Crystallography- Metal Structures and Silicate Chemistry Flashcards
fcc
Bravais lattice: cubic F Aka ccp Point group: m3m Space group: Fm3m Examples: Al, Cu, γ-Fe, Ni
bcc
Bravais lattice: cubic I
Point group: m3m
Space group: Im3m
Examples: Nb, W, α-Fe, Cr
hcp
Bravais lattice: hexagonal P
Point group: 6/mmm
Space group: P6sub3/mmc
Examples: Co, Be, Mg, Ti
Trends in metal structures
hcp most frequent. All alkali metals and typical refractory metals are bcc. Noble metals (Pt group) are fcc. Some metals have small energy difference to the 2nd most favourite structure meaning stacking faults and phase transitions via temperature or pressure are facilitated.
Ordered intermetallic alloy structures
Ordered bcc: system is cubic, Bravais primitive, has one element on corners of cube and the other in the centre, atoms alternate along space diagonal, example NiAl.
Ordered fcc: system is tetragonal, Bravais primitive, has one element for top and bottom faces and other for remaining 4 face centred midway up, alternating layers of elements, example TiAl.
Disordered intermetallic alloys
Do not fulfill strict definition of crystals, can be treated by placing statistical ‘mixed elements’ at lattice points (occupancy factors), then the ordered structures would return to proper bcc and fcc and both be cubic.
Glide systems in alloy structures
Determine the direction of dislocation motion. Normal bcc has (110)[111], normal fcc has (111)[110]. For the ordered intermetallic alloys the Burgers vector is twice as long as for pure metals, so plastic deformation is hindered and alloy is more brittle (not as brittle as ceramics).
Tin structure
Gray α-Tin: stable under 13C, cubic, diamond-structure, semiconductor.
White β-Tin: stable above 13C, tetragonal, is distorted cubic primitive, metallic.
Both are metastable so don’t immediately change as temperature goes through 13C.
Heusler alloy
ABC2. Example MnAlCu2 so no anion. Based on fcc, has all interstitials fully filled, has larger lattice parameter than normal fcc. This is first ferromagnetic alloy known without a ferromagnetic element
What is complex stacking?
Anything other than ABAB, ABCABC, AAAA
Nomenclature for cation/anion stacking sequences
Main (Bravais lattice) stacking is upper case roman letters ABC.
Interstitial stacking is Greek letter αβγ.
A, B and C are same chemistry
α, β, γ are also the same elements as each other
<> denotes infilled interstitials (layers entirely vacant)
Stacking sequences for ZnS wurtzile, ZnS cubic, NiAs hexagonal, CdI2 hexagonal
ZnS wurtzile: AαBβ
ZnS cubic: AαBβCγ
NiAs hexagonal: AγBγ
CdI2 hexagonal: AγB<>
Rare earth elements stacking sequences
Mixed stacking sequences
ABCB
Examples: La, Pr, Nd, Pm
Complex stacking in molybdenite
AβA<>BγB<>. This is height of one unit cell. Is MoS2. System is hexagonal, Bravais primitive. Sheet structure, rather loose S-S cross-layer bonds so good lubricant as can exfoliate into single 2D sheets. Get pairs of hexagonal primitive layers with rather loose packing. Interstitial layers are not oct or tet but are trigonal prismatic
Basics of silicates
Polymerisation via Si-O-Si bonds. Stoichiometry varies within SiOx depending on how many O shared. 1D, 2D, 3D network structures. Tetrahedra robust and angles and distances vary little and are close to ideal mathematical tetrahedron. Al can sub in silica lattices but needs extra caution to balance charge which are often in interstitial positions.
Structures of silicates
Monomer is SiO4 tetrahedron. Dimer is two tetrahedra sharing one O. Trimer 3 tetrahedra forming ring. Can get other rings made of more tetrahedra linked together. Single chain and double chain (2 chains sort of cross-linked). Layer/sheet/net of many hexagon shapes seen in double chains. 3D framework can form a ball.
How to count oxygens in silicates
Bridging oxygen shared by 2 tetrahedra counts 1/2
Non-bridging oxygen is unshared and counts 1
Different Si:O ratios
1: 4- extreme 1, is the monomer where all O non-bridging, e.g CaSiO4, dimensionality is zero.
1: 2- extreme 2, framework SiO2, all O bridging, e.g quartz, dimensionality is 3D.
1: 3- intermediate, chain silicate SiO3, 2 O in line with chain are bridging, out of chain are non-bridging (x2), e.g MgSiO3, dimensionality 1D.
1: 2.5- intermediate, sheet silicate, 3 O in plane of sheet are bridging, other out of plane is non-bridging, e.g Na2Si2O5, dimensionality 2D
Al as tetrahedral framework cation
Can replace Si. E.g NaAlSi3O8 albite feldspar, is 3D framework silicate with 4 bridging O atoms, Na fills large gaps.
Al and Mg as octahedral cation
Either can fill holes in silicate polymer, especially in combination with OH- ions.
Olivine
Mg2SiO4. 3D island silicate. Si:O is 1:4. No bridging O. Oxygen in near hcp packing but trigonal distorted. Si in centre of tetrahedra. Is basically ionic structure between Mg cation and negative SiO4 ions.
Cristobalite
SiO2. 3D framework silicate. All 4 O shared so no non-bridging O. Si:O is 1:2. Alpha phase is tetragonal. Cubic structure and Si atoms sit on diamond lattice. Metastable a time room temperature.
Serpentine and Kaolinite
Mg and Al respectively. Sheet silicates. Has SiOx tetrahedral layer. Oxygens sticking out of bottom of layer are bonded to 2 Mg or Al atoms which are octahedrally coordinated. Mg or Al atoms bonded to OH- ion as well. Lattice contants mismatch between octahedra/tetrahedra and induce curvature/rolling up
Differences between silicate crystal and glass
For normal silicate glass, maintains high Si:O ratio (about 2:1) and has only few non-bridging oxygens, no long range order.
Alkali silicate glass has alkali ions that break Si-O-Si chains and form pairs with non-bridging oxygens. E.g Na+ to SiO4- charge balance
