Download X-ray Source

Chapters 6 & 9
X-ray Diffractometry and
Phase Identification
Wenjea J. Tseng
Department of Materials Science and Engineering
National Chung Hsing University
URL: http://audi.nchu.edu.tw/~wenjea/
Email: [email protected]
Chapters 6 & 9
Introduction
After the work of W.H. and W. L. Bragg on x-ray spectra and crystal
structure, diffractometry passed in to a long period of relative disuse during which photographic recording in cameras was the most
popular method of observing diffraction effects. In the late 1940s,
commercial diffractometer instruments became available and rapidly
became popular because they offered certain particular advantages
over the film techniques.
Bragg’s Law (1912)
William H. Bragg
William L. Bragg
Chapters 6 & 9
Introduction (cont.)
Debye-Scherrer camera method modify the Laue
photography by adopting the Bragg Law into account.
Debye-Scherrer Camera
X-ray in
Camera film is “taped” around the inner
circle of the sampling chamber with
sample being placed at the center.
Sample
position
Chapters 6 & 9
Introduction (cont.)
Problems associated with the camera method include
primarily the lack of quantitative intensity information
due to difficulties in distinguishing overlapping lines with
close two theta values.
Chapters 6 & 9
General Features
A diffractometer is designed with a movable detector replaces the
strip of film. The intensity of a diffracted beam is measured directly
by the electronic x-ray detector, which converts incoming x-rays into
surges or pulses of electric current for further computer processing.
Diffractometers
X-ray
Source
Sample
Chamber
Detector
X-ray
Source
Detector
Sample
Chambe
r
Chapters 6 & 9
Basic Configuration
Sample
Chamber
Detector
X-ray
Source
Chapters 6 & 9
Example
Chapters 6 & 9
Comparison
Tungsten
Five “rings”
Five “peaks”
Difficult to discern
in camera film.
Discernible in
diffracted peaks.
X-ray Source
Chapters 6 & 9
Cross-sectional
schematic
Real cross-sectional
photograph.
Chapters 6 & 9
X-ray Source
Chapters 6 & 9
X-ray Optics
X-ray
Source
Sample
Chambe
r
Detector
How to “focus” the X-ray beam
is a difficult issue.
Use slit to get “parallel” rays.
Sacrifice some x-ray intensity.
Can adjust the slit size.
Chapters 6 & 9
X-ray Optics (cont.)
Slit is needed also on the detector side.
X-ray
Source
Sample
Chambe
r
Detector
X-ray
incoming
slit
X-ray
diffracting slit
Sacrifice some diffracted x-ray intensity.
Can adjust the slit size.
Chapters 6 & 9
X-ray
Source
X-ray Detection
Sample
Chambe
r
Detector
Geiger counter
Voltage
Chapters 6 & 9
Sample Preparation
Flat metal sheet or plate may be examined by diffractometer directly;
however, such materials almost always exhibit preferred orientation
so that relative intensity between diffracted peaks may vary.
Whenever possible, powder samples are best choice for the
diffractometry. Yet, packing of the powdered sample requires
precautions.
Use B-side for
the incoming
x-ray.
Chapters 6 & 9
Example 1
A gel consisting of AgBr crystal is placed within a sample holder for
x-ray examination. The gel surface was initially flat within the
sample holder, while the surface of gel became more concave
because of the drying shrinkage with time.
Time increases
Chapters 6 & 9
Example 2
Aluminum powder was packed level (0 displacement), above
(+ displacement), and below (- displacement) the sample plane. The
observed 2θ values are 38.423o, 38.665o (+0.242o), and 38.281o
(-0.142o), respectively. This represents calculated displacement of
+450 and -260 µm, respectively.
Displacement
of Al powder
Use silicon
powder as an
internal
standard for the
2θ precision.
Chapters 6 & 9
Tips on the Internal Standard
The most frequently used method to solve such a displacement
problem is to include an internal standard in the sample tested. The
selection of the internal standard material is, first of all, the diffracted
peaks from the standard should NOT overlap with that from the
tested sample. Second, the standard sample should NOT react with
the tested sample. Pure materials such as Si, Ag in the powder form
are frequently used.
Chapters 6 & 9
Practice in Phase Identification
A given crystalline substance (e.g., pure element, mixture, or
compound) always produces a characteristic diffraction pattern. An
unknown sample can hence in theory be identified by comparing its
x-ray diffraction pattern with those in the databank.
Qualitative analysis of unknown samples can be accomplished.
Quantitative analysis is also possible because the intensities of the
diffraction lines due to one phase of a mixture (or compound)
depend on the proportion of that phase in the specimen.
Any one powder pattern is characterized by a set of line positions
2θ and a set of relative line intensities I/Imax scaled relative to Imax.
Chapters 6 & 9
XRD Databank
Chapters 6 & 9
Typical JCPDS Card
Chapters 6 & 9
Example
JCPDS
(or PDF)
databank
Experimentally
obtained data
Chapters 6 & 9
Choice of X-ray Source
Chapters 6 & 9
Hanawalt Search Method
Chapters 6 & 9
Hanawalt Search Method
Chapters 6 & 9
Example: One Component
5-490
Chapters 6 & 9
Example: Multi-components
Chapters 6 & 9
Example: Multi-components
Chapters 6 & 9
Remarks
Problems due to inadequate data treatment in finding
the angular positions (i.e., 2θ) of the peak maxima, plus
uncertainty in the value of the experimental wavelength,
all conspire to add errors to the experimental d-values.
In addition, the experimental intensities may be similarly
distorted due to problems of preferred orientation, poor
crystallinity, partially resolved diffraction wavelength
multiplets, and line broadening due to particle size
and/or strain considerations.