X-Ray Powder Diffraction

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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 capillary

sample configuration, spinning and

rocking methods, variable temperature

and humidity conditions, and a unique

sample conveyor system to overcome

sample inhomogeneity effects.

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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 powdery

form, consisting of fine grains of single crystalline

material to be studied. The technique provides

information that cannot be obtained any other way. The

information obtained includes types and nature of

crystalline 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 studying

particles in liquid suspensions or polycrystalline solids

(bulk or thin film materials).

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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 scattering peaks

corresponding to the various d spacings in the crystal

lattice. The positions and the intensities of the peaks

are 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).

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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.

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Single crystal diffraction

Laue‟s method - variable, fixed.

Rotating crystal method - fixed, variable to some 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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Powder diffractionIn this method the crystal is reduced to a

fine powder and is placed in a beam of

monochromatic X-rays. Each particle is a tiny

crystal or an assemblage of smaller crystals

randomly oriented with respect to the the

incident beam.

Powder methods - fixed, variable.

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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 Bragg’s law, n= 2dsin . The integer n refers to the order

of diffraction. For n = 1, (A+B) = and for n = 2, (A+B) = 2 etc.

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• Angles are used to calculate the interplanar atomic

spacings (d-spacings). Because every crystalline

material 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 for

over 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 crystalline

compounds can be made even in complex samples.

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The position (d) of the diffracted peaks also provides information about how the atoms are arranged within the crystalline compound (unit cell size or lattice parameter). The intensity information is used to assess the type and nature of atoms. Determination of lattice parameter helps understand extent of solid solution (complete or partial substitution of one element for another, as in some alloys) in a sample.

The „d‟ and „I‟ from a phase can also be used to quantitatively estimate the amount of that phase in a multi-component mixture.

The width of the diffracted peaks is used to determine crystallite 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.

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Instrument geometriesThere are several ways of collecting XRPD patterns:

Camera methods: Guinier, Debye-Scherrer, Gandolfi,

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The Debye – Scherrer 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 180 and

at the exit the angle is 0.

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Pinhole source

Film located on camera body

Rod shaped sample

Sample rotates to give better “randomness”

Almost complete angular range covered

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View of an instrument Lecture-12

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X-Ray Powder Diffraction Instruments

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