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APPLIED PHYSICS
CODE : 07A1BS05
I B.TECH
CSE, IT, ECE & EEE
UNIT-1: CHAPTER 2.2
NO. OF SLIDES : 20
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S.No. Module LectureNo.
PPT SlideNo.
9 Braggs law. L10 3-9
10 Laue method L11 10-15
11. powder method. L12 16-20
UNIT INDEX
UNIT-I
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X-Ray Powder Diffraction
Lecture-10
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Lecture-10
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X-Ray Powder Diffraction (XRPD) is one
of the most powerful techniques for
analyzing the crystalline nature of solids.
XRPD capabilities include micro-
diffractometry, flat plate or capillarysample configuration, spinning and
rocking methods, variable temperature
and humidity conditions, and a uniquesample conveyor system to overcome
sample inhomogeneity effects.
Lecture-10
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XRPD is perhaps the most widely used X-ray
diffraction technique for characterizing materials. As
the name suggests, the sample is usually in a powderyform, consisting of fine grains of single crystalline
material to be studied. The techniqueprovides
information that cannot be obtained any other way. The
information obtained includes types and nature ofcrystalline phases present, structural make-up of
phases, degree of crystallinity, amount of amorphous
content, microstrain & size and preferred orientation of
crystallites. The technique is also used for studyingparticles in liquid suspensions or polycrystalline solids
(bulk or thin film materials).
Lecture-10
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The term 'powder' means that the crystalline domains
are randomly oriented in the sample. Therefore,when the 2-D diffraction pattern is recorded, it
shows concentric rings of scatteringpeaks
correspondingto the various d spacings in the crystal
lattice. The positions and the intensities of the peaksare used for identifying the underlying structure (or
phase) of the material. This phase identification is
important because the material properties are highly
dependent on structure (think, for example, of
graphite and diamond).
Lecture-10
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Powder diffraction data can be collected using
either transmission or reflection geometry, as
shown below. If the particles in the powder
sample are randomly oriented, both methods
will yield the same results.
Lecture-10
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Single crystal diffraction
Laues method - variable, fixed.
Rotating crystal method - fixed, variable tosome extent.
Why not single crystal methods? It may be difficult to obtain a single crystal.
The usual form of a material may be
polycrystalline. Problems with twinning or phase transitions
complicate structural assignments.
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Lecture-10
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Powder diffractionIn this method the crystal is reduced to a
fine powderand is placed in a beam of
monochromatic X-rays. Each particle is a tinycrystal or anassemblage of smaller crystals
randomly oriented with respect to the the
incident beam.Powder methods - fixed, variable.
Lecture-11
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The diagram shows only two scattering planes, but implicit here
is the presence of many parallel, identical planes, each of which
is separated from its adjacent neighbor by a spacing d.
Constructive interference occurs when (A+B)/= n, coinciding
with Braggs law, n= 2dsin . The integer nrefers to the order
of diffraction. For n= 1,(A+B) = and for n= 2, (A+B) = 2etc.
Lecture-11
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Angles are used to calculate the interplanar atomic
spacings (d
-spacings). Because every crystallinematerial will give a characteristic diffraction pattern
and can act as a unique fingerprint, the position (d)
and intensity (I) information are used to identify the
type of material by comparing them with patterns forover 80,000 data entries in the International Powder
Diffraction File (PDF) database, complied by the Joint
Committee for Powder Diffraction Standards (JCPDS).
By this method, identification of any crystallinecompounds can be made even in complex samples.
Lecture-11
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Lecture-11
The position (d) of the diffracted peaks also providesinformation about how the atoms are arranged withinthe crystalline compound (unit cell size or latticeparameter). The intensity information is used toassess the type and nature of atoms. Determinationof lattice parameter helps understand extent of solidsolution (complete or partial substitution of oneelement for another, as in some alloys) in a sample.
The dand Ifrom a phase can also be used to
quantitatively estimate the amount of that phase in amulti-component mixture.
The width of the diffracted peaks is used to determinecrystallite size and micro-strain in the sample.
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If the sample consists of tens of randomly
oriented single crystals, the diffracted beams
are seen to lie on the surface of several cones.
Lecture-11
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Instrument geometriesThere are several ways of collecting XRPD patterns:
Camera methods: Guinier, Debye-Scherrer, Gandolfi,
Lecture-11
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The DebyeScherrer powder camera
A photographic film is placed around the inner circumference of the camera body.
The incident beam enters through a pinhole and almost the whole diffraction
pattern is recorded simultaneously. At the point of entrance the angle is 180and
at the exit the angle is 0.
Lecture-1
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Pinhole source
Film located on camerabody
Rod shaped sample
Sample rotates to givebetter randomness
Almost completeangular range covered
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Lecture-1
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View of an instrument Lecture-12
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Lecture-10Lecture-10
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X-Ray Powder Diffraction Instruments
Lecture-12
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