Sam - crystal structures Flashcards
Diamond v graphite
diamond: electrically insulating (doesn’t conduct), hard
graphite: conducts electricity, soft, lubricant
How do you convert 3D to 2D?
look along one of the axes
how are lattice points in a unit cell counted?
corner = 1/8
edge = 1/4
face = 1/2
central = 1
what is l-centred?
body centred cubic
what is F-centred?
face centred cubic
which unit cell parameters are equal for orthorhombic crystals?
a, b, c = 3 diff lengths
all angles = 90degrees
which unit cell parameters are equal for tetragonal crystals?
a=b, c
all angles = 90
which unit cell parameters are equal for cubic crystals?
a = b=c
all angles = 90
difference between face centred and body centred?
body = only corners and one in the middle
face = all corners and one on each face too
what packing does face centred do?
cubic close packed (CCP) aka (FCC)
Describe CCP
ABCA
CN= 12
PE = 0.74/74%
Forms anti-cube octahedron (triangles opposite orientations)
stacking inbetween holes, where there isnt an atom beneath
Describe HCP
ABA
CN=12
PE = 0.74/74%
Forms cube octahedron (triangles same orientation)
stacked avoiding the hole between first A &B stack
Describe body centred cubic
ABA
CN = 8
PE = 0.68
often metals, not truly close packed
Describe simple cubic
CN = 6
PE = 0.52
Describe simple cubic
CN = 6
PE = 0.52
Compare properties of HCP/FCC and BCC
HCP/FCC: low corrugation of layers, closer packing so layers slip over each other, FCC more ductile, HCP more brittle
BCC: higher corrugation, cubic structure, tends to be harder than FCC but more ductile than HCP
Two types of alloys?
substitutional: atoms of one element exchanged for another,
interstitial: additional atoms of another element between atoms of the host lattice
What do A and B usually refer to in ionic solids?
A = cation , B=anion
What is a eutactic structure?
when the structure has expanded to accommodate ions in the interstitial sites (the host ions are no longer touching)
radius ratio range for cubic structure
1- 0.732
radius ratio range for octahedral structure
0.732 - 0.414
radius ratio range for tetrahedral structure
0.414 - 0.225
radius ratio range for trigonal structure
0.225 - 0.155
Examples with rocksalt structure
NaCl, MgO, AgCl, KBr, CaO, transition metal monoxides
What close packing and coordination is NiAs?
HCP 6,6 octahedral Ni, trigonal As
Fluorite coordination?
CCP = 8,4
all tetrahedral holes filled with F
all octahedral holes empty
zinc blende/sphalerite coordination?
CCP = 4,4
half tetrahedral holes filled by S-
What structure do III-V semiconductors form?
zinc blende
wurtzite coordination?
same as zinc blende but HCP
HCP, 4,4 coordination with half filled tetrahedral holes w/ anions
coordination of Li3Bi?
FCC with Bi3- in all octahedral, Li+ in all tetrahedral
Li 4 & 6 coordinate
Bi 14 coordinate
coordination of CsCl?
simple cubic array of Cl-, 8,8, cubic hole occupied with Cs, no close packing
NOT body centred cubic as there is no central Cl- as the central lattice point
coordination of ReO3
simple cubic, 6-coordinate octahedral Re6+, linear 2-coordinate O2-, perovskite structure
general formula for spinels
AB2X4, A & B = cations, X = anions
describe the 2,3 spinel structure
Mg2+, Al3+, O2-
FCC oxides, A cations (Mg) in 1/8 tetrahedral holes
B cations in 1/2 octahedral holes
How do you work out the no. of octahedral and tet holes in spinels?
8 formula units per unit cell, no. of oxygens x 8 = oct holes
tet holes = no. of oct x 2
use ratio of A cation to the oxygens (e.g. MgO4 Mg = 1/4) x no. of oxygens to get the no. in the tet holes (then divide this no. by the no. of tet holes)
do the same for B in the oct holes
describe the 4-2 spinel structure
A4+, B2+, X2-
compare inverse spinels to spinels
spinels: A cations in 1/8 tet holes, B cations in 1/2 oct holes
inverse spinels: A cations in 50% of 1/2 oct holes, B cations in tet and the other 50% of 1/2 oct
for d electrons are s electrons included?
YES
describe perovskite
ABX3, FCC, B cations in oct holes, ReO3 structure with A cation at centre
BO6 octahedra in corners
12 coordinate cuboctahedral A
tolerance factor range for hexagonal
1<t<1.06
tolerance factor range for ideal cubic
0.9<t<1
tolerance factor range for orthorhombic pervoskite
0.8<t<0.9
tolerance factor range for other structure types
t<0.8
what is YBCO
1,2,3 superconductor
YBa2Cu3O7
high temp superconductor - oxide loss makes it a semi-conductor rather than superconductor
