Chapter 2 structure of crystalline solidsLecture II (1).pdf

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1. CHAPTER TWO

The Structure of
Crystalline Solids

Lecture Two

2. 3.6 POLYMORPHISM AND ALLOTROPY
Polymorphism:- a phenomenon in which some
materials (metal or non-metal)may have more than one
crystal structure.
When found in elemental solids, the condition is often
termed allotropy.
The crystal structure depends on both the temperature
and the external pressure.
Example:
Pure iron has a BCC crystal structure at room temperature,
which changes to FCC iron at 912
0
C (1674
0
F)


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3. POLY-CRYSTALLINE MATERIALS
composed of a collection of many small crystals or
grains.
Various stages in the solidification of a
polycrystalline
Initially, small crystals or nuclei form at various
positions having random crystallographic
orientations.
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4. The small grains grow by the successive addition from the
surrounding liquid of atoms to the structure of each.
The extremities of adjacent grains interrupt on one
another as the solidification process approaches
completion.
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5. All unit cells interlock in the same way and have the
same orientation.

Single crystals exist in nature and produced artificially
under carefully controlled env’t.


SINGLE CRYSTALS:
when the periodic and repeated arrangement
of atoms is perfect or extends throughout the
entirety of the specimen without
interruption.
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6. Within the past few years, single crystals have become
extremely important in many modern technologies
in particular electronic microcircuits, which employ
single crystals of silicon and other semiconductors.
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7. 3.7 CRYSTAL SYSTEMS
Is a way of classifying crystal structure into groups based
on unit cell geometry &/or atomic arrangements.
Unit cell geometry:- an x, y, z coordinate system is
established with its origin at one of the unit cell corners.
Is defined in terms of 6 parameters:
the three edge lengths a, b, and c, and
the three inter-axial angles , β and r.
Are called lattice parameters of a crystal
structure.
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8. A unit cell with x, y, and z coordinate
axes, showing axial lengths (a, b,and
c) and interaxial angles
On this bases there are 7 possible combinations
of lattice parameters each of which represents a
distinct crystal system.
Cubic system( ):-
has the greatest degree of symmetry, while

Triclinic system( ):- has least
symmetry
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12. CRYSTALLOGRAPHIC POINTS, DIRECTIONS,
AND PLANES
Consider the unit cell and the point P
The coordinates are represented without comma

2.8 POINT COORDINATES

•The position of any point located within a unit cell may be
specified in terms of its unit cell edge lengths (i.e., in terms of
a, b, and c)
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14. E.g.2
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15. E.g.
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16. CRYSTALLOGRAPHIC DIRECTIONS
A crystallographic direction is defined as a line between two
points, or a vector.
The three indices, not separated by commas, are enclosed in square
brackets, thus: [uvw].
By convention, the resultant direction components are reduced
to smallest whole numbers and placed in brackets without
commas; the so-called Miller indices
For example for (½, 1, ½) will be multiplied by 2 and the miller
indices will be [121]
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17. CRYSTALLOGRAPHIC DIRECTIONS
The following steps in the determination of the three directional
indices:
1.A vector of convenient length is positioned such that it passes through the
origin of the coordinate system.
2.The length of the vector projection on each of the three axes is determined
in terms of the unit cell dimensions a, b, and c.
3.These three numbers are multiplied or divided by a common factor to
reduce them to the smallest whole number values.
4.The three indices, not separated by commas, are enclosed in square
brackets, thus: [uvw].
The u, v, and w integers correspond to the reduced projections along the
x, y, and z axes, respectively.
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18. Negative indices are also possible, which are
represented by a bar over the appropriate index.
E.g. the direction would have a component
in –y the direction.
The [100], [110], and [111]
directions within a unit cell
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EX.1

19. EX.2
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21. E.G 3
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23.  For some crystal structures, directions with different
indices are actually equivalent;
this means that the spacing of atoms along each
direction is the same.
E.g. in cubic crystals:
The ff. indices are equivalent
• Equivalent directions are grouped together into a
family, which are enclosed in angle brackets,
In addition, directions in cubic crystals having the
same indices without regard to order or sign are
equivalent. E.g [123] and
This is, in general, not true for other crystal systems
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26. ANISOTROPY
The directionality of physical properties of
substances.
For example, the elastic modulus, and the electrical
conductivity, may have different values in the [100] and
[111] directions.
it is associated with the variance of atomic or ionic
spacing with crystallographic direction.
The extent and magnitude of anisotropic effects are a
function of symmetry.






