SolutionsSolutions
Solid Solution14 Carat Gold
Liquid SolutionVodka
Gaseous Solution
Air
Solution = A homogeneous mixture
AlloysAlloysAn alloy is a material that contains more than one element
and has the characteristic properties of a metal.
Primary Primary ElementElement
Name of AlloyName of Alloy Composition Composition
(by mass)(by mass)PropertiesProperties
Gold, Au 14-Carat Gold 58% Au, 42% Ag
Harder than pure gold
Copper, Cu
Yellow Brass 67% Cu, 33% Zn
Ductile, Takes a polish
Iron, Fe Stainless Steel 80.6% Fe, 18% Cr, 1% Ni, 0.4%
C
Harder and more corrosion resistant
than pure iron
Lead, Pb Plumber’s Solder
67% Pb, 33% Sn
Low melting point (275 °C)
Silver, Ag Sterling Silver 92.5% Ag, 7.5% Cu
Bright surface that is more inert than pure
silver
Substitutional (Homogeneous) Substitutional (Homogeneous) AlloysAlloys
Substitutional AlloySubstitutional Alloy
Two or more types of metal atoms are randomly distributed over the positions occupied in
the host metal.
Atoms should be similar in size (as a rule of thumb, atomic radii should not differ by more than
15%)
Interstitial AlloyInterstitial Alloy
Smaller atoms (typically nonmetals) occupy some of the
holes or interstitial positions in the lattice.
The smaller nonmetal atoms typically bond covalently with the metal atoms, which increases the
hardness and strength (but reduces the ductility)
14-Karat Gold Steel
Au
Ag
Fe
C
Other Types of AlloysOther Types of Alloys
Intermetallic CompoundsIntermetallic Compounds
Intermetallic compounds are not solutions. They have a fixed composition (just like
molecular substances) with well defined properties.
Some examples include Ni3Al which is a strong lightweight alloy used in aircraft engines, and Co5Sm, which is used to
make magnets.
Heterogeneous AlloyHeterogeneous Alloy
A heterogeneous alloy is not homogeneous. It consists of two
or more distinct phases, each with its own composition. The
properties are sensitive to the way a sample was made.
Pearlite shown here is a mixture of essentially pure iron and the
binary phase, Fe3C.
Ni3Al Superalloy Pearlite
Al
Ni
Fe Metal Fe3C
+ +
Semiconductor Solid SolutionsSemiconductor Solid Solutions
Figures taken from “Semiconductor Optoelectronic Devices”, by P. Bhattacharya
The solid solution between GaAs (Band gap, Eg=1.4 eV, Unit cell edge, a=5.65 Å) and AlAs (Band gap, Eg=2.1 eV, Unit cell edge, a=5.66 Å) is among the most important for optoelectronic devices. By forming solid
solutions we can control the band gap.
Heterojunction laser
CdS-CdSe Solid SolutionsCdS-CdSe Solid SolutionsCdS1-xSex solid solutions are excellent pigments (cadmium yellow,
cadmium orange). By controlling the composition we can control the band gap and hence the color.
CdS (ECdS (Egg = 2.4 = 2.4 eV)eV)
CdSe (ECdSe (Egg = 1.7 = 1.7 eV)eV)
Cadmium pigmentsCadmium pigments
CdSCdS1-x1-xSeSexx compositionscompositions
Band Gap vs. CompositionBand Gap vs. CompositionHow does the band gap vary as we change the composition
across a solid solution?
For semiconductors it is not unusual that the band gap will vary (approximately) linearly as the composition changes.
In such cases the band gap of an intermediate composition Eg(int) with composition A1-xBx can be estimated
from the band gaps of the end members, Eg(A) and Eg(B):
g g
g g
E A E Intx
E A E B
ExampleExample
What composition in the CdS1-
xSex solid solution will have a band gap of 2.25 eV?
What color will this compound be?
SolutionThe end member with the larger band gap is A=CdS, while the end member with the smaller band gap is B=CdSe. Using their band gaps we can calculate x that will give a band gap of 2.25 eV.
2.4 2.250.21
2.4 1.7x
The Color WheelThe Color Wheel
UV 100-400 nm 12.4 - 3.10 eVViolet 400-425 nm 3.10 - 2.92 eVBlue 425-492 nm 2.92 - 2.52 eVGreen 492-575 nm 2.52 - 2.15 eVYellow 575-585 nm 2.15 - 2.12 eVOrange 585-647 nm 2.12 - 1.92 eVRed 647-700 nm 1.92 - 1.77 eVNear IR 10,000-700 nm1.77 - 0.12 eV
A semiconductor with a band gap of 2.25 eV will absorb all visible light with energy greater than 2.25 eV. This means it will absorb the violet, the blue and most of the green.
The reflected colors will be red, orange and yellow. Therefore, the color will be orangeorange.