Crystallography- Crystal Structure Gallery

Question 1

Stacking of hcp and fcc

Answer 1

hcp: stacking works ABA along [0001] (c-direction).
fcc: stacking works ABC along [111] (cube diagonal), 4 <111> type directions

Question 2

Interstitial sites in fcc lattice

Answer 2

Split the unit cell into 8 mini cubes. Base fcc lattice positions at faces and corners are the anions (filed circles). Octahedral sites are at the centre of the big cube and the centre of all edges of the big cube. Numbered 1-4 for a mini cube (CN 6). Shown by hollow circles. Tetrahedral sites at centre of each mini cube. Numbered 5-12 over whole big cube. Shown by T with +/- (above/below adjacent close packed plane) and number

Question 3

What interstitial are in NaCl?

Answer 3

All the octahedral sites where Na will be. This is rock salt structure

Question 4

What interstitial are in ZnS?

Answer 4

Only 4 of the possible tetrahedral sites spaced as far apart as possible where the Zn will be. This is sphalerite structure

Question 5

What interstitial are in K2O?

Answer 5

All the tetrahedral positions where the K will be. This is the fluorite structure

Question 6

Information provided on a gallery page for a crystal structure

Answer 6

Stoichiometry, generic name, chemical formula of that mineral.
Crystal system, Bravais lattice, point group, space group.
Packing type, filling of tet/oct sites, bonding type.
Polyhedron representation of structural units inside unit cell and their connection.
Significance of structure, special comments on point and space symmetry.
Other compounds with same structure

Question 7

Rock salt structure

Answer 7

Cubic, NaCl
Has all octahedral positions filled with Na. Derived from fcc of Cl
Empty tetrahedral sites

Question 8

Sphalerite structure

Answer 8

Cubic, ZnS.
Has 4 tetrahedral sites occupied by Zn spaced apart as far as possible.
Based on fcc of S
Empty octahedral sites

Question 9

Diamond structure

Answer 9

Cubic. C diamond
4 tetrahedral sites occupied like sphalerite but different as only one element. Has glide plane with (1/4,1/4,1/4) shift.
All C-C distances equal.
Inversion centre at (1/8,1/8,1/8)
Octahedral sites empty. Based on fcc

Question 10

Why is diamond not close packed?

Answer 10

C is in sp3 hybrid form so is a tetrahedron not a sphere. Total packing density 34% not 74%

Question 11

Why is diamond hardest structure?

Answer 11

C-C hybrid bonds amongst strongest bond in periodic table, short bond length. No layering and no dense slip systems. Awkward intercalation makes dislocation movement difficult

Question 12

Graphite structure

Answer 12

C, graphite
Not based on any close packed structure.
Planes of hexagons of C atoms (2D bonding).
c»a so not dense at all
6 sub 3 screw axis needs 6 layers (3 unit cells high) for one spiral circle.

Question 13

Antifluorite/fluorite

Answer 13

A2X/AX2, K2O.
Cubic system.
All tetrahedral sites filled by element sub 2.
Based on fcc of other element. 
All octahedral sites empty.