Powder Diffraction

Table of Contents:

 

Introduction:

Powder diffraction is one of the most widely used material characterization methods. Over the last 50 years it has been routinely used for crystalline phase "finger-printing". Recently, cheap and powerful computers and dedicated 2nd and 3rd generation x-ray synchrotron sources have transformed powder diffraction into a very powerful structural tool. Powder diffractometry projects the three dimensional reciprocal lattice into one dimensional space. Such projection causes (partial) overlap of peaks with similar lattice spacing. The peak overlap loses structural information and sometimes makes the task of structure solution difficult. Nevertheless, the partial peak overlap is very useful in investigations of subtle symmetry breaks, symmetry breaks that are often missed in routine single crystal measurements. Further, the compaction of the entire reciprocal lattice into one dimension accelerates data collection. With a bright radiation source and appropriate detector many structural changes can be investigated in real time.

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Why Do Powder Diffraction at a Synchrotron Source?:

A diffractometer specifically designed to utilize x-ray radiation from a synchrotron source has several advantages over an instrument utilizing a laboratory x-ray source:
 

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The Powder Diffractometer at SSRL:

The dedicated powder diffractometer is built at one of the focused bending magnet beamlines. The size of the focused beam is 2x1 mm. Approximately 1011 photons/sec are incident on the sample at the energies between Fe K edge (7111 eV) to Zn K edge (9659 eV).
Flux at the Sample

(Click here to enlarge the image)

The dedicated powder diffractometer at SSRL is designed around two large concentric Huber goniometers.
(Click here for other photographs of the diffractometer.)
The 2-theta circle accommodates two detector systems. Either detector can cover a full range of scattering angles (0 to 150; 2q), or the detectors can be used simultaneously to speed data collection. One detector is based on a perfect crystal analyzer (usually Si111). The second detector arm accommodates either a 1 mrad Soller slit, an energy-resolving Ge or diode detector, or a linear position sensitive detector. With the appropriate choice of detector and the monochromator a powder diffraction pattern can be collected with ultrahigh to medium angular resolution.

Diffractometer Resolution:

(Click here to enlarge the image)

The most intense (110) peak of a standard LaB6 flat plate has 10K counts/sec in the ultrahigh resolution configuration whereas the same peak can have more than 300K counts/sec in the moderate resolution configuration.

Samples can be either 25mm wide flat plates, mounted into a cavity in one of the available Si zero background flat plate, or loaded into a 0.3 - 0.5 mm glass/quartz capillary. SSRL is developing sample stages to cover the temperature range from 2 to 1500 K, which will be available users on request.

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Data collected at SSRL

Proposal Submittal and Scheduling Procedures at SSRL 



For more information contact:  Apurva Mehta  or  John Arthur