Introduction to X-ray Powder Diffraction
(prepared by James R. Connolly, for EPS400-002, Introduction to X-Ray Powder Diffraction, Spring 2007)
(Material in this document is borrowed from many sources; all original material is ©2007 by James R. Connolly)
(Updated: 1-Jan-07) Page 3 of 9
• The X-ray tube
• The flat specimen (labeled sample in the diagram)
• The goniometer circle (labeled measuring circle in the diagram) which remains constant through the
analysis and is defined by the position of the target (Cu in our system) in the X-ray tube, the center of the
sample, and the position of the receiving slit (labeled detector diaphragm) on the detector side.
• The X-ray tube, specimen and receiving slit also lie on the arc of the focusing circle. Unlike the
goniometer circle which remains fixed, the radius of the focusing circle is a function of ?-2?, with the
radius decreasing as ? increases.
• The incident angle ? defined as the angle between the incident beam and the sample, and 2? defined as the
angle between the incident and diffracted beams. The detector is moved (rotated) at twice the angular rate
of the sample to maintain the ?-2? geometry.
• A filter (on the diffracted beam side) is used (in this example) to remove all but the desired K? radiation
from the diffracted beam before it enters the detector.
• A slit (labeled aperture diaphragm) on the incident beam side is used to narrow the beam so that it is
confined within the area of the specimen.
The photo above labels the important parts of our Scintag PAD V diffractometer. The following items are noted with differences
between the Scintag and Brukker systems.
• The path AB=BC is the radius of the diffractometer circle.
• The tube position is fixed and the ?-2? geometry is maintained by rotating the sample holder at ½ the angular rate of
the detector.
• There are Soller slits on both the tube and detector side, and two collimating and receiving slits.
• Note the easy-to-read angular indicators and micrometer dials for visually reading ? and 2?.
• The detector on this system also includes a graphite monochromator adjacent to the scintillation detector (off the photo,
top right) eliminating the need for any filters in the system.
Introduction to X-ray Powder Diffraction
(prepared by James R. Connolly, for EPS400-002, Introduction to X-Ray Powder Diffraction, Spring 2007)
(Material in this document is borrowed from many sources; all original material is ©2007 by James R. Connolly)
(Updated: 1-Jan-07) Page 4 of 9
Sample preparation
The Ideal Specimen is a statistically infinite amount of randomly oriented powder with
crystallite size less than 10 ?m, mounted in a manner in which there is no preferred crystallite
orientation.
In this day of automated data collection and analysis, the preparation of your specimen is usually
the most critical factor influencing the quality of your analytical data. Sample preparation is a
significant topic in this course.
Generate Analytical X-rays
A coherent beam of monochromatic X-rays of known wavelength is required for XRD
analysis
Striking a pure anode of a particular metal with high-energy electrons in a sealed vacuum tube
generates X-rays that may be used for X-ray diffraction. By the right choice of metal anode and
energy of accelerated electrons, a known wavelength (i.e., energy) or group of wavelengths will
dominate the X-rays generated. Copper (Cu) X-ray tubes are most commonly used for X-ray
diffraction of inorganic materials. The wavelength of the strongest Cu radiation (K?) is
approximately 1.54 angstroms (?). Other anodes commonly used in X-ray generating tubes
include Cr (K? 2.29 ?), Fe (K? 1.94 ?), Co (K? 1.79 ?), and Mo (K? 0.71 ?).
The full spectrum of radiation produced, and how it is “processed” to get to a (more or less)
monochromatic character will be discussed in more detail later. For most X-ray diffraction
applications, the closer we can get to monochromatic radiation in our X-ray beam, the better our
experimental results will be. The radiation produced in the tube includes K?1, K?2, and K? as
the highest energy X-rays and a whole host of lower energy radiation. We generally use the K?
for our analytical work. The K? radiation is usually removed by use of a filter, a
monochromator or an energy-selective detector. The K?2 radiation is removed from the X-ray
data electronically during data processing.
Direct the X-rays at a Powdered Specimen
An approximately parallel beam of X-rays is directed at the powdered specimen.
In most powder diffractometers systems a series of parallel plates (soller slits) arranged parallel
to the plane of the diffractometer circle and several scatter and receiving slits (arranged
perpendicular to the diffractometer circle) are used to create an incident beam of X-rays that are
(approximately) parallel. Soller slits are commonly used on both the incident and diffracted
beam, but this will vary depending on the particular system. The scatter slits (on the incident
beam side) may be varied to control the width of the incident beam that impinges upon the
specimen and the receiving slits may be varied to control the width of the beam entering the
detector.
Filters for removing K? may be located in the beam path on the generator or dete