
  • Heat in the presence of coal allows more carbon to incorporate
  • Further C atoms can dissolve in fcc structure.
• Rapid cooling gives tetragonal 'martensite' structure, trapping carbon in the interstices.
  • Carbon atoms stop slippage of Fe atoms, hardening the steel.
  • Higher carbon content makes alloy harder, but also more brittle and less ductile.

Interstitial + Substitutional Alloys: Steels

• Steels can have different carbon contents.
• Stainless steel contains 14-18% chromium and 7-9% nickel.
• Nickel steel contains 2-4% nickel.
• High speed steels contain 14-20% tungsten (W), or 6-12% molybdenum (Mo).

Substitutional Alloys: Bronze

• Copper forms a face-centered cubic (ccp) lattice.
• Tin substitution of 1-18% of the copper sites leads to harder material that is less prone to corrosion, resulting in bronzes.
• Some bronzes include 1-25% zinc because Zinc compounds were not isolated in antiquity, these came from minerals like zinc blende
• Sterling silver: 92.5% Ag, 7.5% Cu.
• 14 carat gold: 58% Au, 4-28% Ag, 14-28% Cu.

Brass

• Brasses exhibits as allows if Copper and Zinc.
• As As Zn:Cu ratio approaches 1:1, ẞ-brass forms with a repeating binary pattern.
• It is described as caesium chloride type structure, as resembling body centred cubic, with Zinc and copper atoms.

Alloys in medical devices

• Early medical devices were ceramics, wooden legs, spectacles of various amorphous glasses, metal hooks, etc.
• Modern industry is heavily regulated, requiring high-quality materials with tuneable properties of strength, hardness, biocompatibility
• Early orthopaedic implants used strong stainless steel
• Co-Cr-Mo alloys: developed in dentistry and gained popularity in whole joint replacement
• Modern implants utilise titanium (Ti) and Ti alloys for bone attachment.
  • Better strength weight ratio to steels
  • 3D printing for personalised devices