X-Ray Diffraction of CRYSTALS

8/13/2019 X-Ray Diffraction of CRYSTALS

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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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10/2010

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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11/2011

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.

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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,

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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.

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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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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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X-Ray Diffraction of CRYSTALS

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