Powder X-Ray Diffraction: Assist - Prof.Drnawal Al-Karwi

Powder X- ray Diffraction
Supervised by: Assist.Prof.DrNawal Al-Karwi

by : Nada Talib Jaber
Content
• Introduction to X-ray powder diffraction

• Fundamental principles
• Instrumentation
• Applications
• Strength & Limitations
Introduction
➢ Diffraction is defined as the bending of light around or into the
geometrical shadow of the obstacle.
➢ In powder X-ray diffraction, the diffraction pattern is obtained from a
powder of the material, rather than an individual crystal.
➢ Powder diffraction is often easier and more convenient than single
crystal diffraction as about 1 mg of material is sufficient for the study.
WHAT IS X-RAY POWDER DIFFRACTION (XRD)
▪ X-ray powder diffraction (XRD) is a rapid analytical technique

primarily used for phase identification of a crystalline material
and can provide information on unit cell dimensions
▪ In other methods a single crystal is required whose size is much
larger than microscopic dimensions. However, in the powder
method as little as 1 mg of the material is sufficient for the study.
▪ The analyzed material is finely ground, homogenized, and average
bulk composition is determined.
FUNDAMENTAL PRINCIPLES OF X-RAY
POWDER DIFFRACTION (XRD)
• diffraction is based on constructive interference
of monochromatic X-rays and a crystalline sample.
• These X-rays are generated by a cathode ray tube,

filtered to produce monochromatic radiation,
collimated to concentrate, and directed toward the
sample
• For every set of crystal planes , one or more
crystals will be in the correct orientation to give the
correct Bragg angle to satisfy Bragg's equation.
• Each diffraction line is made up of a large number
of small spots, each from a separate crystal.
• Each spot is so small as to give the appearance of a
continuous line.
• Every crystal plane is thus capable of diffraction.
Diffraction cones and the Debye-Scherrer geometry
X-ray diffraction pattern
• The powdered sample generates the concentric
cones of diffracted X-rays because of the random
orientation of crystallites in the sample.
• Hence, instead of the sample generating only single

diffraction spots, it generates cones of diffracted X-
rays, with the point of all of the cones at the sample.
• The powder diffracts the x-rays in accordance with
Bragg’s law to produce cones of diffracted beams.
These cones intersect a strip of photographic film
located in the cylindrical camera to produce a
characteristic set of arcs on the film.
• When the film is removed from the camera, flattened

and processed, it shows the diffraction lines and the
holes for the incident and transmitted beams.
• The x-ray pattern of a pure crystalline substance
can be considered as a “fingerprint” with each
crystalline material having, within limits, a unique
diffraction pattern.
X-RAY POWDER DIFFRACTION (XRD) INSTRUMENTATION
- HOW DOES IT WORK?
The experimental arrangement of powder crystal method is shown in
Fig.1. its main feature are outlined as below:
(1) A is a source of X-rays which can be made monochromatic by a filter

(2) Allow the X-ray beam to fall on the powdered specimen P through
the slits S1 and S2. The function of these slits is to get a narrow pencil
of X-rays.
.
(3) The X-rays after falling on the powder passes out of the camera
through a cut in the film so as to minimize the fogging produced by the
scattering of the direct beam.
(4) As the sample and detector are rotated, the intensity of the reflected
X-rays is recorded.
(5) When the geometry of the incident X-rays impinging the sample
satisfies the Bragg Equation, constructive interference occurs and a peak
in intensity occurs.
(6) A detector records and processes this X-ray signal and converts the
signal to a count rate which is then output to a device such as a printer
or computer monitor.
Figure 2. Example of an X-ray
powder diffractogram produced
during an X-ray scan. The
peaks represent positions where
the X-ray beam has been
diffracted by
the crystal lattice. The set of d-
spacing (the distance between
adjacent planes of atoms),
which represent the unique
"fingerprint" of the mineral,
can easily be calculated from
the 2-theta (2) values shown.
APPLICATIONS
◼ Identification : The most common use of powder (polycrystalline)

diffraction is chemical analysis . The precipitate:
- This can include phase identification (search/match)
❑ X-ray powder diffraction is most widely used for the identification of
unknown crystalline materials (e.g. minerals, inorganic compounds).
❑ Other applications include :
- identification of fine-grained minerals such as clays and mixed layer clays
that are difficult to determine optically.
- determination of unit cell dimensions.
- measurement of sample purity .
- Most useful for cubic crystal
APPLICATIONS
- Used for determining the complex structure of metals and alloys.

- Useful to make distinction between the allotropic modifications of the same
substance.
STRENGTHS AND LIMITATIONS OF XRAY
POWDER DIFFRACTION (XRD)?
▪ Strengths :-
➢ Powerful and rapid (< 20 min) technique for
identification of an unknown mineral
➢ In most cases, it provides a clear structural
determination.
➢ XRD units are widely available.
➢ Data interpretation is relatively straight forward .
▪ Limitations:
➢ Homogeneous and single phase material is best for
identification of an unknown
➢ Requires tenths of a gram of material which must be ground
into a powder.
➢ For mixed materials, detection limit is ~ 2%of sample
➢ Peak overlay may occur and worsens for high angle
'reflections’.
➢ For unit cell determinations, indexing of patterns for non-
isometric crystal systems is complicated

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Powder X- ray Diffraction

Supervised by: Assist.Prof.DrNawal Al-Karwi

• Introduction to X-ray powder diffraction

▪ X-ray powder diffraction (XRD) is a rapid analytical technique

• These X-rays are generated by a cathode ray tube,

• Hence, instead of the sample generating only single

• When the film is removed from the camera, flattened

(1) A is a source of X-rays which can be made monochromatic by a filter

◼ Identification : The most common use of powder (polycrystalline)

- Used for determining the complex structure of metals and alloys.

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