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27. the degree of anisotropy increases with decreasing structural
symmetry
triclinic structures normally are highly anisotropic.
Where as, Substances in which measured properties are
independent of the direction of measurement are isotropic.
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28. 2.10 CRYSTALLOGRAPHIC
PLANES

In all but, the hexagonal crystal system,
crystallographic planes are specified by three Miller
indices as (hkl).



 Any two planes parallel to each other are equivalent
and have identical indices.
 Steps to determination of the h, k, and l index
numbers:
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29. EX.1
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30. Determine the Miller indices for the plane shown in the
accompanying sketch(a).
EX.2
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31. EX.3.
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32.  Parallel plans has equivalent indices
Representations of a series a. (001), b. (110),and c. (111)
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33. ATOMIC ARRANGEMENTS
a) Reduced sphere
FCC unit cell with
(110) plane
(b) Atomic packing of
an FCC (110)
plane
The atomic arrangement for a crystallographic plane,
depends on the crystal structure.
The (110) atomic planes for FCC and BCC crystal
structures are represented as:
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34. (a)Reduced-sphere BCC unit cell with
(110) plane.
(b)Atomic packing of a BCC (110)
plane.
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35. A “family” of planes contains planes that are
crystallographically equivalent.









i.e. having the same atomic packing;
is designated by indices with enclosed in braces.
For example, in cubic crystals:
all belong to the {111} family
Also, in the cubic system only, planes having the
same indices, irrespective of order and sign, are
equivalent.
both( ) and ( ) belong to the {123}
family.
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36.  HEXAGONAL CRYSTALS
Crystallographic equivalent directions will not have the
same set of indices.
And we use a four-axis, or Miller–Bravais, coordinate
system to describe:



The 3 axes(a1,a2,a3) are on a
single plane (called the basal
plane)
The z axis is perpendicular
to this basal plane.
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37. DIRECTIONAL INDICES WILL BE DENOTED BY FOUR
INDICES, AS [UVTW];

And the 4
th
indices is obtained by the ff. formula:
by convention, the first three indices pertain to
projections along the respective a1,a2,a3 axes in
the basal plane.
E,g, the [010] direction becomes
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38. HEXAGONAL CLOSED PACK
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Study how to determine
crystallographic direction and plane in
hexagonal closed packed structure

39. Why do we care?
 Properties, in general, depend on linear and planar density.
Examples:
 Slip (deformation in metals) depends on linear and planar density

Slip occurs on planes that have the greatest density of atoms in direction with highest density
(we would say along closest packed directions on the closest packed planes)
Linear density (LD) is defined as the number of atoms per unit length whose centers lie on
the direction vector
Linear and Planar Density
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• Planar Density
– Number of atoms per unit area that are centred on a particular
crystallographic plane.

PD = number of atoms cantered on a plane
area of plane

Planar Density
relation between the stacking of
close-packed planes of atoms and the
FCC crystal structure; the heavy
triangle outlines a (111) plane

41. NONCRYSTALLINE SOLIDS
Sometimes such materials are also called amorphous
(meaning literally without form),or
super cooled liquids, in as much as their atomic
structure resembles that of a liquid.

 lack a systematic and regular arrangement of atoms
over a relatively large atomic distances.
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42. (a) crystalline silicon dioxide (b) non-crystalline silicon dioxide.
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43. Unique characteristic of cubic crystals is that planes and
directions having the same indices are perpendicular to
one another.

 However, for other crystal systems there are no simple
geometrical relationships between planes and directions
having the same indices.
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